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According to Andreas Celichius who lived during the tail end of the sixteenth century, a comet is “the thick smoke of human sins, rising every day, every hours, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge.” Carl Sagan, on the other hand, described a comet as a snowball about a mile across made up of frozen organic gases. The snowball part of a comet – its nucleus – is too small to be seen, except when very near the Earth. Every evening stargazers scan the night sky, hoping to see some small insignificant dot that will turn out to be an approaching comet.
If we had to rely on only the nucleus, a comet would be a very unimpressive object – just another dim star. But as a comet approaches the Sun, something remarkable takes place; it grows a tail. The tail of a comet usually appears as a thin white line shooting out form the nucleus. It is brightest at the end attached to the nucleus and spreads out a small bit towards the dim end, finally fading away to nothingness. Pictures of comets showing the tail fanning out from the nucleus like the wake of a ship create a false impression of an object rapidly hurtling through the sky. The slightly blurred background of stars behind the comet heightens this impression. While a comet will move within a single night slowly pass the stars fixed on the celestial sphere, it will not suddenly flash across the sky and its movement will not be detected by merely looking at it.
A comet's movement across the sky is similar to that of the Moon; it may be detected over a period of time. Neither does the tail necessarily trail behind the comet. A comet's tail always points away from the Sun. Whenever a comet is travelling away from the Sun, its tail proceeds it. While a comet’s nucleus may be only a mile across, its tail can stretch out tens of millions of miles. For most of its orbit, a comet is a relatively small ball of frozen gases moving through space.
At a distance of two or three a.u. from the Solar System's center, the Sun begins to affect the comet's appearance. A coma of gaseous and solid particles is formed around the nucleus. Unlike the tiny nucleus, the coma may have a length of hundreds of miles and for some comets approach the size of Jupiter. The Sun constantly emits a flow of electrons and protons that flies off at a speed of hundred of miles a second called the solar wind. Although the mechanism is not yet fully understood, apparently the solar wind interacts with the coma and produces the comet’s tail. The force of the solar wind drives gas particles from the comet’s coma and forms them into a tail much like wind drives a column of smoke emitted from a smoke stack.
A comet may either be periodic or near-parabolic. A periodic comet passes around the Sun in a fixed period of time, anywhere from a year to a couple of centuries. Periodic comets generally orbit in the same direction as the planets and they remain relatively close to the plane of the Solar System. The orbit of a periodic comet is cigar-shaped, with the Sun near the tip. The most famous and brightest of the periodic comets is Halley’s. It is not really fair to call the near-parabolic comets non-periodic. They too circle the Sun in a fixed period of time except that nobody is very likely to be around for more than one passage. A near-parabolic comet may reach as far as 10,000 a.u. from the Sun – well beyond Pluto's orbit – take over 1,000,000 years to complete a single orbit. As it approaches the Sun, the near- parabolic comet travels faster and faster.
A comet 300,000 miles from the Sun (and some get this close) will be travelling at 300 miles a second. A near-parabolic comet will spend only a small fraction of its time within the Earth’s orbit, close enough to the Sun to have a tail. During most of its life, it will be far off beyond the planets. Since every year a few near-parabolic comets are discovered and these discovered can be made only when the comet is at the small portion of its orbit near the Earth, there must be a huge number of comets too far away from us to be detected. One theory is that there are millions of comets in a “cloud” perhaps hundreds of thousands of a.u. from the Sun.
Because of the vast size of their orbits, there is no way to predict the arrival of a near-parabolic comet. One simply has to wait for someone to discover one and spread the word. Several near-parabolic comets are discovered each year in this manner. What makes comet finding so difficult is that the comet must be discovered before a tail has grown at which point it looks like a dim star. Once a skygazer spots a new dot in the sky, he may report his siting and if he's lucky, will receive a confirmation that the dot is not an asteroid or a star or a spot in the eyes of the observer after staring all night into his binoculars, but is actually a comet.
Comets are quite dim and most of the twenty or so that appear every year can only be seen with the aid of a telescope, several through binoculars and perhaps one can be seen with the naked eye. Maybe three or four times a century there will be comet so bright that it may be see during the day. As previously mentioned, the part of the comet most easily seen is its tail. A few comets have a spike is a small needle-like appendage sticking out of the nucleus opposite the tail. Some comets have more than one tail and the tails themselves come in a wide variety of shapes. Changes in the solar wind will cause corresponding changes in comet tails. The tail may be thick or thin. Some tails are almost consistently bright throughout while the brightness of others gradually fades into the background of space. Comet Humason in 1962 had a highly irregular and lumpy coma, but almost o tail at all. Changes in the tail’s appearance can take place over a short period of time. Occasionally a tail will disconnect itself from the coma and a new tail will grow within half an hour. There is even a small chance of a comet splitting in two.
One last thing to keep in mind is that although most comets are dim, a comet’s tail can grow extremely long especially as it approaches the Sun. When a comet's tail spans tens of degrees of the sky, it is a spectacular sight.