The Earth 's motion was described in Newcomb 's Tables of the Sun , which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892 . The Earth 's motion was described in Newcomb 's Tables of the Sun ( 1895 ) , which provide a formula estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892 .