Astronomy

Is there some upper limit in the moon size distribution?

Is there some upper limit in the moon size distribution?


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The planets range hugely in sizes, Jupiter is more than 300 times as massive as Earth. But the sizes of the large moons seem to be uncorrelated with the sizes of the planets they orbit. Why doesn't Jupiter have a giant moon with a mass hundreds times larger than the Earth's Moon's?

Among gas giants, Saturn at a third of Jupiter's mass has the largest of all moons and Triton way out at 30 AU Neptune is the 7th largest of all moons. The theories of the origin of the moons seem to vary much too, without explaining any upper size limit. The Moon was formed by an impact. Triton and maybe Titan were captured. Other large moons were formed along with their planet. Is this upper size limit on moons imaginary, coincidental or is there some reason for it to be this way? I of course mean a hypothetical expected upper size limit for exomoons in general, not only statistics about the Solar System moons.

The wonderful illustration here is from Emily Lakdawalla at The Planetary Society, it is now my desktop background.


Small point, but Ganymede is slightly larger than Titan as moons are measured by their solid surface, though with atmosphere, Titan appears slightly bigger. The drawing is probably accurate, but slightly misleading.

It's worth adding that Titan is out-gassing so it may have, quite some time ago, been larger than Ganymede.

Source

According to wikipedia, the larger, close to Jupiter and Saturn moons are unlikely to have been captured moons.

https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System#Moons

Jupiter and Saturn have several large moons, such as Io, Europa, Ganymede and Titan, which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun.[76] This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries also make formation from collision debris unlikely. The outer moons of the giant planets tend to be small and have eccentric orbits with arbitrary inclinations. These are the characteristics expected of captured bodies

and the 3 listed methods of moon formation are:

Co-formation Impact Capture.

The largest co-formation ratio is Titan to Saturn, about 1 to 4,000 in terms of mass. Ganymede to Jupiter, less than 1 to 12,000. The wiki article points out that ratios of 100 to 1 are likely giant impacts and you're unlikely to see that kind of ratio in co formation, so a conservative estimate, the largest co-formation you mgiht see might be around a few hundred to 1. If you figure the largest planets are about 12-12.5 Jupiters (more than that, they become brown dwarfs, though a cool brown dwarf orbiting a sun might get called a planet too at greater than 13 Jupiter masses), but lets say, 12 Jupiters. That's about 4,000 earth masses. If we look at a co-formation ratio of a few hundred to perhaps over a thousand to 1 as about the maxiumum, there's a possible ballpark answer of a handul of earth masses might be about as big as a co-formation moon is likely to get in a perfect formation situation, And those would probably be pretty rare.

Giant impacts are different. Pluto-Charon could be an impact generated dwarf planet/moon system and Charon is 1/8th the mass of Pluto. How likely that kind of impact is to happen on a super earth is hard to say, but theoretically a super-earth with a giant impact could have a large moon too - a bigger earth with a bigger impact and bigger moon, but there's problems here. Giant planets are likely to form close to the sun and the orbital sphere of influence gets smaller closer to the sun. Also, giant planets might tend to form thick gaseous atmospheres, retaining all their hydrogen and helium, so too big, it might begin to resemble a gas giant, which aren't as good at creating debris when impacted, so there's a few limiting factors here and we might be limited to, certainly several times larger than our moon, but perhaps not as large as co-formation moons, further from the sun.

Finally, moon capture. While there's no examples of this in our solar system, I see no reason why a large planet couldn't capture another planet, though you then have to determine whether it's a co-planet orbit, which I think a captured planet should be considered, or whether the captured object is a moon. - personally I think a captured planet that orbits a planet would be a co-planet system, so I don't like the capture as a method for getting as large a moon as possible, though I suppose it could be looked at either way.

We know almost nothing about moons in other solar systems (exomoons). The observations are just barely possible with current technology, but very difficult. That's likely going to change in the next few years, but for now we know virtually nothing in other solar systems, so, any present predictions on the largest possible moons would have to be modeled or estimated.

A few articles on exomoons:

http://www.astrobio.net/news-exclusive/new-exomoon-hunting-technique-could-find-solar-system-like-moons/

http://discovermagazine.com/2012/jul-aug/06-hunting-moons-outside-the-solar-system

http://www.space.com/25438-exomoon-around-alien-planet-discovery.html

http://phys.org/news/2014-10-exomoons-abundant-sources-habitability.html

http://blogs.scientificamerican.com/life-unbounded/has-an-exomoon-been-found2/


I saw this presentation of the National Academy of Science 2015 where Dr. Robin Canup explains, illustrated by simulations, that many large moons formed sequentially around giant planets. But their growing mass thru accretion caused perturbations in the gaseous planetary disk ("type I interaction") that drew them inwards to collide with the planet within a million years. The total mass of all the satellites to the planet's mass remains fairly constant given (partly) the density of the Solar Nebula. The larger moons crash into the planet while new moons are growing larger. The process stops when the Solar Nebula has been exhausted. So I suppose speculatively that different exoplanetary systems might have different moon size ceilings.

Impact formation size of Earth's Moon, in the middle of the Jovian moons, is a coincidence according to this model.


As the moon orbits the earth, its gravity pulls on the earth’s oceans, causing tides. Since the earth rotates faster than the moon orbits, the tidal bulges induced by the moon are always “ahead” of the moon. For this reason the tides actually “pull forward” on the moon, which causes the moon to gain energy and gradually spiral outward. The moon moves about an inch and a half farther away from the earth every year due to this tidal interaction. Thus, the moon would have been closer to the earth in the past.

Six thousand years ago, the moon would have been about 800 feet (250 m) closer to the earth (which is not much of a change considering the moon is nearly a quarter of a million miles, or 400,000 km, away). So this “spiraling away” of the moon is not a problem over the biblical time scale of 6,000 years, but if the earth and moon were over 4,000,000,000 years old (as big-bang supporters teach), then we would have big problems. This is because the moon would have been so close that it would actually have been touching the earth less than 1.5 billion years ago. This suggests that the moon can’t possibly be as old as secular astronomers claim.

Secular astronomers who assume the big bang is true must invoke other explanations to get around this. For example, they might assume that the rate at which the moon was receding was actually smaller in the past (for whatever reason), but this is an extra assumption needed to make their billions-of-years model work.

The simplest explanation is that the moon hasn’t been around for that long. The recession of the moon is a problem for a belief in billions of years, but is perfectly consistent with a young age.


Contents

The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The radiometric ages range from about 3.16 to 4.2 billion years old (Ga), [5] whereas the youngest ages determined from crater counting are about 1.2 Ga. [6] Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. The few basaltic eruptions that occurred on the far side are old, whereas the youngest flows are found within Oceanus Procellarum on the nearside. While many of the basalts either erupted within, or flowed into, low-lying impact basins, the largest expanse of volcanic units, Oceanus Procellarum, does not correspond to any known impact basin.

There are many common misconceptions concerning the spatial distribution of mare basalts.

  1. Since many mare basalts fill low-lying impact basins, it was once assumed that the impact event itself somehow caused the volcanic eruption. Note: current data in fact may not preclude this, although the timing and length of mare volcanism in a number of basins cast some doubt on it. Initial mare volcanism generally seems to have begun within 100 million years of basin formation. [7] Although these authors felt that 100 million years was sufficiently long that a correlation between impact and volcanism seemed unlikely, there are problems with this argument. [citation needed] The authors also point out that the oldest and deepest basalts in each basin are likely buried and inaccessible, leading to a sampling bias.
  2. It is sometimes suggested that the gravity field of the Earth might preferentially allow eruptions to occur on the near side, but not on the far side. However, in a reference frame rotating with the Moon, the centrifugal acceleration the Moon is experiencing is exactly equal and opposite to the gravitational acceleration of the Earth. There is thus no net force directed towards the Earth. The Earth tides do act to deform the shape of the Moon, but this shape is that of an elongated ellipsoid with high points at both the sub- and anti-Earth points. As an analogy, one should remember that there are two high tides per day on Earth, and not one.
  3. Since mare basaltic magmas are denser than upper crustal anorthositic materials, basaltic eruptions might be favored at locations of low elevation where the crust is thin. However, the far sideSouth Pole–Aitken basin contains the lowest elevations of the Moon and yet is only sparingly filled by basaltic lavas. In addition, the crustal thickness beneath this basin is predicted to be much smaller than beneath Oceanus Procellarum. While the thickness of the crust might modulate the quantity of basaltic lavas that ultimately reach the surface, crustal thickness by itself cannot be the sole factor controlling the distribution of mare basalts. [8]
  4. It is commonly suggested that there is some form of link between the synchronous rotation of the Moon about the Earth, and the mare basalts. However, gravitational torques that result in tidal despinning only arise from the moments of inertia of the body (these are directly relatable to the spherical harmonic degree-2 terms of the gravity field), and the mare basalts hardly contribute to this (see also tidal locking). (Hemispheric structures correspond to spherical harmonic degree 1, and do not contribute to the moments of inertia.) Furthermore, tidal despinning is predicted to have occurred quickly (in the order of thousands of years), whereas the majority of mare basalts erupted about one billion years later.

