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Here are some of the comets we see from earth:
| Name | 1st Seen | Orbit(years) |
| Halley | 240 BC | |
| Tycho Brahe | AD1577 | |
| Lexell | 1770 | |
| Biela | 1772 | |
| Encke | 1786 | |
| Wolf | 1884 | |
| West | 1976 | |
| Yanaka | 1989 |
A comet consists of a nucleus which is thought to be made up of about 25% dust, chunks of rocky or metallic material, and around 75% ice. When a comet passes by the sun on its orbit, it loses some of its matter. The frozen portion of the comet changes from a solid to a gas form. This gas spreads out around the nucleus forming a thin surrounding area called the coma. Sunlight causes the atoms in the coma to gain a pearly glow of light. The light is powered by the gases in the nucleus; so if the amount changes, the comet can either become duller or brighter.
During a comet's orbit, as it approaches the sun, solar winds make a tail attached
to the comet that can be up to 93 million miles long! Another smaller tail made up of
dust can form also, this one being curved. Tails of comets always turn away from the
sun. This means that when the comet is traveling away from the sun, its tails are in front
of it. Some comets, in fact, have been reported to have up to 9 tails!
There are two types of comets: short period and long period. Short period comets are in an orbit for a period of less than 200 years and are part of the inner solar system. Long period comets have orbital periods of millions of years and therefore are at times farther away in our solar system.
Although not often, comets can hit the Earth. An example of this is the Tunguska explosion of 1908
in Central Siberia. The real origin of comets is unknown, but many valid theories have been proposed.
One of these accepted views states that comets are as old as our solar system and are made out of
the left over chunks of the present planets of Uranus and Neptune.
About 5 comets are discovered each year, but most are faint and can not be seen without a telescope.
Some of the brightest comets seen from Earth are Halley's comet, Arend-Roland comet, and Ikeya-Seki comet.
Many people look for comets on their own. They usually discover bright new comets or see comets in
the low western sky in the evening and the eastern sky just before dawn.
People
In the late 15th century comets became a popular site to many of the astronomers of the time. Tycho
Brahe, a Danish nobleman, made many precise observations of the comet of 1577, although the path of
these celestial bodies were still unknown. He has a comet
named after him called the comet Tycho Brahe.
Sir Isaac Newton was the first to decipher the orbital patterns of the comet. Through math he proved
the eliptical motions and orbits of the comets, helping astronomers discover new comets and trace
recorded ones.
Edmund Halley has also contributed to the world of Astronomy. He recorded and studied the movements
of what in the begining seemed to be the successive motion of 5 almost identical comets. Later Halley
realized that these 5 comets were actually only one comet returning each time during a period of
about 75 years. Halley's Comet, as it was refered to, was considered one of the greatest triumphs of
Newton's Theory of gravitation. Halley's Comet was the first comet of which an orbital pattern was
developed.
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 | This page was written by Tracy Chilampath in the Astronomy class of the Summer Science Academy in BCC-Broward County, in July of 1998. |