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The Moon Help

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By — McGraw-Hill Professional
Updated on Aug 28, 2014

Introduction to the Moon

It has not been easy for humanity to develop consistent theories about what happens in the sky. If there were no Moon, it would have been more difficult. If the telescope had never been invented, the puzzle would have been tougher still. The Moon goes through obvious changes even to the most casual observer, but the reasons for these changes were not obvious to most people 50 generations ago. Neither the Sun nor the Moon is a smooth globe. Both have complicated surfaces. The Moon has craters, mountains, plains, and cliffs. The Sun has a mottled surface that is often strewn with spots. The Sun and Moon together perform a cosmic dance that, once in a while, puts on a show to rival anything else in nature.

Earth has countless natural satellites—meteors captured by gravity and orbiting in all manner of elliptical paths. The only natural satellite of significance and the only one that can be detected without powerful observing aids, however, is the Moon. It’s interesting that we have never come up with a better name for Earth’s Moon; we speak about the moons of Jupiter and the moons of Saturn, and then we call our own Queen of the Night “the Moon.” It is as if someone had a daughter and named her “Daughter.” Sometimes the Moon is called “Luna,” but that name conjures up visions of madness and worship and is not used by astronomers.

Double Planet?

The Moon orbits Earth at an average distance of about 384,400 kilometers (238,850 miles). Because the Moon's orbit path is roughly elliptical, sometimes it’s a little closer, and sometimes it’s a little farther away. The Moon’s diameter is 27.2 percent that of Earth, roughly 3480 kilometers (2160 miles). That’s large for a moon relative to its parent planet. The Earth-Moon system is sometimes considered a double planet, and some astronomers think the pair formed that way. But Earth is 81 times more massive than the Moon, and the Moon has essentially no atmosphere. Thus, in planetary terms, the Moon is a dull place.

Perhaps you have seen drawings of the Earth-Moon system and have come to envision the Moon as much closer to Earth than is actually the case. (The drawings in this chapter, except for Fig. 4-1, are examples of such misleading data.) There is a reason for this distortion. If the Earth-Moon system were always drawn true to scale, the illustration would be of little use for most instructive purposes. Earth is a bit less than 12,800 kilometers (7,930 miles) in diameter, and the Moon is about 384,400 kilometers (237,000 miles) away on average. That’s about 30 Earth diameters. If drawn to scale, the Earth-Moon system would look like Fig. 4-1. Think of the Earth and the Moon as pieces of fruit. Suppose that Earth is a 10-centimeter-diameter grapefruit and the Moon is a 27-millimeter-diameter plum (4 inches and 1 inch across, respectively). To make a scale model, you must set the two fruits 3 meters (10 feet) apart.

The Moon and the Sun The Moon Double Planet?

Figure 4-1. Earth-moon system, drawn to true scale.

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