Astronomy

Is Mars brightest from Earth at opposition, perihelion or somewhere in between?

Is Mars brightest from Earth at opposition, perihelion or somewhere in between?


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After reading this article, I was inspired to run some simulations at NASA's JPL Solar System Simulator to see Earth's and Mars' orbits from above.

From what I could visually determine (from the JPEG images), Mars is at opposition on July 26, but its perihelion is sometime around September.

When (and where in their orbits) is Mars brightest from Earth this year?


JPL HORIZONS gives these values for Mars as seen from Earth.ris the Sun-Mars distance in AU,deltais the Earth-Mars distance in AU, andS-O-Tis the Sun-Earth-Mars angle in degrees. I added[]to indicate minimum or maximum values.

Date__(UT)__HR:MN APmag r delta S-O-T 2018-Jul-23 16:00 -2.75 1.40246 0.38911 172.30 2018-Jul-25 14:00 -2.77 1.40106 0.38730 173.27 2018-Jul-27 12:00 [-2.78] 1.39971 0.38600 [173.51] 2018-Jul-29 10:00 [-2.78] 1.39840 0.38522 172.94 2018-Jul-31 08:00 -2.77 1.39714 [0.38497] 171.73 2018-Aug-02 06:00 -2.75 1.39592 0.38523 170.11 2018-Sep-14 12:00 -1.74 1.38147 0.50640 128.93 2018-Sep-16 12:00 -1.69 [1.38144] 0.51602 127.49 2018-Sep-18 12:00 -1.64 1.38147 0.52588 126.08

Mars is at opposition on Jul 27, closest to Earth on Jul 31, and at perihelion on Sep 16. Mars is at peak brightness for several days between opposition and closest approach to Earth. A difference of 0.01 magnitude is imperceptible to the eye and smaller than the uncertainty of most ground-based CCD photometry. Albedo features and dust storms probably contribute at least that much variation in magnitude anyway.


The two main factors affecting the brightness of Mars are its distance from Earth, and the phase (ie how much of the planet is illuminated from the point of view of Earth. Both of these are optimal when Mars is at opposition.

Stellarium has Mars reaching maximum brightness on the 28th of July with a magnitude of -2.78, very close to opposition. The distance of Mars from the sun has only a secondary effect on the brightness.

On that date it will be close to the full moon, but low in the sky for Northern Hemisphere observers.


The key is in "as seen from Earth". Mars is illuminated by the Sun and the distance from the Sun varies only slightly. Therefore, it is always the same brightness. A photo of Mars through a telescope will always use the same exposure no matter how far it is from Earth. The planets do not behave like point sources of light. When Mars is twice as close, it fills 4 times the area in the sky. This exactly counters the inverse square spreading of the light from Mars. The same is true for all the planets. If they got brighter as they get closer, they would get brighter as YOU got closer. The Apollo astronauts would have boiled then vaporized as they approached the Moon. It is a common error. N. D. Tyson makes it with a truly dumb statement near the beginning of his Cosmos series. Something about the brightness of the Moon long ago when it was closer to the Earth. Think of how many people read that script and it still slipped by.

The brightness of an extended object as viewed, is not the same as the illumination it provides. If the Moon filled the sky, it would not look brighter. You would not have to wear welder's goggles to look at it. But the total amount of light reflected on the Earth would be much greater and more like the illumination on an overcast day (and there would never be a full Moon because the Earth would get in the way).

You can easily calculate a brightness compared to the Earth (or Moon) daylight brightness and the ratio of the squares of the distance from the Sun.

When you add that you want brightness as perceived from Earth, it depends on distance and phase. If you think of it as wanting to read by Mars light, it is "brighter" if it is closer and it is "brightest" at "full Mars" or opposition.

There is a gaggle of terms used in optics to describe various ways of describing brightness and intensity and illumination and they get very confusing.


Observing Mars with a Telescope

It is licensed under Creative Commons BY-SA 3.0 and may be freely shared, but must include this attribution.

