Quiz: Kepler's Laws of Planetary Motion

Test your understanding of Kepler's three laws. Answers are at the bottom—try not to peek!

1. What geometric shape does a planet's orbit take, according to Kepler's First Law?
(a) Circle (b) Parabola (c) Ellipse (d) Hyperbola

2. Where is the Sun located within a planetary orbit?
(a) At the center of the ellipse (b) At one focus (c) At both foci (d) Outside the ellipse

3. A planet moves fastest at which point in its orbit?
(a) Aphelion (b) Perihelion (c) The midpoint (d) Speed is constant

4. Which law states that a line from the Sun to a planet sweeps equal areas in equal times?
(a) First Law (b) Second Law (c) Third Law (d) Newton's Third Law

5. If Planet X orbits at 4 AU from the Sun, what is its orbital period in years?
(a) 4 years (b) 8 years (c) 16 years (d) 64 years

6. Kepler's Second Law is a consequence of the conservation of:
(a) Energy (b) Linear momentum (c) Angular momentum (d) Mass

7. Who provided the observational data that Kepler used to discover his laws?
(a) Galileo (b) Copernicus (c) Tycho Brahe (d) Newton

8. In the equation T² = (4π²/GM)a³, what does M represent?
(a) The mass of the planet (b) The mass of the Sun (c) The total mass of the solar system (d) The gravitational constant

Answers

(c) Ellipse. See First Law.
(b) At one focus. The other focus is empty. See Ellipse Geometry.
(b) Perihelion. The closest point to the Sun. See Second Law.
(b) Second Law.
(b) 8 years. T = √(4³) = √64 = 8. See Third Law.
(c) Angular momentum. See Angular Momentum.
(c) Tycho Brahe. See Brahe & Kepler.
(b) The mass of the Sun (or whatever central body is being orbited). See Universal Gravitation and How We Weigh Planets.