Page 391 - 89 history of astronomy_opt

detected a minute periodic error of theory
of twenty-seven days, owing to a period-
ically erroneous position of the centre of
gravity of the earth and moon to which the
position of the observer was referred. This
led him to correct the mass of the moon,
and to fix its ratio to the earth’s mass =
0.012240.
Another method of getting the distance
391