Have you ever observed comets? Can you describe it? What other comets do you know? List their characteristics.

Comet, commonly known as "broom star" in Chinese, is one of the small celestial bodies in the solar system. It consists of frozen substances and dust. It is visible when it is close to the sun. The heat of the sun makes the comet material evaporate, forming a hazy coma around the ice core and a tail composed of thin material flow. Due to the pressure of the solar wind, the tail always points away from the sun. ?

Except when far away from the sun, the long, bright and sparse tail of a comet gives the impression that it is very close to the earth, even in our atmosphere. 1577 tycho pointed out that comets do not show different orientations from different places on the earth: therefore, he correctly concluded that they must be far away. Comets belong to small celestial bodies in the solar system. Whenever a comet approaches the sun, its brightness will increase rapidly. Observations of comets that are quite far from the sun show that they move along a highly elongated ellipse, and the sun is located at a focus of this ellipse, which is consistent with Kepler's first law. Comets spend most of their time away from the sun, where they are invisible. They can only be seen when they are near the sun. There are about 40 comets whose period of revolution is quite short (less than 100 years), so they will appear one after another as the same celestial body. ?

Halley's comet is the first celestial body observed in history. Newton's friend and donor Harley (1656- 1742) realized that it was periodic in 1705. Its cycle is 76 years. Historical records show that it will be observed whenever it passes through the sun from 240 BC, possibly from 466 BC. It was last adopted in 1986. The brightness of a comet is very low when it is far away from the sun, and its spectrum is simply the spectrum of reflected sunlight. When the comet enters within 8 astronomical units from the sun, its brightness begins to increase rapidly and its spectrum changes dramatically. Scientists saw several bright spectral lines belonging to known molecules. This change occurs because the solid matter (comet nucleus) that makes up a comet suddenly becomes hot enough to evaporate, and a gas cloud called coma surrounds the comet nucleus. The ultraviolet rays of the sun make this gas glow. The diameter of coma is usually about 105 km, but the tail is often very long, reaching 108 km or 1 astronomical unit. ?

Scientists estimate that comets usually only a few astronomical units away from the sun will disintegrate within thousands of years. 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, Norman 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).