1. It was occasioned by the fall of an apple,

2. As he sat in a contemplative mood…

3. William Stukeley, Memoirs of Sir Isaac Newton's Life

Few theories can compare in the accuracy of their predictions with Newton's theory of universal gravitation. The predictions of celestial mechanics for the positions of the major planets agree with observation to within a few arcseconds over time intervals of many years. The discovery of Neptune and the rediscovery of Ceres are among the spectacular successes that testify to the accuracy of the theory. But Newton's theory is not perfect: The predicted motions of the perihelia for the inner planets deviate somewhat from the observed values. In the case of Mercury the excess perihelion precession amounts to 43 arcseconds per century. This small deviation was discovered through calculations by LeVerrier in 1845, and it was confirmed by Newcomb in 1882. The explanation of this perihelion precession became one of the early successes of Einstein's relativistic theory of gravitation.

Telescopic observations of planetary angular positions stretching over hundreds of years are needed to detect the excess perihelion precession. However, with the development of radar astronomy it has become possible to measure the distances to the inner planets directly and very accurately by means of the travel time of a radio signal sent from the Earth to the planet and reflected back. With such radar observations of distances, the small deviations from Newton’s theory can be detected after just a few years of observation.