Astronomy 101: From the Sun and Moon to Wormholes and Warp Drive, Key Theories, Discoveries, and Facts about the Universe (Adams 101 Series)

by Carolyn Collins Petersen

Kepler’s Laws of Planetary Motion Kepler applied Brahe’s observations in an effort to solve the problem of Mars’s peculiar-looking orbit. He reasoned that Mars followed an elliptically shaped orbit. This led to his first law of planetary motion: A planet moves in an ellipse around the Sun and the Sun is at one focus of the ellipse. By applying some geometry to the problem, Kepler then devised his second law of orbital motion: A line connected between a planet and the Sun sweeps out equal areas in equal times as the planet orbits the Sun. Kepler then applied his mathematics to calculate orbital periods (that is, how long a planet takes to go around the Sun). His third law is the result: The square of the period of a planet’s orbit is proportional to the cube of the semimajor axis of that orbit.