It is easy to understand how the unexpected sight of a comets long, bright tail in the sky must have terrified ancient civilizations. Today we know that comets are just lumps of ice and rock, traveling from the far reaches of the outer solar system to orbit our sun. As a comet nears the sun the ice melts and gives off jets of gas and releasing clouds of dusty rock particles. From Earth the gas and dust is seen as a dramatic tail, shining be reflected sunlight and stretching for miles.
The time it takes a comet to orbit around the Sun is known as its period. Short period comets take from a few years to decades to orbit the Sun. A long period comet may take thousands of years to return to orbit the Sun.
As a comet nears, the Sun gas is released from its nucleus and forced away from the Sun by the solar wind. The pressure of the Sun’s radiation pushes dust particles back on them and they then form a separate tail.
The solid part of a comet is called the nucleus and is only a few miles across, but it may produce a cloud of dust and gas ten times the size of the Earth.
Some comets take thousands of years to orbit the Sun once, but Comet Encke takes just over 3 years. Comet Biela ,was first seen in 1806, and when it returned in 1845 it was divided into two separate comets.
Comets are named after the people who discover them. Halley’s Comet was named after English Scientist Edmund Halley. Halley believed that comets seen in 1531, 1607, and1682 were in fact the same comet. He suggested that this comet would return in 1758. His prediction proved to correct and it was seen on Christmas day in 1758. Halley’s discovery was important proof that the law of gravity worked for comets also. Halley comet was first sighted over Jerusalem in 66 A.D., and the first recorded sighting of the comet dates from 240 B.C.
No one knows where comets originate, it’s been speculated that they may form a vast cloud surrounding the Solar System. The gravitational effect of passing stars pulls comets from the cloud to fall finally toward the Sun and shine briefly in the sky.