The Moon - Known Facts

About the Moon, what scientific facts are known including trivia about the orbit, surface, and more.

THE MOON

Known Facts: The most obvious feature of the moon to the unaided eye is the night-to-night change in its phases. Parallel rays of light from the sun always illuminate one hemisphere of the moon. Starting from new moon, when the dark side faces us, the fraction of the illuminated face visible on Earth increases until we see all of it (full moon), then decreases back to new moon.

When the moon comes directly between the earth and the sun, all or part of the moon's shadow falls on the earth, totally or partially eclipsing the sun. This doesn't happen at every new moon because the moon's orbit is tilted from the earth's and its shadow often misses the earth completely. At full moon the earth's shadow may fall on the moon, producing a total or partial lunar eclipse.

The moon's greatest direct influence on the earth is through its gravitational pull which produces two tidal bulges on opposite sides of the earth in line with the moon. Water is most attracted toward the moon at the point where the moon is directly overhead; on the opposite side of the earth, the moon attracts the ocean bed away from the ocean, causing a second high tide. Midway between these high tides are the low tides. The positions change as the earth rotates underneath them and the moon moves in its orbit around the earth. The land is not absolutely rigid; therefore, there is a small land tide that follows the same cycle as the water tides.

The sun contributes to the tides, but only about half as much as the moon because of its greater distance. When the sun and moon are in line, as at new or full moon, maximum tides occur, called spring tides. In between, at first and third quarter moon, the solar and lunar tides partly cancel each other to produce the smallest high tides, called neap tides.

The mean diameter of the moon is 2,160 mi., less than the width of the U.S. With gravity only 1/6 that of the earth, the moon retains virtually no atmosphere and no surface water. There is no weather--that is, no wind, rain, or snow--and no weather-induced erosion; surface features on the moon are therefore more nearly permanent than on Earth.

The most striking surface features are the dark round basins, about 200 to 600 mi. in diameter, originally thought to be seas. These imaginatively named maria (Latin for "seas") offer the naked eye a pattern at full moon that forms the face of the "man in the moon." Most mountain ranges are at or near the periphery of the maria; their lofty peaks rise as high as 25,000 ft.

More numerous than the maria are the circular craters, ranging in size from a few feet to more than 100 mi. Some of the large craters have conspicuous light-colored streaks, called rays, radiating from them for hundreds of miles. It appears that these craters were formed when the lunar surface was struck by large solid bodies; ejected material then produced the rays and surrounding secondary craters. There are also a smaller number of volcanically produced craters.

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