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Great Comet

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Great Comet West in 1976
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   Great Comet West in 1976

   A Great Comet is a comet which becomes particularly bright and
   spectacular. While comets are officially named after their discoverers,
   some may come to be referred to as The Great Comet of... the year in
   which they were brightest.

What defines a Great Comet?

   The definition of a great comet could obviously be quite subjective.
   However, any comet which becomes bright enough to be noticed by people
   who are not actively looking for it and becomes well known outside the
   astronomical community may come to be known as a great comet.

Reasons comets become great comets

   The vast majority of comets are never bright enough to be seen by the
   naked eye. They generally pass through the inner solar system unseen by
   anyone except astronomers. However, occasionally, a comet may brighten
   to naked eye visibility, and even more rarely it may become as bright
   or brighter than the brightest stars. How bright a comet becomes
   depends on three main factors.

Size and activity of the nucleus

   Cometary nuclei vary in size from a few hundreds of metres across or
   less to many kilometres across. When they approach the sun, large
   amounts of gas and dust are ejected by cometary nuclei, due to solar
   heating. A crucial factor in how bright a comet becomes is how large
   and how active its nucleus is. After many returns to the inner solar
   system, cometary nuclei become depleted in volatile materials and thus
   are much less bright than comets which are making their first passage
   through the solar system.

Close perihelion approach

   The brightness of a simple reflective body varies with the inverse
   square of its distance from the sun. That is, if an object's distance
   from the sun doubles, its brightness is quartered. However, comets
   behave differently due to their ejection of large amounts of volatile
   gas which then also reflect sunlight and may also fluoresce. Their
   brightness varies roughly as the inverse cube of their distance from
   the sun, meaning that if a comet's distance from the sun is halved, it
   will become eight times as bright.

   This means that the peak brightness of a comet depends significantly on
   its distance from the sun. For most comets, the perihelion of their
   orbit lies outside the earth's orbit. Any comet approaching the sun to
   within 0.5 AU or less may have a chance of becoming a Great Comet.

Close approach to the earth

   For a comet to become spectacular, it also needs to pass close to the
   earth. Comet Halley, for example, is usually very bright when it passes
   through the inner solar system every 76 years, but during its 1986
   apparition, its closest approach to earth was almost the most distant
   possible. The comet became visible to the naked eye, but was definitely
   unspectacular.

   A comet fulfilling all three of these criteria will certainly be
   spectacular. Sometimes, a comet failing on one criterion will still be
   extremely impressive. For example, Comet Hale-Bopp had an exceptionally
   large and active nucleus, but did not approach the sun very closely at
   all. Despite this it still became an extremely famous and well observed
   comet. Equally, Comet Hyakutake was a rather small comet, but became
   bright because it passed extremely close to the earth.

Previous Great Comets

   Great Comets of the past two centuries have included the following:
     * Great Comet of 1811
     * Great Comet of 1843
     * Donati's Comet - 1858
     * Great Comet of 1861
     * Great Comet of 1882
     * Halley's Comet - 1910
     * Great Daylight Comet of 1910
     * Comet Skjellerup-Maristany - 1927
     * Comet Arend-Roland - 1957
     * Comet Ikeya-Seki - 1965
     * Comet West - 1976
     * Comet Hyakutake - 1996
     * Comet Hale-Bopp - 1997

   Retrieved from " http://en.wikipedia.org/wiki/Great_Comet"
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   with only minor checks and changes (see www.wikipedia.org for details
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