The reason that the mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP) is located within the regions of Oceanus Procellarum and the Imbrium basin, a unique geochemical province now referred to as the Procellarum KREEP Terrane. [9] [10] [11] While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon. [12]

Using terrestrial classification schemes, all mare basalts are classified as tholeiitic, but specific subclassifications have been invented to further describe the population of lunar basalts. Mare basalts are generally grouped into three series based on their major element chemistry: high-Ti basalts, low-Ti basalts, and very-low-Ti (VLT) basalts. While these groups were once thought to be distinct based on the Apollo samples, global remote sensing data from the Clementine mission now shows that there is a continuum of titanium concentrations between these end members, and that the high-titanium concentrations are the least abundant. TiO2 abundances can reach up to 15 wt.% for mare basalts, whereas most terrestrial basalts have abundances much less than 4 wt.%. A special group of lunar basalts is the KREEP basalts, which are abnormally rich in potassium (K), rare-earth elements (REE), and phosphorus (P). A major difference between terrestrial and lunar basalts is the near-total absence of water in any form in the lunar basalts. Lunar basalts do not contain hydrogen-bearing minerals like the amphiboles and phyllosilicates that are common in terrestrial basalts due to alteration or metamorphism. [ citation needed ]


Astronomy FAQ Frequently Asked Questions

As with any hobby, amateur astronomy can seem a little intimidating for those who are just beginning. Beginners can have a lot of questions about the the astronomy hobby and about astronomy in general. This astronomy faq page contains answers to some of the most frequently asked questions encountered in the amateur astronomy hobby, ranging from general astronomy to the Sun, the Moon, stars, and the Solar System.

We will be adding more questions and answers as time goes by. You can click on one of the questions below to jump directly to the answer, or browse at your leisure by scrolling down the list. If you have an astronomy question that is not answered here, please feel free to contact us by e-mail and we will try our best to answer the question.

General Astronomy Questions

The Earth

The Moon

The Sun

The Solar System

Stars

The Universe

General Astronomy Questions

What is the difference between astronomy and astrology?

Believe it or not, this is the most asked question that astronomers encounter. Many people do not understand the difference. In ancient times, they were considered one and the same. But the two disciplines were separated during the Age of Reason in the 17th century. Astrology is a practice of using the locations of the planets to look into a person's personality or predict the future. It is not a science and is considered a form of divination. By contrast, astronomy is the scientific study of the universe. Astronomers observe the objects in the night sky to try to determine their composition and learn more about the origin and structure of the universe.

Do I need an expensive telescope to enjoy astronomy?

Many people hesitate to get involved with astronomy because they believe it requires expensive equipment. The only thing you really need to enjoy the night sky is your eyes, a dark viewing location, and some patience. To get a better look at things, a pair of binoculars can provide a really good view. Many people will be surprised how many more stars and objects they can see with a decent pair of 10X binoculars. They collect much more light than the human eye and will bring much dimmer objects into view. You can even see Jupiter’s moons with binoculars. A simple camera tripod to steady the binoculars is also a good idea, since your arms can get tired very quickly.

How does a telescope work?

The primary purposes of a telescope are to gather light and magnify an image. The aperture (opening) of a telescope is larger than that of the human eye and therefore, can gather much more light. This enables us to see dim objects that are too faint to see with the naked eye. The larger the aperture of the telescope, the more light it can gather. Telescopes also use a series of lenses and/or mirrors to magnify the image, enabling us to see more detail.

Why can't I see very many stars at night?

If you live near a big city, you may not be able to see a lot of stars. The reason for this is light pollution. Dust and water vapor in the atmosphere reflects the bright city lights back down towards the ground. This “light pollution” tends to be brighter than some of the dim stars and other deep sky objects, essentially hiding them from view. To truly appreciate the night sky, you must get as far away from city lights as possible. There is no more beautiful sight then the band of the Milky Way stretching across a dark sky. We can all help to combat light pollution by convincing our local authorities to use more efficient light fixtures that shine the light on the ground and block it from going up into the sky.

Space is defined as the area above the Earth’s atmosphere. But there is no specific boundary since the atmosphere gradually thins out as you move farther away from the Earth. However, NASA awards astronaut status to anyone who flies above 50 miles (80 km).

This is another question that gets asked a lot. The blue color of the sky during the day is caused by scattered sunlight. The white light from the Sun is composed of all the colors of the rainbow. During the day, the molecules in the air scatter the blue light from the Sun more that the red light making the sky appear blue. In the evening, however, we see the red and orange colors because the blue light has been scattered away from our line of sight.

Why is the sky dark at night?

Believe it or not, there is no easy answer to this question. Scientists have observed that if the universe was infinitely large, and contained an infinite number of stars, then the night sky should actually be as bright and as hot as the surface of the Sun. But this obviously is not the case. This little brain teaser has come to be known as Olber’s paradox, named after the German astronomer who tried to solve the problem in 1823. The most likely explanation is that the universe is simply not old enough and the observable part of the universe contains too few stars to fill the sky with light. Thus, the night sky is dark.

What is the speed of light?

Light travels at a constant speed of 186,262 miles per second (299,792,458 meters per second). Since the speed of light is constant, it can also be used to measure vast distances. Distances between objects in space are measured in light years. One light year is equal to the distance light travels in a year, which is just under 6 trillion miles (10 trillion kilometers). The speed of light is considered to be the ultimate speed limit in the universe. Scientists believe that it is impossible to travel faster than light because any object traveling at the speed of light would have to achieve infinite mass.

Did galileo invent the telescope?

Many people believe that Galileo invented the telescope, but they are wrong. It was actually a man named Hans Lippershe from Holland who assembled the first telescope. Several years later, Galileo became the first person to use a telescope for astronomical observation. With his early telescope, Galileo observed the craters on the moon, the rings of Saturn, and the moons of Jupiter.

All though the exact age of the Earth is difficult to determine, scientists believe it to be about 4.54 billion years old. This age has been estimated by measuring the amount of radioactive decay in rocks and minerals on Earth, and by observing the elements that exist in the Sun. The age of the Earth is believed to be the same as the age of the Sun and the other planets in the Solar System since they all formed at the same time from the same raw materials.

The mean diameter of the Earth is 7,918 miles (12,742 kilometers). Believe it or not, the Earth is not a perfect sphere. The rotation of the planet causes it to bulge slightly at the equator. The diameter at the equator is 7,926 miles (12,756 kilometers), while the diameter at the poles is only 7,900 miles (12,720 kilometers). Likewise, the circumference of the Earth at the equator is 24,902 miles (40,075 kilometers) while the circumference at the poles is only 24,860 miles (40,008 kilometers). Because the Earth is flatter at the poles and wider at the equator it is actually considered to be an oblate spheroid instead of a perfect sphere. However, if the planet were to stop spinning on its axis, gravity would pull it into a nearly perfect sphere.

What is the Earth made of?

The Earth is actually layered much like the layers of an onion. The outer layer is called the crust. The crust is composed mainly of iron, oxygen, silicon, magnesium, sulfur, and nickel with trace amounts of other elements. The next layer down is the mantle, composed mainly of magnesium and iron. The inner-most layer is called the core, which is composed of two parts. The outer core is believed to be a liquid iron and nickel alloy while the inner core is a solid mass of iron. The inner core is thought to spin within the liquid outer core, creating a magnetic field that protects the Earth from solar radiation.

How much does the Earth weigh?

The Earth is actually weightless because it exists in space. But it does have mass. If we were able to weigh the Earth in its own gravitational field, it would weigh approximately 13.2 septillion pounds. That is 13,200,000,000,000,000,000,000,000 pounds (6,000,000,000,000,000,000,000,000 kilograms). This number assumes we are weighing the planet in Earth's gravitational field. It would weigh more or less on other planets, depending on their mass. But wait, there more. The Earth is actually gaining mass. It is believed that our planet is gaining about 40,000 metric tons of mass each year from space debris that impacts the surface and burns up in the atmosphere.

Why How fast does the Earth rotate?

The Earth rotates once on its axis each day. Because it is so large, it is moving incredibly fast. Its speed, however, depends on the location you are measuring. At the equator, the speed is 1,038 miles per hour (1,670 kilometers per hour). The speed decreases as you travel further north or south. The middle latitudes of the United States and Europe are moving at between 700 and 900 miles per hour (1,125 to 1,450 kph). At the the poles, it is hardly moving at all. At the south pole, you would take an entire day just to rotate once in place.

How fast does the Earth move around the Sun?

The Earth takes one year (365 days) to orbit the Sun. Since it is 93 million miles away from the Sun, it has to be moving extremely fast to cover this vast distance. As it orbits the Sun, the Earth moves at an average of about 67,000 miles per hour (107,300 kilometers per hour). This is only an average because the Earth's orbit around the Sun is not perfectly circular. It is actually slightly elliptical. When the Earth is closer to the Sun, it moves a little bit faster. When it is further away, it moves a little slower.

As the Sun shines on the surface of the Earth, it illuminates one half of the Earth's surface. The other half is in darkness, or night. The dividing line between day and night is known as the terminator. The terminator line passes through any given point on Earth two times a day, once during sunrise and again during sunset. Because of the tilt of the Earth's axis, the polar regions are an exception. They spend parts of the year in perpetual day and night. The terminator is not a sharp line on Earth. It is actually kind of blurry because the atmosphere scatters the sunlight. This is why we have dawn and dusk before sunrise and sunset.

How fast does the terminator move?

The terminator line moves across the surface of the Earth as the Earth rotates on its axis. Therefore, the speed that the terminator moves is the same as the speed that the Earth rotates. At the equator, the speed would be 1,038 miles per hour (1,670 kilometers per hour). The speed decreases as you travel further north or south.

How thick is the Earth's atmosphere?

