sentences:  On the other hand, Wei Pu realized that the old calculation technique for the mean sun was inaccurate compared to the apparent sun, since the latter was ahead of it in the accelerated phase of motion, and behind it in the retarded phase.Hence, he incorporated solar motion into the eclipse theory.The Song Dynasty astronomers of Wei's day still retained the lunar theory and coordinates of the earlier Tang astronomer, mathematician, and mechanical engineer Yi Xing (683-727 AD), which after 350 years had devolved into a state of considerable error.