Measuring shadows means setting up a gnomon and watching the shadow it casts, in order to fix the winter and summer solstices and the "centre of the earth". This was the base of astronomical and calendar measurement in ancient times.
The Taosi site in Shanxi has yielded a painted gnomon ruler and the remains of an observatory, regarded as the earliest known physical evidence of gnomon shadow measurement in China.
The Rites of Zhou records measuring the sun's shadow with an earth gnomon to find the centre of the earth, and shadow measurement thereby became an institutional method.
Guo Shoujing of the Yuan built an observatory at Dengfeng and used a tall gnomon with a long ruler to measure the winter solstice shadow precisely, in the service of the Shoushi calendar.
Shadow Measurement is the class in the chapter on Cosmos on fixing the seasons and directions from the shadow of an upright pole. Set up a gnomon and measure the length of its shadow, and the winter and summer solstices can be fixed, and so can the "centre of the earth". The painted gnomon ruler of Taosi in Shanxi, the earth-gnomon measurement of the Rites of Zhou and the tall gnomon of Guo Shoujing of the Yuan all rest on this method.
