Kepler's First Law states that planetary orbits are ellipses with the Sun at one focus. Which of the following best restates this law?

• A. The Sun is at the center of all planetary orbits.
• B. Planets travel in circular orbits around the Sun.
• C. Planets orbit the Sun in ellipses with the Sun at one focus.
• D. Planets follow parabolic paths around the Sun.

Answer: (C)

The main idea tested is that planetary orbits are elliptical paths with the Sun located at one focus, not at the center of the ellipse. The best restatement captures this precisely: planets orbit the Sun in ellipses with the Sun at one focus. This means the distance to the Sun changes as the planet travels, and the Sun sits at a focal point inside the orbital path.

Why this fits better than the others: the Sun at the center would imply a circular orbit centered on the Sun, which isn’t what the law says. A circular orbit is a special case of an ellipse with zero eccentricity, but the general statement emphasizes the Sun’s position at a focus rather than at the center. Parabolic paths describe unbound trajectories that do not keep planets in orbit, so they don’t match the bound, elliptical orbits described by the law.