Mars constitutes the only planet in the solar system with surface features easily distinguishable from Earth—of the inner planets, Mercury’s small size and proximity to the Sun make viewing exceptionally difficult, while Venus’ surface hides behind a thick layer of clouds. Ώ]

As such, Martian surface features have long captured the imagination of astronomers, even helping lead to the longstanding popular belief of its inhabitation by extraterrestrial civilization. In 1877, Italian astronomer Giovanni Schiaparelli began mapping areas on Mars, including long, dark lines he named “channels”, or “canali." ΐ] Mistranslations of this word to English “canals” along with his own observations encouraged American Percival Lowell to theorize that these dark lines were the work of intelligent beings attempting to irrigate their dying planet. Α]

Although these lines were soon after revealed to be an optical illusion resulting from the mind’s attempts to connect dark areas at the extent of its vision, Β] even amateur astronomers can make out a variety of other surface features with the aid of a telescope.


Mars will be at its closest to Earth before sunrise on July 31 at around 4 a.m. Eastern Time, according to EarthSky.org.

Because Mars and Earth have elliptical orbits rather than perfectly circular ones, opposition and the closest approach between the two planets do not happen at the same time. Mars’s orbit around the sun takes about two Earth years to complete. If the two planets orbited the sun along perfectly circular orbits that were in the same plane, then Mars’s closest approach would happen on the same day as opposition.

In 2003, Mars was at its closest approach to Earth in about 60,000 years, Dr. Zurek said. At that point, it was only about 34.65 million miles away from Earth, according to EarthSky.org.

On average, Mars is about 140 million miles from Earth, according to NASA, and at the farthest, they are about 250 million miles apart, according to Space.com. The close approach happening this month will be the closest since 2003, at a distance of about 35.8 million miles away from Earth, according to NASA.

NASA tends to launch its space missions to Mars during times of closest approach, or every two years. For example, the Mars Exploration Rovers, Opportunity and Spirit, were launched in 2003, the Mars Reconnaissance Orbiter in 2005 and the Mars Phoenix Lander in 2007.

The next closest approach will be in October 2020, and the next super-close opposition will be in September 2035.

Far into the future, Earth and Mars will have another record-breaking meet-up in 2287, when the two will be about 34.60 million miles apart.


2020, August: Mars in Pisces, at Perihelion

Mars is approaching opposition on October 13, 2020. At opposition Mars and the sun are in opposite directions in the sky. As the sun sets in the west, Mars rises in the east. Mars appears in the south around midnight (1 a.m. during daylight time). As morning twilight begins, Mars is low in the western sky, setting before sunrise.

In the sky, Mars appears as an overly-bright star. It is the brightest star in this region of the sky, making its identification easy.

During early August, Mars rises at around 11 p.m., appearing low in the east as midnight approaches. At this time, Jupiter and Saturn are in the south. This giant planet duo starts the evening low in the southeast as the sky darkens.

By early morning, about three hours before sunrise, Mars is part of a quartet of four bright planets that are stretched across the sky from the east to the southwest skyline. Bright Venus is in the east, Mars in the south-southeast, and Saturn and Jupiter in the southwest.

By midmonth, Mars rises about 30 minutes earlier and shines from higher in the eastern sky as midnight approaches. It continues to follow Jupiter and Saturn through the sky.

At this point that the planetary quartet begins to break up. Jupiter disappears below the southwest horizon as Venus climbs into the eastern sky.

By late in the month, when Mars rises around 9:30 p.m. and is well-up in the eastern sky by 11 p.m. By early morning, Jupiter and Saturn have left the sky, as Venus climbs into view.

August 2020: This chart shows the motion of Mars compared to the dimmer stars in southeastern Pisces. During the month, Mars passes near 89 Piscium (89 Psc, m = 5.1), Mu Piscium (μ Psc, m = 4.8), Nu Piscium (ν Psc, m = 4.4) and Omicron Piscium (ο Psc, m = 4.2).

Note that on the accompanying chart, the daily positions of Mars are farther apart than at the end of the month. The planet begins to retrograde next month. Before it reverses course and seems to move westward among the stars, it slows. (A chart in this article shows the retrograde pattern of Mars for this opposition.) The gaps between the daily positions decrease in distance. At the beginning of the month, Mars moves eastward about 0.4° each day. That’s a little less than the apparent size of the moon in the sky. By month’s end, the Red Planet appears to move about half that distance each day.

