Passage II

The shape of a planet's orbit around a star is elliptical. Kepler's third law of motion relates the planet's orbital period (the time the planet takes to complete one orbit) to the ellipse's semimajor axis, , (see Figure 1) and the star's mass. Figure 2 contains a graph of the orbital period versus for 6 of the planets that orbit the Sun. The average orbital speeds of these planets are plotted versus in Figure 3.

Kepler's third law also describes a moon's orbit around a planet. Figure 4 contains graphs of the orbital period versus a for moons orbiting hypothetical planets of various masses.

(Note: In Figures 2-4, 1 AU = 1.5 x 10⁸ km, the average distance from Earth to the Sun, and periods are given in multiples of Earth days or years.)

(Note: In Figure 4, moons' masses are assumed to be insignificant compared to the masses of planets they orbit.)