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Chapter 10: Problem 2

If the Moon always keeps the same face toward the Earth, how is it possible for Earth observers to see more than half of the Moon's surface?

Short Answer

Expert verified
Earth observers can see more than half of the lunar surface due to the effect of lunar libration. This is a result of the Moon's elliptical orbit and tilted axis, which give observers glimpses of the eastern, western and polar edges of the Moon at different times.

Step by step solution

01

Understanding the facts

Firstly, it is necessary to understand that the Moon is tidally locked with the Earth. This means that it rotates around its axis in exactly the same time it takes to orbit the Earth, thereby always presenting the same face towards Earth.
02

Introducing the concept of libration

The key to solving this apparent paradox is the concept of 'libration'. Libration is a small oscillation of an orbiting body that makes it seem to wobble. In the case of the Moon, this occurs because its orbit around Earth is not a perfect circle, but an ellipse. When the Moon is closer to Earth, it moves faster, and its rotation doesn't keep up, allowing observers on Earth to glimpse a bit around the eastern edge. When the Moon is farther from Earth, it moves slower, and the rotation catches up, allowing observers to see a bit around the western edge.
03

Libration in Latitude

Libration also occurs in latitude due to the tilt in the Moons axis. This tilt allows Earth observers to catch a glimpse of the Moon's polar regions at different times.

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Most popular questions from this chapter

NASA is planning a new series of manned missions to the Moon. Compare the advantages and disadvantages of exploring the Moon with astronauts as opposed to using mobile, unmanned instrument packages.

Use the Starry Night Enthusiast \({ }^{\mathrm{TM}}\) program to observe the changing appearance of the Moon. Display the entire celestial sphere by selecting Guides \(>\) Atlas in the Favourites menu. Open the Find pane by clicking the Find tab at the top of the left border of the view window. Click the menu button (the blue colored button with a downward- pointing arrowhead) at the left of the entry in the list for the Moon and select the Magnify option. This will center a magnified image of the Moon in the view as seen from the center of a transparent Earth. Close the Find pane by clicking its tab. Click on the down arrow to the right of the Time Flow Rate control and select hours from the drop-down menu. Then click on the Run Time Forward button (a triangle that points to the right like a Play button). (a) Describe how the phase of the Moon changes over time. (b) Look carefully at features near the left-hand and righthand limbs (edges) of the Moon. Are these features always at the same position relative to the limb? Explain in terms of libration.

Before the Apollo missions to the Moon, there were two diametrically opposite schools of thought about the history of lunar geology. The "cold moon" theory held that all lunar surface features were the result of impacts. The most violent impacts melted the surface rock, which then solidified to form the maria. The opposite "hot moon" theory held that all lunar features, including maria, mountains, and craters, were the result of volcanic activity. Explain how lunar rock samples show that neither of these theories is entirely correct.

In 2005 the SMART-1 spacecraft detected calcium on the lunar surface. Search the World Wide Web for information about the SMART-1 mission and this discovery. How was the presence of calcium detected? What does this tell astronomers about the origin of the Moon?

Describe the differences between the maria and the lunar highlands. Which kind of terrain covers more of the Moon's surface? Which kind of terrain is more heavily cratered? Which kind of terrain was formed later in the Moon's history? How do we know?
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