Why are planets, moons, and the sun all nearly spherical? Why do me earth satellites circle the earth in 90 minutes, while the moon takes 27 days for the trip? And why don’t satellites fall back to earth? The study of gravitation provides the answers for-these and many related questions. As we remarked in Chapter 5, gravitation is one of the four classes of interactions found in nature, and it was the earliest of the four to be studied extensively. Newton discovered in the seventeenth century that the same interaction that makes an apple fallout of a tree also keeps the planets in their orbits around the sun. This was the beginning of celestial mechanics, the study of the dynamics of objects in space. Today, our knowledge of celestial mechanics allows us to determine how to put a satellite into any desired orbit around the earth or to choose just the right trajectory to send a spacecraft to another planet. In this chapter you William the basic law that governs gravitational interactions. This law is universal: gravity acts in the same fundamental way between the earth and your body, between the sun and a planet, and between a planet and one of its moons. We’ll apply the law of gravitation to phenomena such as the variation of weight with altitude, the orbits of satellites around the earth, and the orbits of planets around the sun.