The Earth's atmosphere is an extremely thin sheet of gas that surrounds the planet. All though the planet is nearly 8,000 miles (12,875 kilometers) in diameter, the atmosphere is only 62 miles (100 kilometers) thick. This thin sheet of gas is all that separates us from the cold vacuum of outer space. All though some gas molecules extend out as far as 300 miles (480 kilometers), the majority of the atmosphere's mass lies below 62 miles, which is generally considered to be the edge of space.

What is the Earth's atmosphere made of?

The Earth's atmosphere is made of a variety of different gasses. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.4% carbon dioxide, and trace amounts of other gasses such as neon, helium, and methane. It also contains about 1% water vapor, which forms clouds and storms.

Science is still learning about the composition of the Moon. We know a lot about its surface, but the inside is still a mystery. The outer layer of the Moon is known as the crust, and is composed mainly of silicon, oxygen, magnesium, iron, calcium, and aluminum. Below the crust is the mantle, which is believed to be made primarily of the minerals olivine, orthopyroxene and clinopyroxene. The Moon's mantle is thought to contain more iron than the mantle of Earth. At the center of the Moon is the core, which is believed to be composed of metallic iron with small amounts of nickel and sulfur.

How far is the Moon from Earth?

The distance between the Moon and the Earth averages 238,857 miles (384,403 kilometers). Since the Moon’s orbit is not a perfect circle, its distance varies. At its farthest point, known as apogee, it is 252,080 miles (405,686 km) away. At its closest point, known as perigee, it is 225,621 miles (363,104 km) away.

Is the Moon moving away from the Earth?

Yes, the Moon is gradually moving away from the Earth at the rate of about 1.5 inches (3.8 centimeters) per year. We know this because we can track the Moon’s distance using lasers. The Apollo moon missions left reflective mirrors on the Moon’s surface. By measuring the time it takes a laser beam to travel to the Moon and back, we can calculate the distance using the speed of light. When the Moon first formed, it was much closer to the Earth. Astronomers believe it was about 12 times closer than it is now, which means it would have been much larger in the night sky.

Astronomers believe that the Moon was formed billions of years ago when a small planet the size of Mars collided with the Earth. The foreign planet hit with a glancing blow and ejected a large part of the Earth’s molten mantle into space. Over time, this material coalesced and cooled to form the Moon.

The Moon shines because the light from the Sun shines and reflects from the Moon’s surface. What we think of as Moon shine is actually just reflected sunlight.

Why is the Moon larger when it is close to the horizon?

Although the Moon looks much larger when it is low in the sky near the horizon, this is actually just an optical illusion. It is actually the same size as when it is directly over head. This illusion has been known since ancient times and also happens with the Sun and the constellations. This same illusion works on mountains and tall buildings as well. They appear larger at long distances than they do at closer distances. The reasons for this are complex, but they have something to do with how our brains interpret the sizes of large objects on the horizon. If you don't believe this is only an illusion, you can compare the size of the Moon near the horizon to the size directly over head by holding your finger out at arm's length and comparing the sizes of the Moon with your finger.

What causes the phases of the Moon?

The Moon goes through phases because it is traveling around the Earth. One half of the Moon is always illuminated by the Sun. As the Moon circles the Earth, different amounts of the illuminated part of the Moon are facing us. These phases range from Full Moon (when the Moon is on the opposite side of the Earth then the Sun) to a New Moon (when the Moon is between the Sun and the Earth. It takes about 29 and a half days for a complete cycle, which equals one complete orbit of the Moon around the Earth.

Can you see the flag on the Moon with a telescope?

This is a question that astronomers get asked a lot. Unfortunately, the equipment left behind by the Apollo missions is tiny in comparison to the size of the Moon. Ground-based telescopes, especially those owned by amateur observers, are not capable of resolving objects this small at such extreme distances. Extremely large telescopes could theoretically catch a bright spot of sunlight reflecting from some of the moon landing equipment, although they would not be able to observe the equipment directly.

The answer to this question is a bit complicated. The most recent and most popular definition says that a blue moon is the second of two full moons occurring in the same month. Since the lunar cycle is 29 days and most months have 30-31 days, we eventually find a situation where a full moon occurs at the beginning and the ending of the same month. There is also a second, older definition of a blue moon. This one defines a blue moon as the third full moon in a season with four full moons. Normally there is one full moon each month, so a season such as summer would usually have three full moons. The reason for this is complex, and has to do with the ancient Christian ecclesiastical calendar. This calendar was used to determine important dates such as Easter. Each of the usual 12 full moons of the year had a name associated with the time of year in which they usually occurred. In a year with 13 full moons, the extra full moon was referred to as a blue moon so the calendar could stay on track.

The average distance from the Sun to the Earth is 93 million miles (149 million kilometers). Because the Earth’s orbit around the Sun is not a perfect circle, it varies. At its closest point to the Sun, known as perihelion, the distance is 91 million miles (146 million km). At is farthest point, known as aphelion, the distance is 94.5 million miles (152 million km).

The Sun is an average-sized star that is 865,000 miles (1,392,000 kilometers) in diameter. It is so large that you could fit the planet Earth inside it well over a million times. The Sun actually makes up about 99% of the entire mass of the Solar System. The remaining objects, including all of the planets, moons, comets, and asteroids compose the other 1% of the Solar System.

The core of the Sun is extremely hot at about 27 million degrees Fahrenheit (15 million degrees Celsius). The surface of the Sun is much cooler than the core, at about 9,900 degrees F (5,500 degrees C). For some strange reason, not yet completely understood by scientists, the Sun’s outer atmosphere is hotter than its surface. Known as the corona, its temperature reaches 5 million degrees F (2.7 million degrees C).

How long does it take the light from the Sun to reach Earth?

The light from the Sun travels at the speed of light, 186,282 miles per second. Since the Sun is about 93 million miles from Earth, it takes the light about 8.4 minutes to reach us. This means that when you look up at the Sun, you are actually seeing it the way it looked 8.4 minutes ago. To give you an idea just how close we are to the Sun, the light from the next nearest star, Proxima Centauri, takes 4.3 years to reach the Earth.

What are sunspots and why do they appear dark?

Sunspots are temporary areas on the surface of the Sun that are cooler than the surrounding areas. They are caused by intense magnetic activity that inhibits convection and reduces the surface temperature. Sunspots appear dark on images of the Sun taken with filters because the filter significantly reduces the brightness of the Sun overall. If you could observe a sunspot by itself, away from the rest of the Sun, it would actually be blindingly bright.

The solar wind is a stream of charged particles that are ejected from the Sun in all directions at a very high rate of speed. This constant stream of particles can interact with planets by slowly blowing away their atmospheres. The Earth has a strong magnetic field that deflects the solar wind around the planet and protects our atmosphere. Sometimes the particles from the solar wind can interact with gases in the upper atmosphere of the Earth, causing them to glow. This phenomenon is known as the aurora borealis (northern lights) in the northern hemisphere and the aurora australis in the southern hemisphere.

How many planets are there in the Solar System?

This used to be an easy question. Many of us grew up learning about the nine planets, starting with Mercury and ending with Pluto. But recent discoveries of other Pluto-type objects in the outer Solar System began to call Pluto’s planet status into question. Finally, in 2006, the International Astronomical Union (IAU) decided to change the official definition of a planet. Pluto was reclassifies as a dwarf planet, leaving the total number of planets in our Solar System at eight.

What is the largest planet in the Solar System?

Of the eight planets in our Solar System, the largest is Jupiter. This giant planet is over a thousand times larger than the Earth, and is composed mostly of hydrogen gas. The famous giant red spot on Jupiter is a giant storm system that has been raging for several hundred years and is actually twice the size of the Earth.

What is the smallest planet in the Solar System?

The smallest planet in the Solar System used to be Pluto, with a diameter of 1,441 miles (2,320 kilometers). But in 2006, Pluto was demoted and reclassified as a dwarf planet in. Now Mercury is the smallest planet in the Solar System with a diameter of 3,032 miles. ( 4,879 kilometers).

How old is the Solar System?

Astronomers believe that the Solar System is about 4.6 billion years old. They have determined this age in part by studying meteorites. It is believed that meteors formed at the same time as the rest of the Solar System from a could of dust and gas. When meteors fall to earth as meteorites, a technique called radioactive dating can be used to calculate how old they are. Astronomers also believe the Sun is about middle aged, which means it should continue to shine for about another 5 billion years.

What exactly is a meteor shower?

A meteor shower is an event where a large number of meteors appear to radiate from a common point in the night sky. The meteors are cause by streams of debris left over by comets. This debris is usually no larger than a grain of sand. As the debris enters the Earth’s atmosphere at a high rate of speed, the friction causes the gases to glow. This glowing trail of ionized gas is known as an ionization trail. Meteor showers occur at the same time every year as the Earth passes through the debris field. Meteor showers can be stronger if the comet has recently passed by leaving a fresh stream of debris.

What are asteroids and where do they come from?

An asteroid is a small, rocky or metallic body found in orbit around the Sun. Most of the asteroids in our Solar System are located between the orbits of Mars and Jupiter in an area known as the asteroid belt. They are thought to be the left over remains of a planet that was either destroyed or never fully formed. Asteroids can range in size from a few feet to several miles in diameter. Occasionally, gravity can cause an asteroid to change its orbit and send it on a path towards the inner Solar System. If the asteroid crosses the orbit of Earth, it is known as a near-Earth asteroid, or NEA. NEAs have a small chance of eventually colliding with the Earth. Meteor crater in Arizona is an impact crater nearly a mile across that was caused by an asteroid about the size of a city bus. It is believed that an asteroid about 5 miles in diameter may have been responsible for the extinction of the dinosaurs.

