What did Kepler formulate about planetary motion?

• A. Planets orbit the Sun in circular paths.
• B. Planets orbit the Earth.
• C. Planets orbit the Sun according to three laws.
• D. Planets drift randomly through space.

Answer: (C)

Kepler showed that planets move around the Sun according to three laws. He used detailed observations to challenge the older view that orbits are perfect circles centered on the Earth. The first law states that planets travel in ellipses with the Sun at one focus, not in perfect circles. The second law, equal areas in equal times, means a planet sweeps larger areas when it is nearer the Sun and smaller areas when it is farther away, so its speed varies along the orbit. The third law ties orbital timing to distance: the longer the orbital period, the farther the planet is from the Sun, with the period squared proportional to the semi-major axis cubed. Put together, these laws describe a precise, Sun-centered pattern of planetary motion rather than random paths or Earth-centered orbits.