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The Moon

The moon is the closest natural body and the single natural satellite of the earth. The orbit of the moon around the earth is not circular but elliptical. Thus, the distance of the moon from the earth varies from a maximum distance of 406,685 kilometers to a minimum of 365,410 kilometers. In one day, the moon moves about 12 degrees along its orbit. The moon completes one revolution of the earth in 27.3 days, a period known as a sidereal month.

 A  The moon rotates slowly on its axis, making one complete rotation in a period of time exactly equal to its orbit around the earth.  B  Thus, the moon keeps the same hemisphere or face turned toward the earth at all times. We do not, however, always see only half of the moon’s surface from the earth.  C  The eccentricity of the moon’s orbit allows us to see additional lunar surface through irregular movements called lvibrations, which expose an extra 18 percent of the moon’s surface at one time or another. D 

In 1969, the first humans landed on the moon’s surface in the Sea of Tranquility. Subsequent lunar landings were on the Ocean of Storms and the Sea of Serenity. Despite these watery names, the astronauts had to cope with an environment devoid of water. The dark areas on the moon’s surface are called seas and oceans because early observers assumed the moon was much like the earth. We now know that the seas are dark because they are volcanic basalt flows, mostly of iron silicate. The brighter parts, the mountains, consist of igneous deposits of aluminum and calcium silicates.

Like the earth, the moon has no light of its own; its daylight side deflects the light of the sun. The moon goes through phases, apparent changes in its shape, because it orbits the earth in nearly the same plane as the earth orbits the sun. The eight phases of the moon arise from its changing position in relation to the earth, so the moon cannot be seen. A few nights later, a thin crescent hangs in the evening twilight. At this time, the dark side of the moon is faintly visible because it is illuminated by earthshine, the light of the sun reflected from the earth to the moon, then back again.

The second phase is a waxing crescent moon, followed by the third phase, when the moon forms a right angle with the earth-sun line, and a half moon appears at sunset. During the fourth phase, the moon is more than half but less than fully illuminated, known as a waxing gibbous moon. The waxing gibbous moon is followed by a full moon (fifth phase0, which occurs when the sun, earth, and moon are in opposition, or roughly aligned. At full moon, the rising disk of the moon appears to balance the setting sun in the evening sky. When the moon is just past full, a lunar twilight—seen as a glow in the eastern sky—will precede moonrise.

After the full moon, the moon begins to wane, through a waning gibbous moon (sixth phase), a waning half moon (seventh phase), and a waning crescent moon (eighth phase). Toward the end of the eighth phase, a thin crescent appears at morning twilight, again accompanied by earthshine. Finally, the cycle ends and another begins with a dark moon: another new moon. The lunar cycle takes 29.5 days to complete—a period known as a synodic month or the moon’s synodic period.

At its full phase, the moon’s intensity is about one millionth that of the sun, and it is possible to read a newspaper by the light of the moon. The full moon nearest the autumnal equinox in September is called the Harvest Moon. The Harvest Moon ushers in a period of several successive days when the moon rises in the northeast soon after sunset. This phenomenon gives farmers in temperate latitudes extra hours of light in which to harvest their crops before frost and winter come. The full moon following the Harvest Moon is called the Hunter’s Moon and is accompanied by a similar but less market phenomenon of early moonrise.