What are comets and where do they come from?

Comets are relatively small Solar System bodies composed of ice and dust. Due to this unusual composition, many astronomers refer to them as “dirty snowballs”. It is believed that most comets originate in an area at the outer edge of the Solar System known as the Oort cloud. Occasionally, gravity will disturb a comet’s orbit and send it on a new course toward the inner Solar System. Comets with highly elliptical orbits like this are known as periodic comets, and they return to the inner Solar System at a regular period. The most famous periodic comet is comet Halley, with a period of 76 tears.

Comets are probably best known for their long, luminous tails. These tails are actually plumes of dust and gas that are ejected from the comet as it nears the Sun. Comets are composed of frozen ice and dust. If a comet’s orbit takes it close to the Sun, the solar radiation will cause the volatile materials in the comet to vaporize, carrying some of the dust along with them. As the Sun shines on this vaporized material, known as the coma, it begins to glow. The solar wind pushes the material out away from the comet. Because of this, a comet’s tail always points away from the Sun. Comets will usually have two tails, one formed from ionized gas, and the other formed from dust reflecting the sunlight.

A star is a gigantic, luminous ball of heated gas, or plasma, held together by gravity. They are formed mainly of hydrogen and helium. Stars burn helium in a process called nuclear fusion, where helium atoms are fused together under enormous pressure and temperature to form helium. This process gives off an incredible amount of energy. Stars are very large. The Sun, which is the closest star to the Earth, is so large that you could fit a million Earths inside it.

How far away is the closest star?

This is actually a trick question. The closest star to the Earth would be the Sun, at a distance of about 93 million miles (149 million kilometers). The closest star outside our solar system would be Proxima Centauri. It is located about 4 light years from Earth. That is over 23 trillion miles (38 trillion kilometers).

Why do stars seem to twinkle?

The twinkling of stars is caused by instability of our atmosphere. As the starlight passes through the atmosphere, the movement of the air bends the light slightly and makes the stars twinkle. If you could view the stars from outside the atmosphere, like the space station or the Moon, they would not twinkle.

How many stars are visible in the night sky?

The number of stars visible in the night sky depends on many factors, such as the clarity of the atmosphere, the time of the year, and the amount of light pollution. But on a good night, far away from city lights, you should be able to see about 2,000 stars with the naked eye. Astronomers have calculated that about 6,000 stars should be seen from the darkest locations.

How many stars are there in the universe?

The universe is unbelievably huge. There are billions of galaxies and each galaxy contains billions of stars. The latest estimates from astronomers say that there are a staggering 300 sextillion stars in the known universe. That is a 3 followed by 23 zeros, or 3 trillion times 100 billion. That represents several stars for every grain of sand on Earth.

What is the brightest star in the sky?

The brightest star visible in the night sky is Sirius, located in the constellation Canis Major. With an apparent magnitude of -1.46, it is nearly twice as bright as Canopus, the second brightest star. Apparent magnitude is a measure of a star’s brightness as seen from Earth. The lower the number, the brighter the star. Some of the planets appear like stars and can be brighter than Sirius when they are close to the Earth. Jupiter can have an apparent magnitude of -2.6 when it is close.

The life cycles of stars differ greatly and depend mostly on the star’s size. Large stars burn their fuel faster while smaller stars burn it more slowly. The largest stars burn their fuel so fast that they only last for a few million years. Average sized stars like the Sun live for about 10 billion years. Smaller stars, such as red dwarfs, burn their fuel so slowly that they can live for trillions of years. Our Sun is believed to be about 4.5 billion years old. It should shine for about another 5 billion years.

Just as the lives of stars depends on their size, so does their death. Stars exist in a state of equilibrium because the gravity pulling in on them equals the pressure pushing out. When a star begins to run out of fuel, the outward pressure decreases, and gravity wins. The effect of gravity on the star depends on its size. Smaller stars will eventually lose their outer layers and shrink their cores to form white dwarfs. Larger, more massive stars will experience much more violent deaths. Gravity crushes these stars so fast that a shock wave is created, resulting in a massive explosion known as a supernova. What remains behind depends on the mass of the star. Large stars will form extremely dense objects known as neutron stars. The largest and most massive stars will experience such a tremendous crush of gravity that they will literally be crushed out of existence into what are known as black holes.

What exactly are constellations?

Constellations are arbitrary groupings of stars that seen to form pictures. Ages ago, ancient people looked up at the night sky and thought they could see patterns. Since different constellations are visible at different times of the year, people used these patterns to tell what time of the year it was. Today, we use the constellations to map the night sky and help classify the locations of objects.

How many constellations are there in the night sky?

Throughout the ages, different cultures saw different patterns and images in the stars. Thus, the constellations have changed over time. Today, we divide the sky into 88 different constellations. Astronomers use them to classify objects in the sky. Every star or other object in the night sky falls into one of these 88 constellations.

Are all of the stars in a constellation the same distance from us?

No. Most of the stars in a constellation have no connection with one another. They are simply chance alignments of the stars. The stars are all different distances from us, but because they are so far they all appear flat as seen from Earth. If you could travel far out into space, the constellations would look completely different from your point of view.

Are the constellations permanent?

All of the stars in the sky are moving in relation to the Earth. Since they are so far away, it takes thousands of years to notice their movement. But eventually the movement of the stars will make today’s constellations completely unrecognizable.

This is a question that has puzzled astronomers for many years. The most recent estimates put the age of the universe at between 13 and 20 billion years. However, as new discoveries are made with the Hubble space telescope and other new technologies, these numbers may continue to be revised.

The latest estimates say it is about 156 billion light years across. Since one light year equals about 6 trillion miles, that is a very big number. But since we can only see so far, we may never know exactly how big the universe is. Some astronomers even believe the universe is infinite in size. And there may be even other universes out there beyond ours.

How did the universe begin?

The most widely accepted scientific theory today suggests that the universe began about 14 billion years ago from an infinitely small, dense, and hot state that expanded rapidly. As it continued to expand and cool, hydrogen gas formed into stars and eventually formed into galaxies. This theory is known as the big bang theory and helps to explain why the universe appears to be expanding today. Astronomers have observed that galaxies are moving away from each other. The farther away a galaxy is, the faster it is moving away. The big bang theory states that it is actually the fabric of space that is expanding. Matter is simply going along for the ride like dust on the surface of a balloon. As the balloon expands, the dust gets farther and farther apart.

How will the universe end?

Scientists used to think that the combined gravity from all of the matter in the universe would eventually slow its expansion and cause it to contract back into an infinitely small, dense state as it existed before the big bang. But now, most agree that there is not enough matter in the universe to slow its expansion. This means that it will continue to expand until all of the stars eventually burn out. Eventually, matter will decompose and all that will be left is a cold, dark void. That may sound depressing, but it will take at least 100 trillion years.

How many galaxies are there in the universe?

This is a question that continually puzzles astronomers and is subject to frequent revision. The reason for this is because we can only see so far with the instruments we have available to us today. The most recent estimates by astronomers suggest that there are about 200 billion galaxies in the known universe. However, as new telescopes and new technologies emerge, this number is almost sure to be increased as we gain a better understanding into the true size of the universe.


Earth has two 'hidden moons', but they aren't moons at all

Sadly, this isn't what the Galileo spacecraft saw in 1992.

How many moons does the Earth really have?

On Nov. 6, a flurry of publications ran headlines that suggested our moon had two new neighbours. Some of those headlines were restrained, such as Nat Geo's "Earth has two extra, hidden 'moons'" whereas others were a little more fanciful, like The Weather Channel's "The Earth has not one, but three moons".

The revelation stems back to a paper first published in Monthly Notices of the Royal Astronomical Society on Sept. 1. The paper, by a Hungarian team at ELTE Eötvös Loránd University, confirmed the presence of dust clouds first spotted in 1961 by Polish astronomer Kazimierz Kordylewski. The clouds lie along two points of stability between the Earth and the Moon known as L4 and L5. At these points, the gravitational pull of the Earth and the Moon stabilizes the clouds orbit, so they constantly circle the Earth.

The existence of the Kordylewski clouds, named after the astronomer, was doubted for many years and some suspected that the sun's heavy gravitational effect would leave points L4 and L5 empty. That means confirming the existence of the Kordylewski clouds is big news. However, and I know I'm bursting the celestial bubble here, that doesn't mean we found new moons.

Although there is no agreed upon upper and lower limit for a moon's size, the general definition is a celestial body that orbits a planet. The Kordylewski clouds, based on their position in space, would appear to orbit the Earth, but they are also volatile spaces where gravity pulls dust particles in and out of the system, while its shape and density also vary over time, as more particles are pulled into the orbit.

The research team stops short of calling the clouds a moon, instead referring to them as pseudo-satellites.

"It is intriguing to confirm that our planet has dusty pesudo-satellites in orbit alongside our lunar neighbor," Judit Slíz-Balogh, study co-author told the Royal Astronomical Society.

"The Kordylewski clouds are two of the toughest objects to find, and though they are as close to Earth as the Moon are largely overlooked by researchers in astronomy."

Importantly, though the clouds aren't anything quite as paradigm-shattering as extra moons, they are important features of our cosmic neighbourhood. They provide a point in space suitable for us to park spacecraft or telescopes and may even be viable positions to create an "interplanetary superhighway".

It's cool to think about Earth as sort of a reverse-Tatooine, with three moons illuminating the night sky, but that's not quite the case. The dust clouds are incredibly faint -- part of the reason why they remained only hypothetical for so long -- so don't expect to turn your eyes to the sky and see another pseudo-lunar face staring down at you this evening.

