Comets achieved popularity with the apparition of Halley's in 1986. Sadly, Halley's was a bust, and the chief interest centered around the pronunciation of Edmund's name. After decades of calling him Hey-le, he suddenly became Hal-le. Comets are notoriously unpredictable, and Halley's has not been the only dud. Both Kohoutek and ISON fizzled. There is always hype surrounding comets. ISON was to be the Comet of the Century, and some predicted it would be as bright as a full moon! Being a sungrazer, it failed to survive perihelion.
It was the unpredictability that inspired fear in ancient people. The sun, moon, planets & stars had regular patterns. Comets appeared seemingly for no reason and were considered bad omens.
Comets are stuff left over from the formation of the solar system. Stuff was blown out when the sun turned on. There may be trillions of comets in the Oort Cloud at the edge of the solar system, the region named after Dutch astronomer Jan Oort. Of the 4000 known comets, I have seen 3.
Comets orbit the sun like planets, but their orbits are wildly elliptical. As comets approach the sun, methane and ethane vaporize to form tails millions of miles long.
Comets have 2 parts: heads about 6 miles in diameter and fan-shaped tails. They have 2 kinds of tails: blue gas tails that shine when excited by the solar wind and dust tails that reflect sunlight.
Comets approach the sun head first. When leaving the sun, it is the other way around. The tails take the lead. Either way, tails point away from the sun, driven out by the solar wind.
Comets cause meteor showers. As comets circle the sun, dust gets strung out along their orbits. When our planet crosses these streams, meteoroids no larger than apple seeds enter our atmosphere. They flare up because of friction.
1 The Great Comet of 1744
This one had 6 tails.
2 Halley's Comet 1910
Halley's Comet was recorded as far back as 467 B.C. Edmund Halley's research led him to predict the same comet was returning every 76 years. When it returned in 1758, it made him famous. Halley's is a short-period comet. Long-period comets take thousands of years to return. Halley's was spectacular in 1910, when the earth passed through its tail.
3 Comet Ikeya-Seki 1965
I saw this comet on Halloween morning. Ikeya-Seki was a sun-grazer and split in 2 after emerging from behind the sun. Its tail was 90 million miles long, one of the longest.
4 Comet Bennett 1970
Passing near the Cygnus star cloud, this comet had a tail 10 degrees long. John Bennett discovered it. I was in the army and out of astronomy.
5 Comet West 1976
Comet West was as bright as Jupiter and had a tail 20 degrees long. Richard West discovered it. I was at Peabody College in Nashville and unaware of it.
6 Shoemaker-Levy 9 1994
The Hubble Space Telescope imaged it as it crashed into Jupiter. 21 fragments hit. It was discovered orbiting Jupiter, captured by the planet's gravity. Gene Shoemaker died in a car wreck in Australia.
7 Hyakutake 1996
I saw Hyakutake! Its head was zero magnitude. Its blue gas tail was 12 million miles long. Dust tails are brighter than gas tails.
8 Hale-Bopp 1997
Discovered by Alan Hale and Thomas Bopp, it became the darling of comet buffs. It was brighter than zero magnitude and visible in amateur telescopes for 2 years. Hale-Bopp showed 2 distinct tails. Its blue gas tail and dust tail were widely separated. Hale-Bopp was bright because of its size. Its head was 25 miles in diameter. I saw it from my parents' yard on Aiken Road. Comets are brighter when near the sun because more ice vaporizes.
Astronomers make a big deal out of comets. Theorists have gone so far as suggesting that comets brought water to Earth, that they are responsible for the oceans. Moreover, they maintain that comets may have provided the organic molecules that started life. It sounds absurd, but more and more theories are cosmic rather than Earth-based. Some suggest that the sun stripped the inner planets of ice and gas while the outer planets, KBOs & comets retained theirs. It seems there would have had to be trillions of trillions of trillions of comets impacting Earth to create the oceans.