Comets in the Sky
The ancients, observing that the comet came and went unpredictably, surrounded by a pale mane and tail followed by an extremely changeable, had no doubts: it was something that had upset the celestial order.
The very fact that comets did not follow the movement of the planets, did nothing but strengthen this belief that led to regard comets as generally responsible for serious historical events. Thus, for centuries thought that comets were messengers of misfortune and the appearance of a comet was the cause of great concern in the villages.
In the first century BC JC. the writer Pliny attributed the cause of the bloody war between Julius Caesar and Pompey the passage of a comet. The same happened in many other occasions, also in 1066 when the Duke of Normandy William the Conqueror landed in England and killed King Harold II proclaimed the new king, was spotted another comet. Today we know that it was Halley’s comet, the most illustrious representative of this category of stars, returning periodically.
Leaving aside the superstitions, scientific opinion about the nature of comets, that our ancestors shared, that Aristotle was established about 350 a. JC. The great Greek philosopher formulated the theory that both comets and meteors were nothing to atmospheric phenomena caused by vapors in boiling fell off the Earth and were driven to the top of the atmosphere.
Aristotle’s conviction on the Comets survived for centuries and Galileo himself failed to solve the riddle of the paths of comets, though Tycho had almost total Lobras calculate accurately their 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 influenced by the pull of the Sun but, unlike the planets follow highly elongated paths .
Halley calculated that the appearances of a comet in 1531 produced in 1607 and 1682, were due to the same celestial object and predicted that the comet would return in 1758. Halley did not live long enough to be able to see with their own eyes confirmed the prediction. The comet was submitted in time for the appointment and has since become known by name.
But we got to today. Until recently it was thought that comets were celestial bodies formed by cosmic debris, very similar to meteorites, wandering aimlessly through the solar system. Today, our knowledge of comets have undergone a revolution.
The American astronomer Fred Whipple has formulated a hypothesis that fits perfectly with most astronomical observations. According to Whipple, comets are like “dirty snowballs”, ie they would be formed by a conglomerate of ices (water, ammonia, carbon dioxide) and solid grains constituted of carbon and silicates.
And composite cores, due to its small size, lightweight and compact, they can resist the gravitational pull of the Sun and the planets, but ai the same time are quite volatile to justify ia huge cloud which surrounds the effect of solar heat. This hypothesis would also explain why comets are not visible because they lack the mane and tail.
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Posted in Astronomy | 
Tags: ammonia, atmospheric phenomena, carbon dioxide, Comets, Halley's comet, solid grains, the planets, water