To answer the initial question then: The Earth's only moon is the Moon. So let's get ready to go back .


Contents

Triton was discovered by British astronomer William Lassell on October 10, 1846, [16] just 17 days after the discovery of Neptune. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell did so and discovered Triton eight days later. [16] [17] Lassell also claimed for a period [h] to have discovered rings. [18] Although Neptune was later confirmed to have rings, they are so faint and dark that it is not plausible he actually saw them. A brewer by trade, Lassell spotted Triton with his self-built

61 cm (24 in) aperture metal mirror reflecting telescope (also known as the "two-foot" reflector). [19] This telescope was later donated to the Royal Observatory, Greenwich in the 1880s, but was eventually dismantled. [19]

Triton is named after the Greek sea god Triton (Τρίτων), the son of Poseidon (the Greek god corresponding to the Roman Neptune). The name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, [20] and was officially adopted many decades later. [21] Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as "the satellite of Neptune". Lassell did not name his own discovery he later successfully suggested the name Hyperion, previously chosen by John Herschel, for the eighth moon of Saturn when he discovered it. [22]

Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e. it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons. However, these moons are all much more distant from their primaries, and are small in comparison the largest of them (Phoebe) [i] has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton's orbit is associated with two tilts, the obliquity of Neptune's rotation to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's rotation, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's rotation with a period of about 678 Earth years (4.1 Neptunian years), [4] [5] making its Neptune-orbit-relative inclination vary between 127° and 173°. That inclination is currently 130° Triton's orbit is now near its maximum departure from coplanarity with Neptune's.

Triton's rotation is tidally locked to be synchronous with its orbit around Neptune: it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane. [23] At the present time, Triton's rotational axis is about 40° from Neptune's orbital plane, and hence at some point during Neptune's year each pole points fairly close to the Sun, almost like the poles of Uranus. As Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes were observed in 2010. [24]

Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. Viscoelastic damping from tides alone is not thought to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role. [4] [5] Tidal interactions also cause Triton's orbit, which is already closer to Neptune than the Moon's is to Earth, to gradually decay further predictions are that 3.6 billion years from now, Triton will pass within Neptune's Roche limit. [25] This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn. [25]

Moons in retrograde orbits cannot form in the same region of the solar nebula as the planets they orbit, so Triton must have been captured from elsewhere. It might therefore have originated in the Kuiper belt, [13] a ring of small icy objects extending from just inside the orbit of Neptune to about 50 AU from the Sun. Thought to be the point of origin for the majority of short-period comets observed from Earth, the belt is also home to several large, planet-like bodies including Pluto, which is now recognized as the largest in a population of Kuiper belt objects (the plutinos) locked in orbital step with Neptune. Triton is only slightly larger than Pluto and nearly identical in composition, which has led to the hypothesis that the two share a common origin. [26]

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely eccentric orbit of Neptune's moon Nereid and the scarcity of moons as compared to the other giant planets. Triton's initially eccentric orbit would have intersected orbits of irregular moons and disrupted those of smaller regular moons, dispersing them through gravitational interactions. [4] [5]

Triton's eccentric post-capture orbit would have also resulted in tidal heating of its interior, which could have kept Triton fluid for a billion years this inference is supported by evidence of differentiation in Triton's interior. This source of internal heat disappeared following tidal locking and circularization of the orbit. [27]

Two types of mechanisms have been proposed for Triton's capture. To be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape. [7] An early theory of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely). [7] A more recent hypothesis suggests that, before its capture, Triton was part of a binary system. When this binary encountered Neptune, it interacted in such a way that the binary dissociated, with one portion of the binary expelled, and the other, Triton, becoming bound to Neptune. This event is more likely for more massive companions. [13] Similar mechanisms have been proposed for the capture of Mars's moons. [28] This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects. [29] [30] The event was brief but gentle, saving Triton from collisional disruption. Events like this may have been common during the formation of Neptune, or later when it migrated outward. [13]

However, simulations in 2017 showed that after Triton's capture, and before its orbital eccentricity decreased, it probably did collide with at least one other moon, and caused collisions between other moons. [31] [32]

Triton is the seventh-largest moon and sixteenth-largest object in the Solar System, and is modestly larger than the dwarf planets Pluto and Eris. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and thirteen other known moons, [j] and is also more massive than all known moons in the Solar System smaller than itself combined. [k] Also, with a diameter 5.5% that of Neptune, it is the largest moon of a gas giant relative to its planet in terms of diameter, although Titan is bigger relative to Saturn in terms of mass. It has a radius, density (2.061 g/cm 3 ), temperature and chemical composition similar to those of Pluto. [33]

Triton's surface is covered with a transparent layer of annealed frozen nitrogen. Only 40% of Triton's surface has been observed and studied, but it is possible that it is entirely covered in such a thin sheet of nitrogen ice. Like Pluto's, Triton's crust consists of 55% nitrogen ice with other ices mixed in. Water ice comprises 15–35% and frozen carbon dioxide (dry ice) the remaining 10–20%. Trace ices include 0.1% methane and 0.05% carbon monoxide. [7] There could also be ammonia ice on the surface, as there are indications of ammonia dihydrate in the lithosphere. [34] Triton's mean density implies that it probably consists of about 30–45% water ice (including relatively small amounts of volatile ices), with the remainder being rocky material. [7] Triton's surface area is 23 million km 2 , which is 4.5% of Earth, or 15.5% of Earth's land area. Triton has a considerably and unusually high albedo, reflecting 60–95% of the sunlight that reaches it, and it has changed slightly since the first observations. By comparison, the Moon reflects only 11%. [35] Triton's reddish colour is thought to be the result of methane ice, which is converted to tholins under exposure to ultraviolet radiation. [7] [36]

Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like Earth, into a solid core, a mantle and a crust. Water, the most abundant volatile in the Solar System, comprises Triton's mantle, enveloping a core of rock and metal. There is enough rock in Triton's interior for radioactive decay to maintain a liquid subsurface ocean to this day, similar to what is thought to exist beneath the surface of Europa and a number of other icy outer Solar System worlds. [7] [37] [38] [39] This is not thought to be adequate to power convection in Triton's icy crust. However, the strong obliquity tides are believed to generate enough additional heat to accomplish this and produce the observed signs of recent surface geological activity. [39] The black material ejected is suspected to contain organic compounds, [38] and if liquid water is present in Triton, it has been speculated that this could make it habitable for some form of life. [38] [40] [41]

Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near its surface. [10] [42] [43] Like Pluto's atmosphere, the atmosphere of Triton is thought to have resulted from evaporation of nitrogen from its surface. [26] Its surface temperature is at least 35.6 K (−237.6 °C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. [44] An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. [45] This is colder than Pluto's average equilibrium temperature of 44 K (−229.2 °C). Triton's surface atmospheric pressure is only about 1.4–1.9 Pa (0.014–0.019 mbar). [7]

Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometre in size. [46] Unlike other atmospheres, Triton's lacks a stratosphere, and instead has a thermosphere from altitudes of 8 to 950 km, and an exosphere above that. [7] The temperature of Triton's upper atmosphere, at 95 ± 5 K , is higher than that at its surface, due to heat absorbed from solar radiation and Neptune's magnetosphere. [10] [47] A haze permeates most of Triton's troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from its surface. [7]

In 1997, observations from Earth were made of Triton's limb as it passed in front of stars. These observations indicated the presence of a denser atmosphere than was deduced from Voyager 2 data. [48] Other observations have shown an increase in temperature by 5% from 1989 to 1998. [49] These observations indicated Triton was approaching an unusually warm southern-hemisphere summer season that happens only once every few hundred years. Theories for this warming include a change of frost patterns on Triton's surface and a change in ice albedo, which would allow more heat to be absorbed. [50] Another theory argues that the changes in temperature are a result of deposition of dark, red material from geological processes. Because Triton's Bond albedo is among the highest within the Solar System, it is sensitive to small variations in spectral albedo. [51]

All detailed knowledge of the surface of Triton was acquired from a distance of 40,000 km by the Voyager 2 spacecraft during a single encounter in 1989. [52] The 40% of Triton's surface imaged by Voyager 2 revealed blocky outcrops, ridges, troughs, furrows, hollows, plateaus, icy plains and few craters. Triton is relatively flat its observed topography never varies beyond a kilometre. [7] The impact craters observed are concentrated almost entirely in Triton's leading hemisphere. [53] Analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from an estimated 50 million years old to just an estimated 6 million years old. [54] Fifty-five percent of Triton's surface is covered with frozen nitrogen, with water ice comprising 15–35% and frozen CO2 forming the remaining 10–20%. [55] The surface shows deposits of tholins, organic compounds that may be precursor chemicals to the origin of life. [56]

Cryovolcanism Edit

Triton is geologically active its surface is young and has relatively few impact craters. Although Triton's crust is made of various ices, its subsurface processes are similar to those that produce volcanoes and rift valleys on Earth, but with water and ammonia as opposed to liquid rock. [7] Triton's entire surface is cut by complex valleys and ridges, probably the result of tectonics and icy volcanism. The vast majority of surface features on Triton are endogenic—the result of internal geological processes rather than external processes such as impacts. Most are volcanic and extrusive in nature, rather than tectonic. [7]

