Comets are the member of the Solar System which appears in the sky like a bright ball of light with a long glowing tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun. They revolve around the Sun in highly elliptical orbits. The period of revolution of comets around the Sun is usually very long. Many comets are known to appear periodically. Halley’s comet appears after nearly every 76 years. It was last seen in 1986.