The Moon is Earth's only natural satellite and the fifth-largest planetary satellite in the solar system. The moon is the brightest object in the night sky after the Sun, but gives off no light of its own. Instead, it reflects light from the sun. Like Earth and the rest of the solar system, the moon is about 4.6 billion years old, and is the only celestial body to which humans have traveled and upon which humans have performed a manned landing, during NASA's Apollo program.
The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. However, the Earth and Moon are still commonly considered a planet-satellite system, rather than a double-planet system.
Rotation, Movement & Orbit
The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary line that connects its poles. The moon also orbits Earth. Different amounts of the moon's lighted side become visible in phases because of the moon's orbit around Earth. During events called eclipses, the moon is positioned in line with Earth and the sun. A slight motion called libration enables us to see about 59 percent of the moon's surface at different times.
The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast, Earth takes only 24 hours for one rotation. The moon's axis of rotation, like that of Earth, is tilted. But the tilt of the moon's axis is only about 1.5 degrees, so the moon has no seasons. Another result of the smallness of the moon's tilt is that certain large peaks near the poles are always in sunlight. In addition, the floors of some craters, particularly near the south pole, are always in shadow.
As the moon orbits Earth, an observer on Earth can see the moon appear to change shape. It seems to change from a crescent to a circle and back again. The shape looks different from one day to the next because the observer sees different parts of the moon's sunlit surface as the moon orbits Earth. The different appearances are known as the phases of the moon. The moon goes through a complete cycle of phases in a synodic month.
The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last quarter. When the moon is between the sun and Earth, its sunlit side is turned away from Earth. Astronomers call this darkened phase a new moon. The next night after a new moon, a thin crescent of light appears along the moon's eastern edge. The remaining portion of the moon that faces Earth is faintly visible because of earthshine, sunlight reflected from Earth to the moon. As the moon changes from new moon to full moon, and more and more of it becomes visible, it is said to be waxing. As it changes from full moon to new moon, and less and less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is called crescent. When it looks larger than a half moon, but is not yet a full moon, it is called gibbous.
Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A lunar eclipse occurs when Earth gets directly, or almost directly, between the sun and the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a full moon. A solar eclipse occurs when the moon gets directly, or almost directly, between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can occur only during a new moon.
The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The Moon is divided into two types of terrain. There are large, dark plains called maria and also brighter, undulating, heavily cratered highland regions. Maria are found almost exclusively on the near side of Moon. The term comes from the smoothness of the dark areas and their resemblance to bodies of water. The maria are cratered landscapes that were partly flooded by lava when volcanoes erupted. The lava then froze, forming rock. Since that time, meteoroid impacts have created craters in the maria.
The lighter-colored regions of the Moon are rugged, cratered highlands known as terrae, or just highlands, since they are higher than most maria. The highlands are the original crust of the moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets. Many craters in the terrae exceed 25 miles in diameter. The largest is the South Pole-Aitken Basin, which is 1,550 miles in diameter. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins.
The vast majority of the moon's craters are formed by the impact of meteoroids, asteroids, and comets. Craters on the moon are named for famous scientists. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer. There are about half a million craters with diameters greater than 1 km on the moon. The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.
The shape of craters varies with their size. Small craters with diameters of less than 6 miles have relatively simple bowl shapes. Slightly larger craters cannot maintain a bowl shape because the crater wall is too steep. Material falls inward from the wall to the floor. As a result, the walls become scalloped and the floor becomes flat.
Surrounding the craters is rough, mountainous material, crushed and broken rocks that were ripped out of the crater cavity by shock pressure. This material is called the crater ejecta blanket. Farther out are patches of debris and, in many cases, irregular secondary craters, also known as secondaries. Those craters come in a range of shapes and sizes, and they are often clustered in groups or aligned in rows.
Crater rays are light, wispy deposits of powder that can extend thousands of miles or kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes the powder into the upper surface layer.
Craters larger than about 120 miles across tend to have central mountains. Some of them also have inner rings of peaks, in addition to the central peak. The appearance of a ring signals the next major transition in crater shape -- from crater to basin. Basins are craters that are 190 miles or more across. The smaller basins have only a single inner ring of peaks, but the larger ones typically have multiple concentric rings.