The shining celestial bodies with tails are called ‘comets’. They are the members of the solar system and also revolve round the Sun in definite orbits. Every comet has two parts — a head and a tail. Most comets are very big in size. Many of them are so big that their heads alone are many times bigger than the sun. Their tails are millions of miles long. Comets are made up of rocks, dust and gases. Their tails contain ammonia, methane, water vapour and ice particles. As the sun-light falls on them, they shine. Some comets are very bright and can be seen with naked eyes but others can be observed only with the help of telescopes. Scientists have been constantly studying these comets with the help of telescopes. It is estimated that on an average, 9 new comets are discovered every year. Halley’s comet is the biggest amongst the comets discovered so far. It comes close to the Earth after every 75 years and only then, it becomes visible. This was seen for the first time by the famous astronomer Edmund Halley of England in 1682, and was named after him. This was subsequently observed, in 1758, 1835 and 1910. Between September 1909 and July 1911, many studies were conducted about this comet with the help of powerful telescopes. It was last seen, in 1986, when space probe Giotto took its close-up photographs. It is estimated that this will be again visible in the year 2061. Some comets come near the Earth after few years, but there are others, which take thousands of years to do so, as they take thousands of years to complete one revolution around the Sun. Sometimes, the comets approaching the Earth are crushed to dust and then meteoric dust falls on the Earth. It is now believed that comets are born out of the dust generated by the bursting of volcanoes from some planets or satellites. Some scientists contend that they were formed along with the solar system. But, till now the scientists are not certain about their origin.