What is Kepler's first law of planetary motion? A. The period of a planet's orbit is proportional to its distance from the sun. B. Planets have circular orbits. C. Planets have elliptical orbits. D. The eccentricity of a planet's orbit is proportional to its distance from the sun. Which of these is correct about the moon's motion around the earth? A. Centrifugal force acting on the moon balances the force of the earth's gravity. B. The moon exerts no gravitational force on the earth. C. The moon is in free fall. D. There is no gravity acting on the moon because it is in orbit