Kepler's First Law: Which of the following correctly summarizes the description of planetary orbits?

• A. The Sun is at the center of the exact path of every planet.
• B. Orbits around the Sun are perfectly circular.
• C. Orbits are circular with the Sun at one focus.
• D. Orbits are elliptical with the Sun at one focus.

Answer: (D)

Kepler's First Law tells us that planetary orbits are ellipses with the Sun at one of the foci. An ellipse has two foci, and the Sun sits at one of them, so the distance between a planet and the Sun changes as the planet moves around the orbit. This explains why planets speed up when they are closer to the Sun (perihelion) and slow down when they are farther away (aphelion).

Orbits being perfectly circular would place the Sun at the center of the path, not at a focus, which is why the option describing circular paths is incorrect. The idea of a circular orbit with the Sun at a focus is contradictory, since a circle has no distinct focus like an ellipse does. Some planets have nearly circular orbits, meaning their ellipses have very small eccentricity, but they are still ellipses with the Sun at a focus.