Cosmic Quest: Ancient Chinese Astronomy

In the last Cosmic Quest post we talked about astronomy in its infancy in ancient Babylon, Egypt and Greece. This time we’ll focus on astronomy in ancient China.

Ancient Chinese astronomy is interesting because it was free of Indo-European influence and thus is based on different principles. Although there is no evidence that early Chinese astronomers were concerned with how the Universe works, their calculations and predictions of celestial events were surprisingly precise.

In ancient China, astronomers recorded regular events like solar and lunar eclipses, while astrologers interpreted these events and tried to apply them to everyday life. Emperors played an important role in this, as the tradition dictated that they receive their mandate from heavens. This Mandate of Heaven made astronomy a powerful political tool and emperors consulted astrologers before major decisions. Sometimes, if astronomers made false predictions of celestial events, emperors would behead them! Still, with time, they got better at it, to their relief. The first human record of a solar eclipse dates back to 2136 BC and with time, Chinese astronomers not only recorded about 1600 solar and lunar eclipses, but the accuracy of their predictions, backed by a very good motivation, became unparalleled for that era.

Of course, keeping time with calendars was the probably the first use of astronomy in ancient China. Their year was divided into twelve months with added extra month every two or three years. Because there are 12.37 lunar months in a solar year, this extra month was necessary in order to have seasons always in the same months. Imagine having a summer in July one year and then in a few years in October, then in April after a few more years. This is called a lunisolar calendar.

Unlike Indo-Europeans who used ecliptic and the horizon as reference points, ancient Chinese astronomers used circumpolar stars - stars that never set and would be visible during the day if there wasn’t for the Sun outshining them. Their constellations system is somewhat similar to zodiac constellations, but rather than reflecting movement of the Sun, it was based on the movement of the Moon. Ancient Chinese divided the sky into four regions, each of them having seven so-called mansions. With these 28 mansions, they tracked the Moon’s movement across the sky during one sidereal month, 27.32 days, which is the time it takes from one full Moon to the next.

Shi Shen is one of the most notable ancient Chinese astronomers, having catalogued 809 stars in 122 constellations and making earliest known observations of sunspots. He lived in the fourth century BC. Gan De is another famous Chinese astronomer, who, together with Shi Shen, made accurate observations of the five planets visible to the naked eye. Two of them divided the celestial sphere into 365.25 degrees after the number of days in a year, which Chinese astronomers also accurately measured, whereas other civilisations took 360 as a number of degrees in a circle, originally used by Babylonians.

Chinese astronomers took interest in recording non recurring phenomena, like supernovae, meteor showers and comets. They observed a supernova in 1054, which created what we now know as Crab Nebula. A star seemingly appeared in the sky out of nowhere and remained there for two years before disappearing again and was visible with the naked eye, as bright as Jupiter. Interestingly, there is no record of the event in European or any other cultures except Anasazi Indians. These “guest stars”, as the ancient Chinese astronomers called them, were catalogued over centuries and this has enabled modern astronomers to know exactly where to look for remnants of supernovae.

For such precise observations, Chinese astronomers used tools like armillary spheres. They are made of a sighting tube and circles marked with degrees, enabling the user to measure the position of a star.

Another example of their excellent measuring capabilities is Zu Chongzhi, astronomer who lived 429-500 CE. He measured a year to last 365.24281481 days, under a minute difference compared to modern measurements. With this measurement, he was able to successfully predict an eclipse four times during 23 years.

Shen Kuo, who lived in the 11th century, used solar and lunar eclipses to back up his view that the Sun and the Moon were spheres on different paths and distances. Together with his colleague Wei Pu, he made precise measurements of the distance between the polestar and true north. They also charted the exact coordinates of the planets as a part of extensive observations that lasted five years. They theorised about nature of planets’ motion, including retrograde motion.

Only in 3rd century CE did Chinese astronomy receive some influence from the outside, namely from India, but the more significant impact came during the Tang Dynasty (618-907 CE). Later, it was also influenced by Persian and Arab astronomy from the 10th to the 14th century.

As has Chinese civilisation, Chinese astronomy progressed uninterrupted for over 4 milennia. It played a significant political role and was often supported by an emperor, as evident from many observatories built during different dynasties. Chinese astronomy remains to be supported today by the government that wishes for China to remain major factor in exploring the space.

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