Johannes Kepler showed that the orbits of the planets were ellipses and not circles. Kepler's First Law states that the orbits of the planets are

• A. The orbits are circles with the Sun at the center.
• B. The orbits are ellipses with the Sun at one focus.
• C. The orbits are parabolas with the Sun at the focus.
• D. The orbits are hyperbolas with the Sun at the center.

Answer: (B)

The main idea is that planetary orbits are ellipses, not circles. An ellipse has two focal points, and the Sun sits at one of them. This placement means the distance between a planet and the Sun changes as the planet travels around the orbit, so the planet speeds up and slows down along the path in a way that preserves equal areas swept in equal times. A circle would place the Sun at the center, giving a constant distance and not matching what Kepler observed. The other shapes described—parabolas and hyperbolas—are unbound trajectories, not the bound orbits of the planets. So the description that fits Kepler's First Law is ellipses with the Sun at one focus.