The law under the image on the left shows the force of attraction (F) between two astronomical objects of masses M and m, which are separated by the distance r.

The gravitational constant G was measured years later by Henry Cavendish. Click on  to see how it was done.

An interesting point to remember is that, as the force of attraction between the two  is mutual, both bodies move around their common centre of mass. However, when one of the astronomical objects is much greater than the other, the distance between the centre of mass and the centre of the large object is small.

Click on and change the masses of the star and planet to check this statement.