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© Copyright 2001, Jim Loy
On the left we see my drawing of a comet. The comet is moving from left to right in the picture, very slowly from our point of view here on earth, as it doesn't appear to be moving unless we view it from night to night. A comet is a chunk of rock and ices. As it gets close to the sun, the light from the sun heats the ices which evaporate and form a cloudy coma around the rocky nucleus. The nucleus is very tiny (just a few kilometers in size) compared to the large coma. The solar wind and the sun's light push the dust and gasses surrounding the comet away from the sun, forming one or two tails (one of dust, and one of gasses). The tail gets longer (sometimes millions of kilometers) the closer the comet gets to the sun.
In the picture, the comet is moving roughly toward its tail. The tail does not indicate its direction of motion. It just points away from the sun. So, as the comet approaches the sun, from the outer regions of the Solar System, the tail trails behind it. And when the comet goes away from the sun, the tail goes ahead of the comet.
There are many comets. But few are bright enough to see without a telescope. Some astronomers complained that most of the really bright comets were only seen in centuries past. Then, near the end of the twentieth century, several bright comets appeared, and that century became the century of the comet. There were more bright comets, in just a few years, than at any such time period in all of recorded history.
The most famous comet is Halley's Comet (or Comet Halley). Edmond Halley did not discover it, but instead predicted its return. He had noticed from old records that the comets of 1531, 1607, and 1682 had similar orbits. The first two of those years are 76 years apart, and the last two are 75 years apart. He assumed that they were the same comet, and predicted its return in 1758. Halley died in 1742. In 1758 the world waited for the famous returning comet. It began to seem that Halley was wrong, when it finally appeared right at the end of the year. Halley's Comet last appeared in 1986, when it passed a relatively great distance from Earth, which made it look unimpressive.
The word "comet" comes from the Greek for "long hair." The really bright ones are Long Period Comets. It is well known that Mark Twain (Samuel Clemens) was born in 1835, a year in which Halley's Comet appeared; and that he died in 1910, the next year that Halley's Comet returned. It is possible that The Tunguska Event was a small comet that hit the Earth. Also the disaster that wiped out the dinosaurs may have been caused by a comet (see T. rex and the Crater of Doom - by Walter Alvarez).
Here are a couple of photos of comets, from Broderbund's ClickArt. I will try to figure out which comets they were.
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