Historic comet observations in China

Chinese records of comets are the most extensive and accurate from the ancient and medieval periods. They stretch back across three millennia. They were observed in such great detail because of their astrological significance. However, they are now of great use to modern astronomers. The accuracy is great enough to allow orbital elements to be calculated and this has been done for many comets. Most notably, the ancient orbits of Halley's Comet have been determined using Chinese data, a feat not possible from modern data due to a close approach of the comet to Earth in the ninth century.

Chinese records

Ancient Chinese records of comet observations are the most extensive historical records in existence. They are far more complete than European observations.[1] The earliest confirmed Chinese comet observation is from 613 BC,[2] but there is also a possible sighting of Halley's comet in 1059 BC.[3] However, this may not be an actual sighting, but the result of later back calculation.[4]

Early records call comets po-hsing (孛星, "bushy star"—or "sparkling star"[5] according to Yeomans et al.). Later on, a distinction is made between po-hsing and hui-sing (彗星, "broom star"), that is, comets without, and with a tail respectively. The broomstick here being a metaphor for the tail and the broomhead the head of the comet. Chinese astronomers were the first to observe that comet tails point away from the sun. They knew this by at least 635 AD, many centuries before the phenomenon was observed in the West.[6] Other words for comet include sao-hsing ("brush star"), thien chhan ("edging-in star"), phêng-hsing ("sailing star"), chhang-hsing ("long star"), and chu-hsing ("candle-flame star").[7]

The Chinese records are not only the most extensive from ancient times, they are also the most accurate, often to within half a degree of right ascension. Western measurements did not overtake them for accuracy until the fifteenth century—in 1456 AD Paolo Toscanelli tracked Halley's comet's progress to within a fraction of a degree.[2]

Finding the time of a Chinese observation is a bit more problematic. This can be very important for fast moving objects like comets. However, the dates are recorded, and the time can be estimated to within one or two hours by considering when the Chinese observers would have had good viewing conditions.[8] Comets that actually have a tail can be described as po-hsing when they are in opposition and the tail is not visible, giving a further clue.[7]

Chinese records of comet brightness are superior to Western observations to an even later date. The West did not overtake Chinese astronomers in this respect until the twentieth century, at least in the matter of diligent recording of brightnesses.[8]

List of records

The following documents contain substantial comet observations. Many other local documents have useful information and have been incorporated into various compilations.

Primary sources

Compilations

One of the earliest compilations is a Chinese list in Wen Hsien Thung Khao by Ma Tuan-Lin in the thirteenth century. This includes a list of comets up to 1222 AD. Various European authors used this list as a basis for their own lists starting with Antoine Gaubil who translated it from the Chinese and later extended the list to 1644 AD. The most complete European list was published by John Williams (1797–1874) in 1871. Williams list contains 372 comets from 613 BC to 1621 AD.[1] A Chinese compilation published in 1988, A Comprehensive Collection of Ancient Chinese Astronomical Records (Zhonguo gudai tianxiang jilu zongji), has over a thousand comet observations.[10]

Importance to modern astronomers

The Chinese records have modern importance to science because they allow astronomers to deduce the past orbits and brightnesses of comets.[2] This is important because comet orbits are not always entirely regular. They can be changed by gravitational encounters with the planets. Without ancient records, such past changes would be very difficult to determine.[9]

A feature of the Chinese records that makes them particularly useful for modern scientists and scholars is that there is a high degree of consistency in the methods of observation and recording over a very long period. Chinese astronomers were still using naked eye observation in 1835 AD, long after the invention of the telescope.[8]

Around forty comets have had their orbits calculated based entirely on ancient Chinese records.[1]

Halley's comet

Chinese observations of Halley's comet are of especial importance to astronomers because of the important place this comet had in their growing understanding of them.[7] The earliest confirmed sighting was in 240 BC (in the Shih-chi)[9]—with a continuous record after 164 BC.[11] A comet observed in 467 BC may have been Halley, but there is insufficient information to be sure.[12] The most accurate records of Halley begin in 12 BC.[9]

A computer calculation of the past orbits of Halley using numerical integration[13] could not continue past 837 AD because a very close approach to Earth made calculation of orbits prior to that too inaccurate. The researchers had started with accurate European measurements from 1759, 1682 and 1607 AD and calculated backwards. However, no accurate European records exist for 837 AD so the researchers looked to Chinese records. From these they obtained a good estimate of the perihelion of Halley for that year. They used that, together with perihelions found from Chinese data for 374 and 141 AD, to provide constraints for their calculation.[9] They were finally able to calculate the orbits of Halley back to 1404 BC.[14]

Studies of historical changes in brightness of Halley's comet have also been carried out using ancient Chinese data. There has been no regular recording of the brightness of Halley in the West prior to the 1910 apparition. The Chinese records, on the other hand, are almost complete from 12 BC to 1835 AD.[8]

Comets in Chinese astrology

Ancient Chinese astrology set great store by celestial omens and comets were an important omen. Comets were thought to signify an imbalance of yin and yang.[15] Chinese emperors employed observers specifically to watch for them. Some important decisions were made as a result. For instance, Emperor Ruizong of Tang abdicated after a comet appearance in 712 AD.[2]

References

  1. 1 2 3 Needham, p.430
  2. 1 2 3 4 5 Stephenson and Yau, p. 30
  3. ↑ Xu, Pankenier & Jiang, p. 107
  4. ↑ Needham, p. 408
  5. ↑ Yeomans, Rahe & Freitag, p. 71
  6. ↑ Stephenson and Yau, p. 30
    • Needham, p. 432
  7. 1 2 3 Needham, p. 431
  8. 1 2 3 4 Stephenson & Yau, p. 32
  9. 1 2 3 4 5 6 Stephenson & Yua, p. 31
  10. ↑ Xu, Pankenier & Jiang, pp. 107, 109
  11. ↑ Yeomans, Rahe & Freitag, p. 62
  12. ↑ Needham, p. 432
    • Yeomans, Rahe & Freitag, p. 68
  13. ↑ Yeomans & Kiang, 1981
  14. ↑ Yeomans & Kiang, p. 633
    • Xu, Pankenier & Jiang, p. 110
  15. ↑ Needham, p. 32

Bibliography

Further reading

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