Science Olympiad Solar System Practice Test 2026 - Free Solar System Practice Questions and Study Guide

Kepler's First Law of Planetary Motion states that the orbits of planets are ellipses with the Sun located at one of the two foci. This was a groundbreaking realization that shifted the understanding of planetary motion from the earlier belief in circular orbits, which had the Sun at the center. By describing the orbits as ellipses, Kepler acknowledged the variation in distance between the planets and the Sun throughout their orbits, showcasing the gravitational influence of the Sun on the planets as they move.

This law emphasizes that the pathway of each planet around the Sun is not a perfect circle but rather an elongated shape, creating varying speeds as the planet moves closer to or further from the Sun due to the nature of elliptical orbits. This means that planets travel faster when they are closer to the Sun (at perihelion) and slower when they are further away (at aphelion). Understanding this elliptical motion was a critical advancement in the field of astronomy, as it combined observational data—such as that of Tycho Brahe—with mathematics to provide a more accurate model of solar system dynamics.