One of the largest cryovolcanic features found on Triton is Leviathan Patera, [57] a caldera-like feature roughly 100 km in diameter seen near the equator. Surrounding this caldera is a volcanic dome that stretches for roughly 2,000 km along its longest axis, indicating that Leviathan is the second largest volcano in the solar system by area, after Alba Mons. This feature is also connected to two enormous cryolava lakes seen north-west of the caldera. Because the cryolava on Triton is believed to be primarily water ice with some ammonia, these lakes would qualify as stable bodies of surface liquid water while they were molten. This is the first place such bodies have been found apart from Earth, and Triton is the only icy body known to feature cryolava lakes, although similar cryomagmatic extrusions can be seen on Ariel, Ganymede, Charon, and Titan. [58]

The Voyager 2 probe observed in 1989 a handful of geyser-like eruptions of nitrogen gas and entrained dust from beneath the surface of Triton in plumes up to 8 km high. [33] [59] Triton is thus, along with Earth, Io, Europa and Enceladus, one of the few bodies in the Solar System on which active eruptions of some sort have been observed. [60] The best-observed examples are named Hili and Mahilani (after a Zulu water sprite and a Tongan sea spirit, respectively). [61]

All the geysers observed were located between 50° and 57°S, the part of Triton's surface close to the subsolar point. This indicates that solar heating, although very weak at Triton's great distance from the Sun, plays a crucial role. It is thought that the surface of Triton probably consists of a translucent layer of frozen nitrogen overlying a darker substrate, which creates a kind of "solid greenhouse effect". Solar radiation passes through the thin surface ice sheet, slowly heating and vaporizing subsurface nitrogen until enough gas pressure accumulates for it to erupt through the crust. [7] [46] A temperature increase of just 4 K above the ambient surface temperature of 37 K could drive eruptions to the heights observed. [59] Although commonly termed "cryovolcanic", this nitrogen plume activity is distinct from Triton's larger scale cryovolcanic eruptions, as well as volcanic processes on other worlds, which are powered by internal heat. CO2 geysers on Mars are thought to erupt from its south polar cap each spring in the same way as Triton's geysers. [62]

Each eruption of a Triton geyser may last up to a year, driven by the sublimation of about 100 million m 3 (3.5 billion cu ft) of nitrogen ice over this interval dust entrained may be deposited up to 150 km downwind in visible streaks, and perhaps much farther in more diffuse deposits. [59] Voyager 2 's images of Triton's southern hemisphere show many such streaks of dark material. [63] Between 1977 and the Voyager 2 flyby in 1989, Triton shifted from a reddish colour, similar to Pluto, to a far paler hue, suggesting that lighter nitrogen frosts had covered older reddish material. [7] The eruption of volatiles from Triton's equator and their deposition at the poles may redistribute enough mass over the course of 10,000 years to cause polar wander. [64]

Polar cap, plains and ridges Edit

Triton's south polar region is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Little is known about the north pole because it was on the night side during the Voyager 2 encounter, but it is thought that Triton must also have a north polar ice cap. [44]

The high plains found on Triton's eastern hemisphere, such as Cipango Planum, cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape. The plains are dotted with pits, such as Leviathan Patera, which are probably the vents from which this lava emerged. The composition of the lava is unknown, although a mixture of ammonia and water is suspected. [7]

Four roughly circular "walled plains" have been identified on Triton. They are the flattest regions so far discovered, with a variance in altitude of less than 200 m. They are thought to have formed from eruption of icy lava. [7] The plains near Triton's eastern limb are dotted with black spots, the maculae. Some maculae are simple dark spots with diffuse boundaries, and others comprise a dark central patch surrounded by a white halo with sharp boundaries. The maculae typically have diameters of about 100 km and widths of the halos of between 20 and 30 km. [7]

There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles. [65] Many also appear to be tectonic in nature and may result from extension or strike-slip faulting. [66] There are long double ridges of ice with central troughs bearing a strong resemblance to Europan lineae (although they have a larger scale [14] ), and which may have a similar origin, [7] possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized. [14] These faults with parallel ridges expelled from the interior cross complex terrain with valleys in the equatorial region. The ridges and furrows, or sulci, such as Yasu Sulci, Ho Sulci, and Lo Sulci, [67] are thought to be of intermediate age in Triton's geological history, and in many cases to have formed concurrently. They tend to be clustered in groups or "packets". [66]

Cantaloupe terrain Edit

Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain" because of its resemblance to the skin of a cantaloupe melon. Although it has few craters, it is thought that this is the oldest terrain on Triton. [68] It probably covers much of Triton's western half. [7]

Cantaloupe terrain, which is mostly dirty water ice, is only known to exist on Triton. It contains depressions 30–40 km in diameter. [68] The depressions (cavi) are probably not impact craters because they are all of similar size and have smooth curves. The leading hypothesis for their formation is diapirism, the rising of "lumps" of less dense material through a stratum of denser material. [7] [69] Alternative hypotheses include formation by collapses, or by flooding caused by cryovolcanism. [68]

Impact craters Edit

Due to constant erasure and modification by ongoing geological activity, impact craters on Triton's surface are relatively rare. A census of Triton's craters imaged by Voyager 2 found only 179 that were incontestably of impact origin, compared with 835 observed for Uranus's moon Miranda, which has only three percent of Triton's surface area. [70] The largest crater observed on Triton thought to have been created by an impact is a 27-kilometre-diameter (17 mi) feature called Mazomba. [70] [71] Although larger craters have been observed, they are generally thought to be volcanic in nature. [70]

The few impact craters on Triton are almost all concentrated in the leading hemisphere—that facing the direction of the orbital motion—with the majority concentrated around the equator between 30° and 70° longitude, [70] resulting from material swept up from orbit around Neptune. [54] Because it orbits with one side permanently facing the planet, astronomers expect that Triton should have fewer impacts on its trailing hemisphere, due to impacts on the leading hemisphere being more frequent and more violent. [70] Voyager 2 imaged only 40% of Triton's surface, so this remains uncertain. However, the observed cratering asymmetry exceeds what can be explained on the basis of the impactor populations, and implies a younger surface age for the crater-free regions (≤ 6 million years old) than for the cratered regions (≤ 50 million years old). [53]

The orbital properties of Triton were already determined with high accuracy in the 19th century. It was found to have a retrograde orbit, at a very high angle of inclination to the plane of Neptune's orbit. The first detailed observations of Triton were not made until 1930. Little was known about the satellite until Voyager 2 flew by in 1989. [7]

Before the flyby of Voyager 2, astronomers suspected that Triton might have liquid nitrogen seas and a nitrogen/methane atmosphere with a density as much as 30% that of Earth. Like the famous overestimates of the atmospheric density of Mars, this proved incorrect. As with Mars, a denser atmosphere is postulated for its early history. [72]

The first attempt to measure the diameter of Triton was made by Gerard Kuiper in 1954. He obtained a value of 3,800 km. Subsequent measurement attempts arrived at values ranging from 2,500 to 6,000 km, or from slightly smaller than the Moon (3,474.2 km) to nearly half the diameter of Earth. [73] Data from the approach of Voyager 2 to Neptune on August 25, 1989, led to a more accurate estimate of Triton's diameter (2,706 km). [74]

In the 1990s, various observations from Earth were made of the limb of Triton using the occultation of nearby stars, which indicated the presence of an atmosphere and an exotic surface. Observations in late 1997 suggest that Triton is heating up and the atmosphere has become significantly denser than when Voyager 2 flew past in 1989. [48]

New concepts for missions to the Neptune system to be conducted in the 2010s were proposed by NASA scientists on numerous occasions over the last decades. All of them identified Triton as being a prime target and a separate Triton lander comparable to the Huygens probe for Titan was frequently included in those plans. No efforts aimed at Neptune and Triton went beyond the proposal phase and NASA's funding on missions to the outer Solar System is currently focused on the Jupiter and Saturn systems. [75] A proposed lander mission to Triton, called Triton Hopper, would mine nitrogen ice from the surface of Triton and process it to be used as propellant for a small rocket, enabling it to fly or 'hop' across the surface. [76] [77] Another concept, involving a flyby, was formally proposed in 2019 as part of NASA's Discovery Program under the name Trident. [78] Neptune Odyssey is a mission concept for a Neptune orbiter with a focus in Triton being studied as a possible large strategic science mission by NASA that would launch in 2033 and arrive at the Neptune system in 2049. [79]


Contents

Knowledge of contributions to Pythagorean astronomy before Philolaus is limited. Hippasus, another early Pythagorean philosopher, did not contribute to astronomy, and no evidence of Pythagoras's work on astronomy remains. None for the remaining astronomical contributions can be attributed to a single person and, therefore, Pythagoreans as whole take the credit. However, it should not be presumed that the Pythagoreans as a unanimous group agreed on a single system before the time of Philolaus. [8]

One surviving theory from the Pythagoreans before Philolaus, the harmony of the spheres, is first mentioned in Plato’s Republic. Plato presents the theory in a mythological sense by including it in the legend of Er, which concludes the Republic. Aristotle mentions the theory in De Caelo, in which he presents the theory as a "physical doctrine" that coincides with the rest of the Pythagorean cosmology, rather than attributing it to myth. [8]

Zhmud summarizes the theory thus:

1) the circular motion of the celestial bodies produces a sound 2) the loudness of the sound is proportional to their speed and magnitude (according to Achytas, the loudness and pitch of the sound depends on the force with which it is produced 3) the velocities of the celestial bodies, being proportional to their distances from the earth, have the ratios of concords 4) hence the planets and stars produce harmonious sounds 5) we cannot hear this harmonious sound.

