The value of G was first determined by Henry Cavendish in 1798 while the Greek mathematician/scientist Hipparchus (190 BC - 120 BC) found that the radius of the moon's orbit was between 60 and 80 Earth radii. (It is actually about 59 Earth radii). The period of the Moon's orbit is about 29 days. From the period and radius you can find the speed of the Moon in its orbit. What can be determined if the radius and period of the Moon's orbit and the value of G are known