Because Mars’ orbit is not a perfect circle, Mars is not necessarily closest to the sun or closest to Earth at opposition. Mars is closest to the sun (perihelion) on August 2. Our planet is closest to the Red Planet on October 6, followed by opposition a week later.

Use a binocular to track Mars’ eastward motion in the starfield. Here are dates to note:

  • August 1: Mars starts the month 1.2° to the upper right of 89 Piscium (89 Psc).
  • August 2: Mars is closest to the sun (perihelion), 1.38 Astronomical Units from the sun. (An Astronomical Unit – AU – is equal to Earth’s average distance from the sun, about 93 million miles or 150 million kilometers). At this time the Earth – Mars gap is still 0.63 AU. Mars continues to brighten in our sky as we get closer to it.
  • August 4: Mars passes 0.3° above 89 Psc.

As midnight approaches the moon is 2.1° to the lower right of Mars that is about 13° in altitude in the east.

  • August 8: Before midnight, look eastward for Mars, 2.1° to the upper left of the gibbous moon that is 73% illuminated. They’ll still be together in the morning.
  • August 14: The planet is 1.0° to the lower left of Mu Piscium (μ Psc).
  • August 22: Mars passes 0.5° below Nu Piscium (ν Psc).
  • August 31: Mars ends the month 2.7° to the lower right of Omicron Piscium (ο Psc).

For more about where to locate the planets in August, here is a semi-technical description of their locations for each day.


Events Around Opposition

Because Mars is near our planet, it moves around the sun at about half the speed of Earth. Our planet catches and moves past Mars in a little over two years. So compared to most other observations of the planets, the intervals between oppositions seem long.

Figure 3: Orbital Chart. This chart shows the overhead view of Earth and Mars during 2018

The two charts (Figure 1 and Figure 3) show two different views of the same events. The former chart shows the view of what we see from our viewpoint on Earth. The latter shows the orbital paths of the two planets as viewed from above the solar system. (Click the charts to see them larger.)

Here are the events as noted on the charts:

  • April 27, 2018: Mars is over 81 million miles from Earth. It is brighter than any other “star” in the region, except for Jupiter which is 60 degrees to the west of Mars. Mars rises in the southeast at about 1:30 a.m.
  • Between these two dates, Mars appears to move eastward compared to the starry background. In June Mars appears to slow down its eastward motion.
  • June 26, 2018: Mars stops moving eastward and begins to slowly move westward compared to the starry background. It is 44 million miles away and has grown five times in brightness. The planet rises at about 11 p.m. It is important to note the Mars appears as a star and during this opposition, it cannot be seen as a planetary globe with the unassisted eye. A telescope is needed to see its shape.
  • July 27, 2018: Mars is at opposition. In one month it nearly doubles in brightness. It is nearly as bright as Jupiter, yet appears starlike without a telescope. It is 35.8 million miles away. It rises in the southeast at sunset and sets in the southwest at sunrise. It is highest in the south around midnight. This is the best time to view the planet through a telescope.
  • July 31, 2018: Mars is closest to Earth (closest approach). It is 35.7 million miles away. Mars is not necessarily closest at opposition because of its elliptical orbit. Mars is still moving toward perihelion gradually getting closer to the sun each day.
  • August 27, 2018: Mars stops retrograding. It is 41 million miles away. It is still bright but its intensity is noticeably diminished. Earth is now pulling away from Mars. Mars begins moving eastward again compared to the starry background
  • September 15, 2018: Mars is a its solar perihelion, 47 million miles away from Earth.
  • October 13, 2018: (The last date charted.) Mars is 62 million miles away from Earth and distinctly dimmer than it was at opposition. As Earth moves away from Mars, the Red Planet appears to pick up speed as it moves eastward.

One of the biggest challenges of our ancestors was to explain the retrograde motion of Mars (Figure 1), Jupiter, and Saturn. From a sun-centered explanation of the solar system, these outer planets seem to stop and backup as a faster moving Earth catches and passes them. From Figure 3, note that the planets do not stop at any time in their orbital motions.