Philolaus (c. 470 to c. 385 BC) was a follower of the pre-Socratic Greek philosopher Pythagoras of Samos. Pythagoras developed a school of philosophy that was both dominated by mathematics and "profoundly mystical". [3] Philolaus has been called one of "the three most prominent figures in the Pythagorean tradition" [4] and "the outstanding figure in the Pythagorean school", who may have been the first "to commit Pythagorean doctrine to writing". [5] Most of what is known today about the Pythagorean astronomical system is derived from Philolaus's views. [8] Because of questions about the reliability of ancient non-primary documents, scholars are not absolutely certain that Philolaus developed the astronomical system based on the Central Fire, but they do believe that either he, or someone else in the late fifth century BC, created it. [5] Another issue with attributing the whole of Pythagorean astronomy to Philolaus is that he may have had teachers who were associated with other schools of thought. [8]

In the Pythagorean view, the universe is an ordered unit. Beginning from the middle, the universe expands outward around a central point, implying a spherical nature. In Philolaus’s view, for the universe to be formed, the "limiters" and "unlimited" must harmonize and be fitted together. Unlimited units are defined as continuous elements, such as water, air, or fire. Limiters, such as shapes and forms, are defined as things that set limits in a continuum. Philolaus believed that universal harmony was achieved in the Central Fire, where the combination of an unlimited unit, fire, and the central limit formed the cosmos. [9] [10] It is presumed as such because fire is the "most precious" of elements, and the center is a place of honor. Therefore, there must be fire at the center of the cosmos. [6] According to Philolaus, the central fire and cosmos are surrounded by an unlimited expanse. Three unlimited elements: time, breath, and void, were drawn in toward the central fire, where the interaction between fire and breath created the elements of earth and water. Additionally, Philolaus reasoned that separated pieces of the Central Fire may have created the heavenly bodies. [9]

In Philolaus's system, these heavenly bodies, namely the earth and planets, revolved around a central point. His could not be called a Heliocentric "solar system", because in his concept, the central point that the earth and planets revolved around was not the sun, but the so-called Central Fire. He postulated that this Central Fire was not visible from the surface of Earth—at least not from Greece.

Philolaus says that there is fire in the middle at the centre . and again more fire at the highest point and surrounding everything. By nature the middle is first, and around it dance ten divine bodies—the sky, the planets, then the sun, next the moon, next the earth, next the counterearth, and after all of them the fire of the hearth which holds position at the centre. The highest part of the surrounding, where the elements are found in their purity, he calls Olympus the regions beneath the orbit of Olympus, where are the five planets with the sun and the moon, he calls the world the part under them, being beneath the moon and around the earth, in which are found generation and change, he calls the sky.

However, it has been pointed out that Stobaeus betrays a tendency to confound the dogmas of the early Ionian philosophers, and occasionally mixes up Platonism with Pythagoreanism. [1]

According to Eudemus, a pupil of Aristotle, the early Pythagoreans were the first to find the order of the planets visible to the naked eye. While Eudemus doesn’t provide the order, it is presumed to be moon – sun – Venus – Mercury – Mars – Jupiter – Saturn – celestial sphere, based on the mystically "correct" order accepted in the time of Eudemus. It is likely that the Pythagoreans mentioned by Eudemus predate Philolaus. [13]

In this system the revolution of the earth around the fire "at the centre" or "the fire of the hearth" (Central Fire) was not yearly, but daily, while the moon's revolution was monthly, and the sun's yearly. It was postulated that the earth's speedy travel past the slower moving sun resulted in the appearance on earth of the sun rising and setting. Farther from the Central Fire, the revolution of the planets was slower still, and the outermost "sky" (i.e. stars) probably fixed. [4]

The Central Fire defines the center-most limit in the Pythagorean astronomical system. It is around this point that all heavenly bodies were said to rotate. Wrongly translated as Dios phylakê or "Prison of Zeus", a sort of hell, [4] the Central Fire was more appropriately called "Watch-tower of Zeus" (Διος πυργος) or "Hearth-altar of the universe" (εστια του παντος). [14] Maniatis claims that these translations more accurately reflect Philolaus's thoughts on the Central Fire. Its comparison to a hearth, the "religious center of the house and the state", shows its proper role as "the palace where Zeus guarded his sacred fire in the center of the cosmos". [9]

Rather than there being two separate fiery heavenly bodies in this system, Philolaus may have believed that the Sun was a mirror, reflecting the heat and light of the Central Fire. [15] Johannes Kepler, a sixteenth–seventeenth century European thinker, believed that Philolaus's Central Fire was the sun, but that the Pythagoreans felt the need to hide that teaching from non-believers. [16]

In Philolaus's system, the earth rotated exactly once per orbit, with one hemisphere (presumed to be the unknown side of the Earth) always facing the Central Fire. The Counter-Earth and the Central Fire were thus never visible from the hemisphere where Greece was located. [17] There is "no explicit statement about the shape of the earth in Philolaus' system", [18] so that he may have believed either that the earth was flat or that it was round and orbited the Central Fire as the Moon orbits Earth—always with one hemisphere facing the Fire and one facing away. [4] A flat Earth facing away from the Central Fire would be consistent with the pre-gravity concept that if all things must fall toward the center of the universe, this force would allow the earth to revolve around the center without spilling everything on the surface into space. [5] Others maintain that by 500 BC most contemporary Greek philosophers considered the Earth to be spherical. [19]

The "mysterious" [4] Counter-Earth (Antichthon) was the other celestial body not visible from Earth. We know that Aristotle described it as "another Earth", from which Greek scholar George Burch infers that it must be similar in size, shape, and constitution to Earth. [20] According to Aristotle—a critic of the Pythagoreans—the function of the Counter-Earth was to explain "eclipses of the moon and their frequency", [21] and/or "to raise the number of heavenly bodies around the Central Fire from nine to ten, which the Pythagoreans regarded as the perfect number". [5] [22] [23]

Some, such as astronomer John Louis Emil Dreyer, think the Counter-Earth followed an orbit so that it always was located between Earth and Central Fire, [24] but Burch argues it must have been thought to orbit on the other side of the Fire from Earth. Since "counter" means "opposite", and opposite can only be in respect to the Central Fire, the Counter-Earth must be orbiting 180 degrees from Earth. [25] Burch also argues that Aristotle was simply having a joke "at the expense of Pythagorean number theory" and that the true function of the Counter-Earth was to balance Earth. [5] Balance was needed because without a counter there would be only one dense, massive object in the system—Earth. The universe would be "lopsided and asymmetric—a notion repugnant to any Greek, and doubly so to a Pythagorean", [26] because Ancient Greeks believed all other celestial objects were composed of a fiery or ethereal matter having little or no density. [5]


Hypothesis Testing: Upper-, Lower, and Two Tailed Tests

The procedure for hypothesis testing is based on the ideas described above. Specifically, we set up competing hypotheses, select a random sample from the population of interest and compute summary statistics. We then determine whether the sample data supports the null or alternative hypotheses. The procedure can be broken down into the following five steps.

H0: Null hypothesis (no change, no difference)

H1: Research hypothesis (investigator's belief) α =0.05

Upper-tailed, Lower-tailed, Two-tailed Tests

The research or alternative hypothesis can take one of three forms. An investigator might believe that the parameter has increased, decreased or changed. For example, an investigator might hypothesize:

  1. H1: μ > μ 0 , where μ0 is the comparator or null value (e.g., μ0 =191 in our example about weight in men in 2006) and an increase is hypothesized - this type of test is called an upper-tailed test
  2. H1: μ < μ0 , where a decrease is hypothesized and this is called a lower-tailed test or
  3. H1: μ ≠ μ 0, where a difference is hypothesized and this is called a two-tailed test .

The exact form of the research hypothesis depends on the investigator's belief about the parameter of interest and whether it has possibly increased, decreased or is different from the null value. The research hypothesis is set up by the investigator before any data are collected.

The test statistic is a single number that summarizes the sample information. An example of a test statistic is the Z statistic computed as follows:

When the sample size is small, we will use t statistics (just as we did when constructing confidence intervals for small samples). As we present each scenario, alternative test statistics are provided along with conditions for their appropriate use.

The decision rule is a statement that tells under what circumstances to reject the null hypothesis. The decision rule is based on specific values of the test statistic (e.g., reject H0 if Z > 1.645). The decision rule for a specific test depends on 3 factors: the research or alternative hypothesis, the test statistic and the level of significance. Each is discussed below.

  1. The decision rule depends on whether an upper-tailed, lower-tailed, or two-tailed test is proposed. In an upper-tailed test the decision rule has investigators reject H0 if the test statistic is larger than the critical value. In a lower-tailed test the decision rule has investigators reject H0 if the test statistic is smaller than the critical value. In a two-tailed test the decision rule has investigators reject H0 if the test statistic is extreme, either larger than an upper critical value or smaller than a lower critical value.
  2. The exact form of the test statistic is also important in determining the decision rule. If the test statistic follows the standard normal distribution (Z), then the decision rule will be based on the standard normal distribution. If the test statistic follows the t distribution, then the decision rule will be based on the t distribution. The appropriate critical value will be selected from the t distribution again depending on the specific alternative hypothesis and the level of significance.
  3. The third factor is the level of significance. The level of significance which is selected in Step 1 (e.g., α =0.05) dictates the critical value. For example, in an upper tailed Z test, if α =0.05 then the critical value is Z=1.645.

