Before we can answer this question, we must look at exactly what a comet is. As you probably know, a comet looks rather like a star with a long, milky ‘tail’ stretching across the sky. In fact these ‘tails’ are not tails at all because they always point away from the sun and do not trail behind.
A comet seems to be made up of three main parts. The nucleus in the center of the star like part is composed mainly of ice and dust particles, surrounded by a coma of much smaller particles and gas. The coma results from the melting of the ice and other substances as the comet approaches the sun on its extremely stretched out elliptical orbit. The sun’s radiation also tends to force some of these tiny piece of ice away from the comet and out into space, which explains why the tail is always directed away from the Sun.
A scientist called Halley, after whom a particularly bright comet is named discovered how comets moved. He also predicted when some comets which had already been seen would return. For example, Halley’s comet made its closest. Approach to Earth for 76 years as predicted in 1986.
Why are comets associated with disaster? It is true that a comet appeared before the Norman conquest of Britain in 1066, and that two more comets were seen before the Great Fire of London in 1666 and the Great Plague in the previous year, but in world terms these cannot be regarded as major events. Perhaps it is the result of people’s belief that what happens in their own country is of world importance that has led to this idea that comets predict disaster.
On the other hand, it has been suggested that the Star of Bethlehem that told of the birth of Christ was in fact a comet. It does seem unlikely, however, that a body of ice and gas moving around the sky in a quite regular orbit can have anything to do with the events here on Earth. This is particularly true of comets because they have such a low density. Although they do not have enough gravitational pull of affect the bodies they pass. In fact, their own orbits may be completely changed.