Theory and observations thus coinciding, Newton was justified in declaring that the force that pulls the moon towards the earth and keeps it in its orbit, is the familiar force of gravity, and that this varies inversely as the square of the distance.)} It was to appear in due time that Newton's hypothesis was perfectly valid and that his method of attempted demonstration was equally so.

The difficulty was that the earth's proper dimensions were not at that time known.

A wrong estimate of the earth's size vitiated all the other calculations involved, since the measurement of the moon's distance depends upon the observation of the parallax, which cannot lead to a correct computation unless the length of the earth's radius is accurately known.

Newton's first calculation was made as early as 1666, and it was not until 1682 that his attention was called to a new and apparently accurate measurement of a degree of the earth's meridian made by the French astronomer Picard.

The new measurement made a degree of the earth's surface 69.10 miles, instead of sixty miles.
Learning of this materially altered calculation as to the earth's size, Newton was led to take up again his problem of the falling moon.