The following figures illustrate the rejection regions defined by the decision rule for upper-, lower- and two-tailed Z tests with α=0.05. Notice that the rejection regions are in the upper, lower and both tails of the curves, respectively. The decision rules are written below each figure.

Rejection Region for Upper-Tailed Z Test (H1: μ > μ0 ) with α=0.05

The decision rule is: Reject H0 if Z > 1.645.

Upper-Tailed Test

Rejection Region for Lower-Tailed Z Test (H1: μ < μ0 ) with α =0.05

The decision rule is: Reject H0 if Z < 1.645.

Lower-Tailed Test

Rejection Region for Two-Tailed Z Test (H1: μ ≠ μ 0 ) with α =0.05

The decision rule is: Reject H0 if Z < -1.960 or if Z > 1.960.

Two-Tailed Test

The complete table of critical values of Z for upper, lower and two-tailed tests can be found in the table of Z values to the right in "Other Resources."

Critical values of t for upper, lower and two-tailed tests can be found in the table of t values in "Other Resources."

Here we compute the test statistic by substituting the observed sample data into the test statistic identified in Step 2.

The final conclusion is made by comparing the test statistic (which is a summary of the information observed in the sample) to the decision rule. The final conclusion will be either to reject the null hypothesis (because the sample data are very unlikely if the null hypothesis is true) or not to reject the null hypothesis (because the sample data are not very unlikely).

If the null hypothesis is rejected, then an exact significance level is computed to describe the likelihood of observing the sample data assuming that the null hypothesis is true. The exact level of significance is called the p-value and it will be less than the chosen level of significance if we reject H0.

Statistical computing packages provide exact p-values as part of their standard output for hypothesis tests. In fact, when using a statistical computing package, the steps outlined about can be abbreviated. The hypotheses (step 1) should always be set up in advance of any analysis and the significance criterion should also be determined (e.g., α =0.05). Statistical computing packages will produce the test statistic (usually reporting the test statistic as t) and a p-value. The investigator can then determine statistical significance using the following: If p < α then reject H0.

  1. P-values summarize statistical significance and do not address clinical significance. There are instances where results are both clinically and statistically significant - and others where they are one or the other but not both. This is because P-values depend upon both the magnitude of association and the precision of the estimate (the sample size). When the sample size is large, results can reach statistical significance (i.e., small p-value) even when the effect is small and clinically unimportant. Conversely, with small sample sizes, results can fail to reach statistical significance yet the effect is large and potentially clinical important. It is extremely important to assess both statistical and clinical significance of results.
  2. Statistical tests allow us to draw conclusions of significance or not based on a comparison of the p-value to our selected level of significance. Remember that this conclusion is based on the selected level of significance ( α ) and could change with a different level of significance. While α =0.05 is standard, a p-value of 0.06 should be examined for clinical importance.
  3. When conducting any statistical analysis, there is always a possibility of an incorrect conclusion. With many statistical analyses, this possibility is increased. Investigators should only conduct the statistical analyses (e.g., tests) of interest and not all possible tests.
  4. Many investigators inappropriately believe that the p-value represents the probability that the null hypothesis is true. P-values are computed based on the assumption that the null hypothesis is true. The p-value is the probability that the data could deviate from the null hypothesis as much as they did or more. Consequently, the p-value measures the compatibility of the data with the null hypothesis, not the probability that the null hypothesis is correct.
  5. Statistical significance does not take into account the possibility of bias or confounding - these issues must always be investigated.
  6. Evidence-based decision making is important in public health and in medicine, but decisions are rarely made based on the finding of a single study. Replication is always important to build a body of evidence to support findings.

We now use the five-step procedure to test the research hypothesis that the mean weight in men in 2006 is more than 191 pounds. We will assume the sample data are as follows: n=100, =197.1 and s=25.6.

The research hypothesis is that weights have increased, and therefore an upper tailed test is used.

Because the sample size is large (n>30) the appropriate test statistic is

In this example, we are performing an upper tailed test (H1: μ> 191), with a Z test statistic and selected α =0.05. Reject H0 if Z > 1.645.

We now substitute the sample data into the formula for the test statistic identified in Step 2.

We reject H0 because 2.38 > 1.645. We have statistically significant evidence at a =0.05, to show that the mean weight in men in 2006 is more than 191 pounds. Because we rejected the null hypothesis, we now approximate the p-value which is the likelihood of observing the sample data if the null hypothesis is true. An alternative definition of the p-value is the smallest level of significance where we can still reject H0. In this example, we observed Z=2.38 and for α=0.05, the critical value was 1.645. Because 2.38 exceeded 1.645 we rejected H0. In our conclusion we reported a statistically significant increase in mean weight at a 5% level of significance. Using the table of critical values for upper tailed tests, we can approximate the p-value. If we select α=0.025, the critical value is 1.96, and we still reject H0 because 2.38 > 1.960. If we select α=0.010 the critical value is 2.326, and we still reject H0 because 2.38 > 2.326. However, if we select α=0.005, the critical value is 2.576, and we cannot reject H0 because 2.38 < 2.576. Therefore, the smallest α where we still reject H0 is 0.010. This is the p-value. A statistical computing package would produce a more precise p-value which would be in between 0.005 and 0.010. Here we are approximating the p-value and would report p < 0.010.


Earthshine Is Reflected Sunlight

Although only a small part of the Moon is directly illuminated by the Sun at the end of the Waning Crescent Moon phase, the rest of the Moon is sometimes also faintly visible. The reason is that Earth reflects sunlight as a faint glow onto the Moon. This phenomenon is called earthshine or the Da Vinci glow, and it is most noticeable in April and May.

A Waning Crescent Moon with earthshine, and the planet Venus close by.


Installation

There are many differences between a desktop environment and a minimalist production environment. At a guess, on my desktop there sit menu links to over a hundred useful or fun programs—from a rather overburdening array of addictively joyous games to utilities for burning CDs and reading RSS feeds. Regularly, there is a new set of updates. In fact, once in a blue moon, I sit at home with a glass of whisky and a dubious smile on my face installing the newest and the best. Package managers such as YUM (RPM) and APT (DEB) support this craving. Under this context, I would be foolish to compile and deploy new sources by hand. Updating all the software and seeking dependencies would not be achievable or pleasant. Therefore, for installing astronomy packages in a desktop environment, I would strongly recommend the use of a package manager. For a server environment, where you want a minimum of software tailored for specific tasks, the need is less stringent and perhaps even counter-productive.

For installing astronomy packages in a desktop environment, I would strongly recommend the use of a package manager

There are three main methods for installing software within a GNU/Linux environment. The primary method is the use of a package. Packages not only contain the software but are also structured to place the software correctly within the target GNU/Linux distribution. Complexities such as dependencies and file structures and placing menu options with the GNOME or KDE X windows environments are understood. Upgrading is trivial. Therefore, I will use this approach to install Stellarium and Celestia, even at the risk of stating the obvious. Zooming into the details of installation: there are two main competitors in the packaging domain RPM and Debian packages. For Debian, packages apt-get or tools sitting on top of apt-get do most of the heavy lifting. For RPM, YUM is currently my tool of choice. Both methods are best suited to a live internet connection, and if you are automatically updating your system every night through a cron job, then you will probably need a reasonably fast internet connection at that.

The second approach is to download the tar files and compile by hand. If you are lucky, things work out via a couple of standardized commands such as configure , make , make test , make install . If you are unlucky, patching and library dependencies can make for an evening of dependency hell.

The third and final approach is the use of an executable custom executable (normally called "installer") that copies the right files in the right spot, and sits outside of DEB or RPM package control. However, I won’t be using this method in this article.

There are numerous GNU/Linux distributions, and the vast majority understand either RPM or DEB packaging I will, therefore, describe both means of installation for Stellarium and Celestia. The target operating systems being both Fedora Core 5 for YUM/RPM and Ubuntu Breezy for APT.


Asteroid Ore

A long, long time ago.
On 3/30/2020, CCP pushed a previously announced but unscheduled update that changed moon ore distribution across all types of space. Prior to this change, moons provided both regular asteroid ores (but in a variant that gave a bonus 15% yield, a variant that was only available from moons) as well as moon-specific ores as mentioned elsewhere in this article. After the change, all asteroid type ores were completely removed from moons, and the refining yield of moon ores was adjusted to only give moon goo, with R4 (Ubiquitous) ore still providing Pyerite or Mexallon as well - all other normal minerals were removed.

Moon-specific Ore

In addition to the classic minerals that other ores provide, some materials required for T2 construction can only be found in moon ores. Like the normal asteroid ores, moon ores also have basic, improved, and excellent quality types. However, moon ore quality more strongly affects the reprocessed minerals received. The improved ores yield a 15% bonus, while the excellent ores provide a 100% bonus on minerals received through reprocessing.

There are 5 classes of moon ore, as shown in the table below. All classes are available in low and null-sec systems, but High sec and Wormhole systems may only have R4 (Ubiquitous) ores.

Ubiquitous Common Uncommon Rare Exceptional
R4 R8 R16 R32 R64
High Sec
Low Sec
Null Sec
Wormhole

Moon-specific ores are often of mixed quality, with the same moon pull containing both regular and improved quality of the same ore. However there is a chance that, at the time of detonating a moon chunk to create the asteroid field, that there will be a bright blue flash - this has come to be known as a "jackpot" and indicates that the moon ores for that particular extraction will all be of the excellent quality instead.

Moon Ore Refining

The following tables show the minerals and special materials present in 1000 m 3 of each moon ore. Values are for the basic ore. Any decimal values from the improved ores (+15%) are rounded up to the next whole number.


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