Table 1: Some properties of Mars, 2018

The table above summarizes the preceding text . The magnitude column shows the brightness of Mars on a numerical scale. Smaller numbers indicate that the planet is brighter. The scale becomes more negative numerically for brighter objects. (The sun’s magnitude is -26.5. After all it is so bright that it causes daytime.) From April 27 through July 27, Mars increases in brightness 2.48 magnitudes. To the eye that corresponds to a 10 times increase in brightness. Each step on the magnitude scale is about a 2.5 times change in brightness. In comparison here are magnitudes of some other bright stars:

  • Sirius, -1.44 (the brightest star in the night sky)
  • Arcturus, -0.05
  • Betelgeuse, 0.45
  • Aldebaran, 0.87

An exciting viewing opportunity occurs during the next year. With this opposition occurring in July 2018, it is easily observed. The next martian opposition is October 13, 2020 with the next perihelic opposition September 15, 2035 at a distance of 35.4 million miles from us. Happy observing!


What, exactly, is an opposition?

Mars orbits the sun at a greater distance than Earth. As the distances increase, the orbital period also increases, so Mars takes about 26 months to complete one orbit around the sun. Due to these different orbital speeds, every two years or so, Earth passes between Mars and the sun. This means that Mars and the sun are on directly opposite sides of Earth.

Also, because Mars is directly opposite the sun during opposition, Mars rises as the sun sets, and it sets as the sun rises. As a result, the Red Planet shines prominently in our night sky. In fact, during opposition, Mars is at its brightest and near its maximum apparent size in telescopes.

However, since both planets have elliptical orbits, some Earth-Mars encounters are closer than others. The last time Mars aligned with the sun and Earth in 2018, the Red Planet was slightly closer to Earth, at a distance of only 35.8 million miles (57.6 million km) away.

During the Mars opposition in 2003, the Red Planet was even closer, at a distance of only 34.6 million miles (55.8 million kilometers) from Earth. This was the closest the two planets had come to each other in almost 60,000 years, and this record won't be broken until Aug. 28, 2287, according to NASA. However, the Red Planet's next-closest approach will occur in 2035.

In comparison, when Mars is on the other side of the sun and thus at its greatest distance from Earth, it is about 250 million miles (401 million km) away. The average distance between the two planets is roughly 140 million miles (225 million km).


Seeing the red planet at its peak

Here's how to find and see Mars this week. A few days before opposition, Mars will rise in the east at about 9:35 p.m. local time (depending on your latitude). It will climb higher until nearly 2 a.m. local time, when it will reach an elevation of about 20 degrees (or two outstretched fist diameters) above the southern horizon. This is the best time to view the planet in a telescope, because it will then be shining through less of Earth's distorting atmosphere. An open southern vista is essential, because 20 degrees is lower than many trees and buildings.

You can use your astronomy app to see where Mars will rise at your location and to determine where along the horizon to look for it. Ensure that the app is using your current location and time and engage the Compass mode. (In the SkySafari app, this is done by tapping the Compass icon on the toolbar.) Hold your device up to the sky and move it around until Mars is centered on the display.

If Mars has not yet risen, open the time control toolbar and advance the time until Mars appears above the horizon. Using the app's compass mode, note the time and direction where Mars will be visible for your site. You may wish to shift your location if a building or tree will be hiding the planet.

In the Stellarium Mobile app, use a finger swipe to place the southeastern sky in view and tap the time labeled in the lower-right corner of the display. Use left and right finger motions to alter the time until Mars appears. For Star Walk 2 app users, display the southeastern sky and tap the Clock icon to open the time controls. Adjust the time until Mars appears. Star Walk 2's search function will display an arrow pointing to Mars. Move the device in the direction the arrow points until Mars is displayed.

Since Mars is close to opposition, it will descend into the west in the wee hours and set around dawn. Mars will be rising about 5 minutes earlier every night. On Friday night, it will rise by about 9:15 p.m. local time, and the following Tuesday it will rise before 9 p.m. Opposition occurs at 1 a.m. EDT on Friday morning, so Thursday evening will be a fine time to look at it, too.

