Definitions
A comet is an icy, small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail.
A coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate. This gives a comet a fuzzy appearance when viewed in telescopes and distinguishes it from stars.
Basics
A comet is a ball of mostly ice that moves around in outer space. They are often described as "dirty snowballs". They are very different from asteroids. In the solar system, the orbits of comets go farther than Pluto. Most are very far away from the Sun, but some come near enough to Earth for us to see at night. They have long "tails", because the Sun melts the ice. A comet's tail does not trail behind it, but points directly away from the Sun, because it is blown by the solar wind.
The hard centre of the comet is the nucleus. It is one of the blackest things in the solar system. When light shone on Comet Halley's nucleus, the comet reflected only 4% of the light back to us.
Periodic comets visit again and again. Non-periodic or single-apparition comets visit only once.
People have seen some comets when they broke into pieces: Comet Biela was one example. Another comet was seen when it hit a planet: Comet Shoemaker-Levy 9 hit Jupiter in 1994. Some comets orbit (go around) together in groups. Astronomers think these comets are broken pieces that used to be one object.
In old times, people used to be scared of comets. They didn't know what they were, or where they came from. Some thought that they were fireballs sent from demons or gods to destroy the earth. They said that each time a comet appeared, it would bring bad luck with it. Whenever a comet appeared, a king would die. One of these examples is shown in the Bayeux Tapestry, when Halley's Comet returned. Comets were also known to end wars and thought to bring famine.
It was not until the Renaissance when scientists started to look at comets with less superstition and base their observations on science. Tycho Brahe reasoned that comets did not come from the earth, and his calculations showed that comets must be six times farther than the earth is from the moon.
Edmond Halley (whom Halley's Comet is named after) reasoned that comets are periodic, that is, they appear once every several hundreds of years. This led to the first prediction of a comet's return, Halley's Comet. In honor of this prediction, Halley's Comet was named.
Isaac Newton also studied comets, but he thought that comets were do to "vapours rising from the soil". In other words, he thought that gasses came up from the ground to form comets. Newton later said that comets make U-turns around the sun. He asked Edmond Halley to publish the research in his book Philosophiae Natrualis Principia Mathematica. Before Newton said this, people believed that comets go in to the sun, then another comes out from behind the sun.
All this new information and research gave people confidence, but some still thought that comets were messengers from the gods. One 18th century vision said that comets were the places that hell was, where souls would ride, being burned up by the heat of the sun and frozen by the cold of space.
Although today we know a lot about comets, there will always be people who will be afraid when a comet returns.
Topics of Interest
A comet is a small solar system body bigger than a meteoroid that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.
Halley's Comet or Comet Halley (officially designated 1P/Halley) is the most famous of the periodic comets, and is visible every 75 to 76 years. Many comets with long orbital periods may appear brighter and more spectacular, but Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet that might appear twice in a human lifetime. During its returns to the inner solar system, it has been observed by astronomers since at least 240 BC, but it was only recognized as a periodic comet in the eighteenth century when its orbit was computed by English astronomer Edmond Halley, after whom it is named. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04.
A Comet tail and coma are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.
In astronomy, a coma (from the Latin word for "hair") is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate.
Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma. The volatile material contained in the comet nucleus evaporates away, and all that remains is inert rock or rubble that can resemble an asteroid. Comets may go through a transition phase as they come close to extinction. A comet may be dormant rather than extinct, if its volatile component is sealed beneath an inactive surface layer.
A Great Comet is a comet that becomes exceptionally bright; there is no official definition, often the term will be attached to comets that become bright enough to be noticed by casual observers who are not actively looking for them, and become well known outside the astronomical community. Great Comets are rare; on average only one will appear in a decade. While comets are officially named after their discoverers, Great Comets are sometimes also referred to by the year in which they appeared great, using the formulation "The Great Comet of...", followed by the year.
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion - sometimes within a few thousand kilometres of the Sun's surface. While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, strong evaporation and tidal forces they experience often lead to their fragmentation.
Comets have, through the centuries, appeared in numerous works of fiction. In earliest times they were seen as portents, either of disaster or of some great historical change. As knowledge of comets increased, comets came to be imagined not just as symbols, but as powerful forces in their own right, capable of causing disaster. More recently, comets have been described as destinations for space travelers.
Comet vintages are years during which an astronomical event, involving generally a "Great Comet", occurs prior to harvest. Throughout the history of wine, winemakers have attributed successful vintages and ideal weather conditions to the unexplained effects caused by the comets. Some of the most heralded vintages in the last couple of centuries—such as the 1811, 1826, 1839, 1845, 1852, 1858, 1861, 1985 & 1989 vintages—have coincided with a notable appearance of a comet. The term "comet wine" is sometimes used in the wine world to describe a wine of exceptional quality in reference to the high reputation that comet vintages haves. The 1811 comet vintage, coinciding with the appearance of the Great Comet of 1811, has perhaps the most notoriety. The 1811 Château d'Yquem has exhibited what wine experts like Robert Parker have described as exceptional longevity with Parker scoring the wine a perfect 100 points when tasted in 1996. The 1811 vintage of Veuve Clicquot is theorized to have been the first truly "modern" Champagne due to the advancements in the méthode champenoise which Veuve Clicquot pioneered through the technique of remuage.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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