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The ancients, observing that the comets appeared and disappeared in an unpredictable way, surrounded by a pale hair and followed by an extremely changeable tail, had no doubts: they were something that came to disrupt the celestial order.
The very fact that the comets did not follow the movement of the planets, did nothing but strengthen this belief that led the comets to be responsible for generally serious historical events. Thus, for centuries it was considered that comets were messengers of misfortunes and the appearance of a comet was a cause of great concern in the villages.
In the 1st century BC JC the writer Plinio attributed the cause of the bloody war between Julius Caesar and Pompey to the passage of a comet. The same happened on many other occasions; also in 1066, when the Duke of Normandy William the Conqueror landed in England and killed King Harold ll proclaiming himself new king, another comet was seen. Today we know that it was Halley's comet, the most illustrious representative of this category of stars, who returns periodically.
Superstitions aside, the scientific opinion about the nature of comets, which our ancestors shared, was what Aristotle established around 350 BC. JC The great Greek philosopher formulated the theory that both comets and meteors were nothing more than atmospheric phenomena caused by boiling vapors that broke off the Earth and were propelled to the top of the atmosphere.
Aristotle's conviction about comets survived for centuries and Galileo himself failed to solve the enigma of comet trajectories, although Tycho Brahe had already managed to almost completely calculate his enormous distances from Earth.
Only in the second half of the seventeenth century, thanks to the studies of Newton and Halley, it was possible to know that comets are under the influence of the force of attraction of the Sun, but that, unlike the planets, follow extremely elongated trajectories .
Halley calculated that the occurrences of a comet produced in 1531, 1607 and 1682, should be attributed to the same celestial object and predicted that the comet would return in 1758. Halley did not live long enough to see the prediction confirmed with his own eyes. The comet appeared on time for the appointment and has since been known by name.
But we come to our days. Until a few years ago it was thought that comets were celestial bodies formed by cosmic residues, very similar to meteorites, wandering aimlessly through the solar system. Today our knowledge about comets has undergone a revolution.
The American astronomer Fred Whipple has formulated a hypothesis that perfectly matches most astronomical observations. According to Whipple, comets are like "dirty snowballs", that is, they would be formed by an ice conglomerate (water, ammonia, carbon dioxide) and solid grains consisting of carbon and silicates.
The nuclei thus composed, due to their small size, light and compact, are able to resist the gravitational force of the Sun and the planets, but at the same time they are quite volatile to justify the huge cloud from which they are surrounded by the effect of solar heat This hypothesis would also explain why comets are not visible when they lack hair and tail.
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