If the clouds stay away, the planet will be impossible to miss. It will be brighter than anything nearby, except the moon. On Thursday evening, the nearly full moon will be sitting 10 degrees to the upper right of Mars. On Friday night, the full moon will hop to sit to Mars' upper left. By Tuesday, the night of closest approach, the moon will have moved well away from the Red Planet. The red coloration will be obvious — as opposed to Saturn's mere tint of yellow. The planet will not rapidly move location, or flash or blink. Anything doing that is a plane — keep hunting.

The farther south you live, the higher Mars will climb. An observer in Florida will see Mars nearly halfway up the sky after midnight. And someone at the latitude of Sydney will see Mars directly overhead at midnight!


Mars Is Now The Closest To Earth It Will Be For Another 15 Years

If you are blessed with clear skies this week, you can catch Mars at its best and brightest. The Red Planet and Earth are currently at the closest distance they will be for another 15 years. It can be seen this week as a clear red dot close to the Moon, the crimson hue of the celestial body visible even without binoculars or a telescope.

The two planets will be roughly 62.1 million kilometers (38.6 million miles) apart, with the closest point occurring on Tuesday, October 6. Mars won't get as close to us again until the year 2035.

It's not the closest that it has gotten in recent years – in 2018, Earth and Mars were just 57.6 million kilometers (35.8 million miles) from each other – but it's still pretty close.

The closest distance recorded was back in 2003 when Mars was 55.7 million kilometers (34.6 million miles) away. This distance was the closest the two planets have been in 60,000 years, and we will have to wait until August 28, 2287, for this record to be broken.

This is still not the closest distance possible. That would happen when Earth is at its aphelion, the furthest point from the Sun in a planet’s orbit, and Mars is at its perihelion, the closest point to the Sun. That special alignment would take the two planets to a distance of just 54.6 million kilometers (33.9 million miles), but it has never been recorded yet.

The opposite of the closest approach is when the two planets are at their aphelion on opposite sides of the Sun. The furthest distance that they can be is about 401 million kilometers (250 million miles) apart.

The next close approach will be in two years, but the gap between the two planets will become larger and larger until 2029. Despite this, every close approach is an exciting period for astronautics. Space agencies around the world time the launch of missions to Mars to occur when the distance between the two planets is smallest, so the travel time is shortest. Three missions were sent this summer, including NASA’s Perseverance rover.

The Rosalind Franklin Rover, a joint mission from the European Space Agency and Russia's Roscosmos, was also due to launch this summer but had to be put back due to the pandemic. It is expected to fly in the next launch window in late 2022.

If you are blessed with clear skies this week, you can catch Mars at its best and brightest. The Red Planet and Earth are currently at the closest distance they will be for another 15 years. It can be seen this week as a clear red dot close to the Moon, the crimson hue of the celestial body visible even without binoculars or a telescope.

The two planets will be roughly 62.1 million kilometers (38.6 million miles) apart, with the closest point occurring on Tuesday, October 6. Mars won't get as close to us again until the year 2035.

It's not the closest that it has gotten in recent years – in 2018, Earth and Mars were just 57.6 million kilometers (35.8 million miles) from each other – but it's still pretty close.

The closest distance recorded was back in 2003 when Mars was 55.7 million kilometers (34.6 million miles) away. This distance was the closest the two planets have been in 60,000 years, and we will have to wait until August 28, 2287, for this record to be broken.

This is still not the closest distance possible. That would happen when Earth is at its aphelion, the furthest point from the Sun in a planet’s orbit, and Mars is at its perihelion, the closest point to the Sun. That special alignment would take the two planets to a distance of just 54.6 million kilometers (33.9 million miles), but it has never been recorded yet.

The opposite of the closest approach is when the two planets are at their aphelion on opposite sides of the Sun. The furthest distance that they can be is about 401 million kilometers (250 million miles) apart.

The next close approach will be in two years, but the gap between the two planets will become larger and larger until 2029. Despite this, every close approach is an exciting period for astronautics. Space agencies around the world time the launch of missions to Mars to occur when the distance between the two planets is smallest, so the travel time is shortest. Three missions were sent this summer, including NASA’s Perseverance rover.

