Gravitational Perturbations However, the small deviations from this ideal picture have consequences if careful measurements are made. These small deviations from the simplified picture are called perturbations . They can be calculated systematically using Newton's laws of motion and gravitation from the positions of the known masses in the Solar System. If we account carefully for all known gravitational perturbations on the motion of observed planets and the motion of the planet still deviates from the prediction, there are two options: 1. Newton's Law of Gravitation requires modification, 2. There is a previously undetected mass that is perturbing the orbits of the observed planets. We shall see that the history of astronomy following the introduction of the Law of Gravitation by Newton gives examples of both. The Discovery of the New Planet Neptune In 1846, the planet Neptune was discovered after its existence was predicted because of discrepancies between calculations and data for the planet Uranus. Astronomers found the new planet almost exactly at the position predicted by the
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