A.D. 1066, on the eve of Norman invasion of England, Comet Halley returned. At that time, people looked at this strange celestial body with a long tail in the night sky with mixed feelings and thought it was a war warning and omen given by God. Later, the Normans conquered England, and Commander Norman's wife embroidered the scene of Halley's Comet's return on the tapestry as a souvenir. China folks call comets "bad luck" and "disaster". In this way, the appearance of comets is associated with human wars, famines, floods, plagues and other disasters.

Comet is a small cloud-like celestial body with low mass, which orbits the sun in a flat orbit (very few in a near-circular orbit).

There are three kinds of orbits of comets: ellipse, parabola and hyperbola. Comets with elliptical orbits are also called periodic comets, and comets with other two orbits are also called aperiodic comets. Periodic comets are divided into short-period comets and long-period comets. Generally speaking, a comet consists of a head and a tail. The comet head includes two parts: the nucleus and the coma, and some have comet clouds. Not all comets have structures such as nucleus, coma and tail. In ancient China, the shape of comets has been studied a lot. There are 29 pictures of comets on the silk book unearthed from Mawangdui ancient tomb in Changsha. It is clearly stated in Tian Wenzhi in the Book of Jin that comets don't shine because they reflect sunlight and are seen by us. Their tails are facing away from the sun. Comets are huge, but their mass is pitifully small, even the mass of big comets is less than one tenth of that of the earth. Because a comet is composed of frozen impurities and dust, it is only a small cloud-like spot when it is far away from the sun; When it is close to the sun, the comet tail is produced because of the evaporation, gasification, expansion and eruption of the solidified body. The tail is huge, hundreds of millions of kilometers long. It has different shapes, some more than one, and generally extends away from the sun, and the closer it is to the tail of the sun, the longer it is. There are many comets in the universe, but only about 1600 has been observed so far.

You can extract a paragraph.