Moon

I

Introduction

Moon, name given to the only natural satellite of Earth. The Moon is the second brightest object in Earth’s sky, after the Sun, and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 12 percent of the light that falls on it. This reflectivity, called albedo, of 0.12 is similar to that of coal dust.

The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes. This period is called a lunar month. The Moon rotates once on its axis in the same period of time that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the “near side”) is always turned toward Earth.

II

The Moon Seen from Earth

As the Moon orbits Earth in a counterclockwise direction, Earth itself rotates counterclockwise (from west to east) on its axis and revolves around the Sun in a counterclockwise orbit. All of these motions combined determine when and how the Moon appears in the sky to an observer on Earth. Seen from a single spot on Earth, the Moon rises about 50 minutes later every day. Since the Moon has moved 13.8 degrees further in its orbit in 24 hours, the Earth has to turn an extra 13.8 degrees on its axis for the Moon to rise above the horizon again.

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The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. When it is less than half illuminated, it is said to be in crescent phase. The Moon is said to be waning as it progresses from full to new, and to be waxing as it proceeds from new to full.

The Moon is in the sky about 12 hours a day. At new moon it is in the sky during daylight hours, rising just after dawn. At full moon it is visible throughout the night, rising at sunset. The phases of the Moon match its position in the sky. New moon is noticeable when the Moon is close to the western horizon at sunset. The full moon occurs when the Moon is above the eastern horizon at sunset about 14 days later. The dark phase of the Moon occurs when the Moon is in the daytime sky with its shaded night side facing Earth. Its unseen presence can be revealed in a spectacular way if the dark Moon passes directly in front of the Sun. When this happens, the view of the Sun is blocked and the Moon’s shadow falls on a small region of the surface of the Earth, an event called a solar eclipse.

By a cosmic coincidence, the apparent sizes of the disk of the Moon and the disk of the Sun are approximately the same (within about 0.5 of a degree) when seen from Earth. If the Moon’s orbit lay exactly in the plane of Earth’s orbit around the Sun, a solar eclipse would occur somewhere on Earth every month at new moon. However, the Moon’s orbit is tilted 5.1 degrees with respect to the plane of Earth’s orbit around the Sun. As a result, solar eclipses occur only about 2 to 5 times a year. Partial eclipses, when the Moon only partially covers the disk of the Sun, happen more often than total eclipses.

Another type of eclipse results when Earth comes directly between the Sun and the Moon. Lunar eclipses happen at full moon about twice a year and are visible over large areas of Earth. The round shadow of Earth passes over the Moon, giving it a red or copper hue from sunlight filtered through Earth’s atmosphere.

At any one time, an observer on Earth can see only 50 percent of the Moon’s entire surface. However, an additional 9 percent can be seen from time to time around the edges because the viewing angle from Earth changes slightly as the Moon moves through its elliptical orbit. This slight relative motion is called libration.

III

Surface of the Moon

Early observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge.

The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Micrometeorites also slowly erode surface features and alter the lunar soil. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Small amounts of gas from deep in the Moon may still reach the surface. Scientists have also recently discovered possible evidence of ice in permanently shadowed areas of the surface. Such ice could have come from comet impacts.

A

Craters

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full Moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

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