## Newton Kepler and Gravity

Johannes Kepler was an accomplished mathematician, but he is best known for writing down the first correct empirical description of planetary motion in the solar system, and in particular for noting that the planets move along ellipses with the Sun at one of the foci. Kepler's observations would not have been possible without the impressive astronomical data collected by Kepler's teacher, Tycho Brahe (Figure 3.1).

Tycho Brahe's contributions to astronomy were enormous. He designed and built new instruments, meticulously calibrating them and checking their accuracy periodically, which ultimately revolutionized astronomical instrumentation. While earlier astronomers had been content to observe the positions of heavenly bodies at certain points of their orbits, Brahe and his students observed these bodies throughout their orbits. As a result, Brahe was the first to notice many orbital anomalies. Without such a complete and accurate set of observations, Kepler could not have discovered his empirical laws for planetary motion.

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