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Introduction For 200 years, astronomers were satisfied that Newton's model of gravity successfully explained the orbits of the planets in the solar system. However, as observations improved, it became apparent that Newtonian gravity did not completely describe the fine details in Mercury's orbit. Was something wrong with the model? Was there something else in the solar system? How can we fully explain Mercury's orbit? Part 1: Mercury's Orbit and Newton's Law of Universal Gravitation 1. We often model Mercury's orbit around the Sun as shown below. Describe this orbit, using the ideas developed by Kepler and Newton. Mercury Sun Simplified model of Mercury's orbit around the Sun 2. Astronomers have observed Mercury's orbit over many centuries and have noticed that its orientation slowly changes, as shown below. Physicists call this change precession and say that Mercury's orbit precesses. What could cause the precession of Mercury's orbit? Sun Mercury Mercury's orbit slowly precesses