The Rosalind Franklin Rover, a joint mission from the European Space Agency and Russia's Roscosmos, was also due to launch this summer but had to be put back due to the pandemic. It is expected to fly in the next launch window in late 2022.


Mars is at opposition, so what does that mean?

You may have heard that Mars reached opposition on 27 July 2018. But what does that mean? It means that Mars is bright and easy to find in the night sky. It’s called opposition because that’s when Mars is 180 degrees away from – so directly in line with – the sun. When the sun is setting, Mars is rising and will cross the sky all night, setting at sunrise.

Opposition is also when the planet’s distance to the Earth reaches a relative minimum. Because it’s closer, it appears bigger and brighter in our sky. Already since spring, we’ve seen oppositions of Jupiter (9 May), then Saturn (27 June), so it’s been a good summer for planet viewers. (Uranus, Neptune and Pluto have also reached opposition this year, but they are all dim enough that most casual star gazers won’t see them at all.)

Planetary orbits drive opposition and oppositions of Mars are a bit more complicated than those of others because the Martian orbit is much more elliptical than the orbits of planets such as Jupiter and Saturn. As astronomer Johannes Kepler described in the early 1600s, planets follow elongated circular paths – ellipses – rather than perfectly-circular paths around the sun.

Mars at opposition. The Red Planet has a more elliptical orbit than other planets, which means its distance from the Earth at opposition can vary greatly. Image: Jim Lattis, UW–Madison

When opposition occurs near Martian perihelion (when Mars is at the nearest point in its orbit to the sun), Mars is also quite close to Earth. But when opposition occurs near Martian aphelion (when it is at its farthest point from the sun), it is likely to be a relatively great distance from Earth. Those differences in distance can be dramatic.

For example, Mars is near perihelion for this year’s opposition – a so-called perihelic opposition – and at its closest it came within 58 million kilometers of Earth. Compare that to the opposition of March 2012, an aphelic opposition, when Mars was never fewer than 100 million kilometers from Earth. Because of that difference in distance, Mars at this year’s opposition is nearly three-and-a-half times brighter for the Earthbound observer than it was at opposition in 2012.

However, even perihelic oppositions effectively never happen precisely at perihelion, so there are small differences from one event to the other. For instance, at the opposition of 27 August 2003, the minimum Earth-Mars distance was just shy of 56 million kilometers. That means Mars appeared up to 10 percent brighter in 2003 than in 2018.

This is nothing to get excited about, really, but it excited people nevertheless. It turns out the opposition of 2003 brought it slightly closer to Earth than in all previous Mars oppositions in the previous 60,000 years. This slight edge in “favorability” of that opposition was responsible for a frenzy of interest among astronomy enthusiasts around the world. But the dramatic nature of that event was more about its historical context than its intrinsic virtues.

Headlines declaring the closest opposition in 15 years – which would be this year’s – just don’t get the same amount of attention.

Artist’s illustration of Mars at opposition. Note that the sun, Earth and Mars align, with the Earth in the center. Not to scale. Credit: NASA/JPL-Caltech

Despite the favorability of both the 2003 and 2018 oppositions, neither one was particularly easy to observe for would-be viewers in the northern temperate regions of the Earth. The perihelion of Mars’ orbit lies in the direction of the southern parts of the ecliptic (the plane of Earth’s orbit), roughly toward such constellations as Capricornus and Aquarius. Hence, perihelic oppositions favor southern observers and always occur low in the sky, near the horizon, relative to us northerners.

In 2003, Mars at opposition was nearly 16 degrees south of the celestial equator, making it inconveniently low on Wisconsin horizons, and in 2018, Mars at opposition is more than 25 degrees south, so far worse than it was in 2003. In practical terms, this means that even the best of (northern) telescopes look at Mars through our dense and unsteady atmosphere, making the reddish disk blurry and unsteady even on a good night. The really good Martian oppositions favor the southern hemisphere, while we northerners get the best views of the far less-favorable aphelic oppositions, which occur in northern parts of the ecliptic, out in the direction of constellation Leo, for example.

