Chapter 18 Section IV Comet

The "Spring and Autumn Classics" records: "In the autumn and July of the fourteenth year of Lu Wengong (613 BC), there was a star in the Big Dipper." Xingbo (bo Bo) is a comet. This is the earliest record of Halley's Comet in the world. .Comet Halley returns on average every 76 years.From the seventh year of Qin Shihuang (240 BC) to the second year of Xuantong of the Qing Dynasty (1910 AD), with an interval of 2149 years, Halley's Comet returned 29 times, each time China has detailed records.JR Hind, a modern western astronomer, used these continuous observation data to calculate the orbit of Halley's Comet, and found that the orbital inclination was gradually decreasing, from 170° in the Han Dynasty to 162° in the middle of the 19th century.This discovery has attracted the attention of the astronomy community.Dr. Brady of the University of California in the United States published a paper in 1972 arguing that there is a tenth planet in the solar system. The evidence is that the orbital change of Halley's Comet is the result of the planet's long-term perturbation.Blondy also predicted the position of the planet, but neither the Greenwich Observatory in the United Kingdom nor the Lick Observatory in the United States found it in the predicted position.

Comets are members of the solar system, and there are three types of orbits: elliptical, parabolic, and hyperbolic.Comets with the latter two orbit types make a single turn around the sun and are gone forever.Only comets that move in elliptical orbits will return, which are called periodic comets, and Halley's Comet is a periodic comet.Because the eccentricity of the comet's elliptical orbit is relatively large, and some are close to 1, it is only possible to see it with the naked eye when the comet travels near the perihelion.The structure of the comet is also very special. The central part of the comet head is dense and bright, which is called the nucleus.Surrounding it is the haze that evaporates from the nucleus of the comet, called the coma.Under the action of solar wind and solar light pressure, the coma extends in opposite directions, forming one or several bands of light, called comet tails.The closer you are to the sun, the greater the pressure and the longer the comet tail.As a result, comets will take on various forms as they approach and leave the perihelion.In addition, the size of the nucleus and the amount of coma of each comet are different, so it is impossible to describe all the forms with only one or two schematic diagrams.

It is precisely because of the diversity of comet shapes that the ancients thought they were different types of celestial bodies and named them respectively.For comets with long and straight tails, they are called sweeping stars or comets. In fact, the word "comet" means broom.For comets with slightly shorter and slightly curved tails, they are called Pui Xing or Fu Xing.The hook-shaped comet is called Chiyou's flag.Comets with several tails are very rare, and they are called Wu Can, Jail Han or Zhao Ming. "Book of Jin · Tianwen Zhi" has a passage about the phenomenon of comet tail: "The comet has no light, and it is light when it shines on the sun. Therefore, when it is seen at night, it points to the east, and when it is seen in the morning, it points to the west. In the north and south of the sun, it points with the sun. Its awn, long or short." Clearly pointed out the relationship between the sun and the direction of the comet tail.Such insights are difficult to come up with without a basis of multiple observations.

Comets also split, although they are rare, but they are also recorded in ancient Chinese books. "New Book of Tang Astronomy" wrote: "In October of the third year of Tang Qianning, there were three guest stars, one big and two small, in the void and danger, they met and separated at the same time, and they followed each other eastward, like a fight. Three days, and the second small star is gone. The big star will be empty and dangerous." The guest star here refers to the comet.This passage describes the situation where a comet that has split into three parts comes and goes between the empty and dangerous places.