Comets are snowballs made of carbon dioxide, methane or water ice that have very elliptical orbits around the sun. Many of them come closer to the sun than any of the inner planets, and range farther from the sun than the farthest planets. Some comets get so close to the sun that they fall in and are devoured, but some, like the comet Kahoutek, take many centuries to make a complete orbit around the sun. The period of this comet is about 75,000 years. Since they seemed to come from nowhere, a lot of societies believed comets to be bad omens.
The most famous comet is Halley’s comet. It wasn’t exactly discovered by English astronomer Edmond Halley, as the comet was seen over the Battle of Hastings in 1066, but he calculated its period, which is 76.3 years. Because of this, the comet not only bears his name, but humans now understand that comets are predictable.
A comet has an icy core called a nucleus. The nucleus of Halley’s Comet is only about nine miles long and five miles wide. As the comet approaches the sun it begins to warm up, and a halo or coma begins to form around it. The Latin word “coma” means hair, and not unconsciousness, as the Greek word “coma” means. When comets come close to the sun, they develop very long tails as the sun heats them up. It’s only then that they can be seen from earth with the naked eye. The tail forms when the solar wind blows material away from the nucleus, and can be 100 million miles long, though the Great Comet of 1843 was said to have a tail of about 200 million miles long!
After the comet loops around the sun and begins its return journey, its tail actually blows in front of the nucleus. As it leaves the sun’s warmth, the tail diminishes, and then disappears altogether. Some astronomers believe comets originate in the Oort cloud, a cloud of icy bodies around the solar system. One famous comet is the Great Comet that appeared in 1811 and will take 3,000 years to return. Other famous comets are Encke’s Comet, which has a period of 3.3 years, and the Great Comet of 1864, which has an orbit that takes 2,800,000 years to complete.