1577 - The Danish astronomer Tycho Brahe observes a comet and concludes its orbit must be elliptical. This means that stars and planets can't be attached to spheres, or the comet would have broken through them.
1609 - German astronomer Johannes Kepler measures the movements of the planets. He discovers that the planets circle the Sun in elliptical, or oval, orbits--not in circles. He also says that planets don't revolve around their orbits at the same speed. The closer they are to the Sun, the faster they move. His mathematical descriptions of the planetary orbits are now called Kepler's Laws.
