I looked at it and it seems that what first led him to his conclusion was his theories.

"

In 1679, Newton received a letter from Robert Hooke. Hooke was asking Newton about the motion of the planets as described by Kepler. Hooke believed he had figured out a mathematical explanation of why the planets moved the way they did. Kepler had explained how the planets moved, but couldn’t explain why. Newton thought about this idea but failed to write Hooke back. Hooke began to boast that he had solved the problem of planetary motion in 1680. Newton knew that Hooke’s calculations were wrong but did not want to embarrass Hooke.

Edmund Halley then visited Newton and asked him about the problem of planetary motion. Newton told Halley that he had already calculated the answer and the effects of gravity on planetary motion. This is known as the inverse-square law. In 1686, Newton would write the book for which he is probably the most famous. Principia included the inverse-square law, the 3 laws of motion, and the Law of Gravitation.

Newton’s Law of Gravitation had many implications. First, it helped explain the motions of the planets. Second, it helped explain the curvature of the Earth. There was a wide debate on where the Earth was flattened and where it was curved. Many believed that the Earth was flattened at the equator and rounded at the poles. Many believed just the opposite.

Newton’s Law of Gravitation implied that the Earth would be flattened at the poles and rounded at the equator. Rene Descartes believed the opposite. Jacques Cassini, the son of Giovanni, made measurements that seemed to agree with Descartes. Later, two other explorers would make journeys towards the equator, which lasted 10 years. Their measurements backed Newton and proved that the Earth is flattened at the poles. "

And I believe that those were 'the guys'

http://www.saao.ac.za/assa/html/his-exp-arcmeridian.html