Why do historical records always mention comets?

Qin Shihuang only recorded a "great hunger" in three, seventeen and nineteen years, which is not much. There are no more records of comets than anywhere else, only seven years, nine years and thirteen years. According to the table of comets visible to the naked eye by Ichiro Hasegawa, in the past 200 years, comets with brightness above 6 have appeared1701–1900, with an average of 1. 13. It is generally believed that the frequency of comets is similar from ancient times to the present, so this frequency should also be suitable for before the Han Dynasty. According to this frequency, it is entirely possible to record the biography of Qin Shihuang.

So what's the special significance of Sima Qian's Records of Natural Disasters? Nothing special. It's the duty of historians. Historians are not only responsible for recording the rulers' speeches and actions, but also for observing astronomical phenomena, making calendars and divining the good and bad of state affairs according to astronomical phenomena. Among Li Zhouguan's pure Zongbo, there was an official named Zhang Bao, who was juxtaposed with Mahavamsa, Miniature History, Civil History and Foreign History. His duty is "to be in charge of the stars, to observe the changes of the stars, the sun and the moon, and to distinguish between good and bad." Understanding fate and personnel with the help of celestial phenomena is a very basic way of thinking of the ancients, and it was not until the rise of the theory of divination or the theory of the induction of heaven and man by Dong Shi that it became popular. There are many records in Spring and Autumn Annals that "food is the sky" and "there are stars in the morning". Sima Qian just inherited the tradition of historians and truthfully recorded the astronomical phenomena in the history books of past dynasties. He particularly emphasized this part, not because of his own thoughts.

Only short-period comets such as Halley's can accurately calculate its past position, while long-period comets (accounting for more than 80% of the total number of comets visible to the naked eye) cannot be calculated. Moreover, even short-period comets are extremely difficult to calculate. It is necessary to consider the gravitational perturbation of the planet and the non-gravitational effect of the comet itself. The latter only has empirical formulas, and it is impossible to determine comets that have only been recorded once or twice. For most comets, it is good to be able to go backwards for hundreds of years. If it is farther away, there will be a great accumulated error, and it may even enter chaos. Without the assistance of other reliable historical materials, it is impossible to accurately determine whether Sima Qian recorded a real comet or whether other celestial phenomena were regarded as comets.