Zhou bi suan jing"An Analysis and Study", by Christopher Cullen, 1996, Cambridge University Press
The Zhou bi is a collection of ancient Chinese texts on astronomy and mathematics. The time when it was assembled was probably under the Han dynasty during the first century B.C., or it could even be 1000 years older.
The title means "The Gnomon of Zhou Dynasty". Recall that a gnomon is a pole for measuring shadow lengths. In distinction to the spherical cosmography of the Greeks, the Zhou bi follows the "gai tan" cosmography, an umbrella like heaven that rotates about a vertical axis above a plane earth. It is unique in that it is the only rationally based and mathematical account of a flat earth cosmos. It is also noted that ancient Chinese astronomical theory and practice appears to have had no significant connection with Babylon, Greece or Egypt.
"The Foundation of Chinese astronomy is attributed to the need to make calendars. A legendary Emperor Yao in remote antiquity commision his two star clerks, Xi and He, to measure celestial phenomena to set up a calendar so that the seasons could be determined." This lunar based calendar is probably the oldest known, from about 2000BC. In Greece Plato's commission set Eudoxus off(See chapter 3 of our textbook). In China - the need for a calendary set astronomy off.
Tasks of Imperial Astronomers
- used a lunar/solar techinique to keep the calendar in step with the seasons
- tabulate the motion of the planets
- predict lunar eclipses and give advance warning for solar eclipses
- Emperor was supposed to keep heaven and earth in harmony. He had to do his earthly rituals in synchronization with the state of the heavens. Even a few hours off was viewed as a possible disaster.
- They had to advise on lucky or unlucky days.
The Zhou bi deals with mathematical astronomy and not with "portent astronomy". Detailed records at 35deg N. latitude were kept of non predictable phenomena, for example, comets, meteor showers, novae. It's interesting to compare these detailed records from China with Western records. As we discussed in "objects in the sky" the non predictable phenomena for the Greeks would fall in the category of atmospheric phenomena, like clouds, rather than celestial phenomena.
The astronmers divided the sky into 28 lunar lodges around the celestial equator of varying sizes. The lodges were bigger than the Babylonian zodiacal band (+- 8 degrees around the ecliptic), and were used to establish chinese constellations. Nevertheless, the moon in a "lodge" is always within about 5 degrees of the ecliptic. The astronomers divided the sky into seven "hengs". These are circles of different radii. There are three important hengs as we can see in these scans from the Zhou bi figures.
The "Yellow road" is the path of the sun. The sun moves along the yellow road during the year and it is carried around by the daily rotation of the sky. The observer at 35 deg N is a distance of 103000 li away from the POLE. Think of the pole as the handle of an umbrella. As you turn the handle of the umbrella looking up you see this curved umbrella like sky rotating. Since the sun is always out in the sky you might wonder how the astronomers explained the sunrises and sunsets. This is explained by the green circle which has a radius of 106000 li about the observer. The sun is assumed to be visible only when the rotation of the sky places the sun within the green circle.
The second figure highlights the celestial equator as the red circle. When the sun is on heng 7(WS) this is the winter solstice location. The sun spends the least amount of time during the daily rotation of the sky within the green circle so winter days have least sunlight. When the sun is on heng 1 it spends more time during the day within the green circle giving the season of most sunlight. The equator is on heng 4. The places where the yellow road and the equator cross are the times of the equinoxes (EQ).
The length of chinese units varies depending upon the Emperor. You can find a table of chinese units in Hugh Thurston's book(p. 87). For example, in Tang times 1 li is about 0.44km.