Quick Answer: How Did Chinese Calendrical Astronomy Work?
Chinese calendrical astronomy brought the Moon’s monthly cycle and the Sun’s seasonal cycle into one lunisolar calendar (阴阳合历, yīnyáng hélì). The calendar assigned month starts to new moons. Solar terms marked stages of the seasonal year, and an extra lunar month in some years kept the two cycles in step.[1]
- Twelve lunar months add up to about 354 days. The seasonal year is about 365 days.[1]
- New moons and principal solar terms (中气, zhōngqì) help determine where an extra month falls.[2][3]
- Astronomers measured phenomena such as the Sun’s changing noon shadow and used calculations to assign dates.[5][6]
A calendar month and a solar term can begin on different days because they track different motions.
A month can begin with a new moon even while the winter solstice falls near its middle. Their relationship shows the two kinds of time that astronomers brought together when they made a calendar.
Why Twelve Lunar Months Fall Short
From one new moon to the next takes about 29 and a half days on average. Twelve such cycles amount to about 354 days. A seasonal year runs to about 365 and a quarter days. If every year had twelve lunar months, the dates would move through the seasons at roughly eleven days per year.[1]
Adding a lunar month at intervals compensates for that difference. The regular Chinese calendar months still follow new moons in years that have an extra month.[2]
Where the Extra Month Fits
The 24 solar terms mark successive positions of the Sun along its apparent annual path. Twelve alternate terms are principal terms. In a calendar year that requires an extra month, the principal terms help determine which month receives the leap label. The precise rules matter because a lunar month can occasionally contain two principal terms.[2][3]
A reform in the late second century BCE anchored the winter solstice in the eleventh month. Oracle-bone evidence of intercalation is considerably older. Evidence for adding a lunar month therefore predates the surviving account of that particular placement rule.[2]
How Astronomers Measured the Year
At the Yuan observatory at Dengfeng in Henan, a tall structure worked with a horizontal scale to measure the shadow of the Sun. Near a solstice, noon shadows change only slightly from day to day. Observers used the measurements with calculations to estimate when the seasonal turning point occurred.[4][6]
Other instruments served other tasks. Armillary spheres could be used to measure positions in the sky. Celestial globes represented positions on a spherical model. Both help explain the range of tools used in Chinese astronomy.
What a Calendar Reform Changed
Guo Shoujing and his colleagues completed the Shoushi calendar (授时历, Shòushí Lì) in 1280. Its methods related solar and lunar motion to computations for dates, eclipses and other astronomical phenomena. A reform could change how astronomical events were calculated and how those calculations were expressed in an official calendar.[5]
Compare Chinese eclipse records with comet records. A solar eclipse can be related to a new moon and calculated. A comet report typically describes an appearance, date and position written down when the observer saw it.
FAQs About Chinese Calendrical Astronomy
Did the Chinese calendar use only the Moon?
It combined lunar months with the seasonal year. The solar terms and added months kept those cycles aligned.[1][2]
Does the winter solstice always mark the start of a month?
It can fall within a lunar month. In the system established in the late second century BCE, the eleventh month contains the winter solstice.[2]
What is the Shoushi calendar?
It was a Yuan calendrical system completed in 1280. It contained methods for calculating lunar and solar events, including eclipses.[5]
Sources
[1] Hong Kong Observatory. Calendar
[2] United States Naval Observatory. Explanatory Supplement to the Astronomical Almanac
[3] NASA Goddard Space Flight Center. Calendars and their History
[4] ICOMOS. Historic Monuments of Dengfeng advisory evaluation
[5] Chinese Academy of Sciences, National Astronomical Observatories. Research on the Shoushi calendar
[6] Proceedings on the History of Mathematics. Calendars, Interpolation, Gnomons and Armillary Spheres