25.2 The Earth-Moon System

Astronaut Alan Shepard, shown in Figure 5, was commander of the Apollo 14 mission. He was not a golf pro, but he loved golf so much that he brought the game all the way to the moon. Shepard wondered how far he could hit a golf ball on the moon. So he attached the head of a golf club to the handle of an instrument for collecting rocks. Swinging with just one arm, he hit a golf ball a few hundred meters, much farther than it would have gone on Earth.

Why did the golf ball go so far on the moon? The moon is only about one quarter of Earth's diameter and has much less mass than Earth. As a result, the force of gravity on its surface is only one sixth that on Earth. Also, the moon has virtually no atmosphere to slow the ball down.

Earth's Moon

At an average distance of about 384,400 kilometers, the moon is our nearest neighbor in space. Unlike Earth, the moon has no atmosphere. The moon's gravity is too weak to hold onto gas molecules, which simply float away into space. The lack of an atmosphere allows the moon's surface temperature to vary tremendously. In direct sunlight, the average surface temperature of the moon is very high, about 130ºC. At night, however, the average surface temperature plummets to a chilly ‒180ºC.

Rocks on the moon's surface date back 3.2–4.5 billion years. Studying such ancient rocks, which are rare on Earth, give scientists clues about the early history of our planet.

Figure 5 Apollo 14 commander Alan Shepard planted an American flag on the moon's surface in 1971. Inferring Why did the flag need to be supported by a horizontal rod?

---

End ofPage 800

Table of Contents