This doesn’t mean you shouldn’t look for an opportunity to see Mars this summer, of course, but if Mars viewing is a priority, pack up your telescope and make for parts closer to the equator! But also make a note of October 2020, the next opposition of Mars. While it won’t be a record-breaker by any means in terms of distance, it will happen considerably farther north, about 5 degrees north of the celestial equator. Even if it will be a bit farther away, in the clear skies of October views of Mars should be breathtaking.

NASA’s Hubble Space Telescope photographed Mars on July 18, 2018, near its closest approach to Earth since 2003. The planet was observed near opposition, when the Sun, Earth and Mars are lined up, with Earth sitting in between the Sun and Mars. This proximity gives the Red Planet its brightest appearance in the night sky since the 2003 opposition. Credit: NASA


Mars at Opposition

Mars, the Red Planet, will reach opposition on April 8th. That’s its closest approach to Earth in its orbit around the Sun, and your best chance for a good view through telescopes or binoculars. It won’t look that large in any common binoculars, but you’ll definitely see the bright “reddish” disc of the planet. And you don’t need to catch it exactly on April 8th, as the view will be excellent for most of April.

As the graphic shows, the orbit of Mars is distinctly elliptical compared to Earth’s orbit. That means some oppositions bring Mars closer to Earth than others. Earth’s orbit takes it around the Sun in 1 year, but Mars takes 1.88 years (that’s in “Earth years”). Since they both orbit the Sun in the same direction, Earth and Mars are like “runners” on a track, with the faster Earth “lapping” Mars once every 2.135 years. Their motions are not synchronized, so opposition can occur anywhere along the Mars orbit shown in the graphic.

Note: The Sun and planets are not drawn to the same scale as the orbits, and the elliptical shape of Mars’s orbit has been exaggerated for the purposes of this discussion.

The closest oppositions are known as perihelic oppositions. Perihelion (for any planet) is the point of closest approach to the Sun. For Mars, that distance is about 207,000,000 km (129,000,000 miles). Since Earth orbits the Sun at a nearly constant distance of 150,000,000 km (93,000,000 miles), a perihelic opposition brings Mars within about 58,000,000 km (36 million miles) of Earth. That last happened in 2003.

The farthest oppositions are known as aphelic oppositions. Aphelion (for any planet) is the point of greatest distance from the Sun. For Mars, that’s about 249,000,000 km (153,000,000 miles). During an aphelic opposition, Mars will be about 97,000,000 km (60,000,000 miles) from Earth.

Since the apparent size of any planet varies depending on its distance from the observer, Mars exhibits the greatest change in size of all the planets as seen from Earth. It will appear nearly twice as large at a perihelic opposition as it does at an aphelic opposition. The opposition on April 8, 2014 is about midway between those two as indicated in the graphic.

On April 8, Mars will be about 93,000,000 km (57,000,000 miles) from Earth, with an apparent size of 15″ (arcseconds). That’s about 60% of its maximum apparent size. It will shine at magnitude -1.5, and be the brightest object in that area of the sky. It will also appear distinctly “reddish” in color compared to nearby stars.

If you look toward the east, about 15° above the horizon, you can’t miss it. And Mars will continue rising higher as the night progresses. But if you do need help locating it, click on the thumbnail below for a sky chart generated using my astronomy app. The two bright stars nearby (Arcturus and Spica) should help you zero in.

For comparison, Arcturus has a magnitude of -0.04, and Spica +1.04. Mars will not only be the brightest object in that part of the sky, it will also slightly outshine Sirius (M = -1.47), the brightest star in the sky. Look over your right shoulder to see Sirius that evening. There will also be a First Quarter Moon up that night, shining with approximate magnitude -10.0, but it won’t interfere with your view of Mars.

One final note: For the sake of comparison, the inset at upper left in the graphic shows Mars at its largest apparent size (about 25″). The icons used to show Mars on its orbit are scaled down to show Mars at its smallest apparent size, when Mars and Earth are on opposite sides of the Sun, and at their greatest separation of 377,000,000 km (234,000,000 miles). Its apparent size is a mere 5″ at that distance. That’s a factor of 5 difference, and that’s why, if you want to see Mars, oppositions are your best opportunity.


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