Astronomy Practice Exam

Who is known for stating the laws of planetary motion?

1. Galileo Galilei

2. Johannes Kepler

3. Nicholas Copernicus

4. Tycho Brahe

The correct answer is: Johannes Kepler

Johannes Kepler is recognized for formulating the laws of planetary motion, which describe the orbits of planets around the Sun. His first law states that planets move in elliptical orbits with the Sun at one focus, challenging the long-held belief in circular orbits. The second law, often referred to as the law of equal areas, explains that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time, indicating that a planet moves faster when it is closer to the Sun. The third law establishes a relationship between the period of a planet's orbit and its average distance from the Sun, expressing that the square of a planet's orbital period is directly proportional to the cube of the semi-major axis of its orbit. Kepler's work laid the foundation for Newton's theories and significantly advanced our understanding of celestial mechanics. The other figures mentioned played important roles in the history of astronomy but did not formulate these laws. Galileo contributed to observational astronomy, Copernicus proposed the heliocentric model, and Tycho Brahe made precise measurements of celestial bodies, which supported Kepler's later laws.