SECULAR VARIATION OF EARTHS ROTATION - INFERRED FROM THE CHINESE ANCIENT SHOUSHI CALENDAR (AD 1281)

The Shoushi calendar (epoch of AD 1281, Yuan dynasty) is famous and ve ry accurate in ancient China. It has evolved perfect and complete theo retical models of solar system objects, such as solar and lunar motion s during that period. Almost every part of this work corresponds to th e modern astronomical yearbooks. Compiled by native Chinese astronomer s, it sums up through their studies many real observing results. The m athematical methods were adopted in this calendar before the foundatio n of Newton's mechanical system. It is presented in this paper that th e indirect system is also very useful to recover the real observing hi storical material. By selecting these calculating results, we may sum up the integral data of the secular variation of the Earth's rotation from 1000 BC to AD 1500.