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The above photography is reprinted with permission of NASA.
Although there are periodic comets such as Halley’s whose appearance can be predicted years in advance, most comets come as a surprise. This is because during most of a comet's orbit (it can take a millions years to complete a single orbit), it is well beyond the borders of the Solar System.
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's description is more accurate: 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.
As a comet approaches the Sun, it begins to interact with electrons and protons being emitted by the Sun and a tail pointing away from the comet is created. The closer to the Sun, the longer the tail. Sometimes the tail can even split into two.
The tail gives the false impression of the comet quickly moving across the sky in photographs. A comet's movement is very slow within a single night. There are generally several naked eye comets visible every year, but only a very few every decade can be clearly seen with their tail.
Lepus