The Moon

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The Moon: the closest astronomical object to our planet, the only natural satellite of Earth, and the only other astronomical object that has been visited by people - the Apollo 11 astronauts first landed on the moon July 20, 1969.

The Galileo spacecraft sent back this image of the Moon as it headed into the outer solar system. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. (Credit: Galileo Mission, JPL and NASA)Credit: Galileo Mission, JPL and NASA

Full Moon. The Galileo spacecraft sent back this image of the Moon as it headed into the outer solar system. The distinct bright ray crater at the bottom of the image is the Tycho impact basin.
The Moon was formed around 4.6 billion years ago, at the same time that the earth was forming. It has a diameter of 3,476 kilometers and is an average distance of 384,400 kilometers from the Earth - that's 1.3 light seconds. It takes 27.322 days to complete one orbit around the Earth, and its mass is 1.23% of the Earth's mass.

The Moon has a small iron-rich core, but is composed mostly of rock. Its heavily cratered surface was caused by the bombardment of asteroids when the solar system was young, about 500-700 million years after its formation. Volcanic activity that continued until approximately 2 billion years ago is responsible for the basalt lava floes that flooded the surface, cooled, and solidified into level plains. These plains are known as "seas" though they contain no water. Not only does the Moon lack water, it also has no permanent atmosphere. The pull of gravity at the surface of the Moon is only 1/6 as strong as gravity's pull at the Earth's surface. This force is too weak to permanently retain a blanket of gas around the Moon. There is, however, a very light temporary atmosphere of sodium, and potassium, which is constantly refurnished by the solar wind.

This image of the Moon was taken by the Galileo spacecraft as it passed by. It is a composite of images taken in three different colors. The color scheme is exaggerated to emphasise composition differences. Blue areas are titanium rich, orange areas are titanium poor and purple areas are iron poor. (Credit: Galileo Mission, JPL and NASA via Astronomy Picture of the Day)Credit: Galileo Mission, JPL and NASA via Astronomy Picture of the Day

The Colorful Moon. This image of the Moon was taken by the Galileo spacecraft as it passed by. It is a composite of images taken in three different colors. The color scheme is exaggerated to emphasise composition differences. Blue areas are titanium rich, orange areas are titanium poor and purple areas are iron poor.

The tidal forces between the Earth and the Moon slowed the Moon's rotation until it became locked with its orbit around the Earth. The Moon therefore always keeps the same side of its surface facing the Earth. As the Moon orbits the Earth, the entire half facing the sun is illuminated, but the amount of this lighted half that we can see changes. Thus the Moon seems to change shape or phase. The far side of the Moon is lighted part of the time and dark part of the time as well; hence it is incorrect to refer to it as the "dark side of the Moon" (sorry, Pink Floyd).

Apollo 16 metric camera image the eastern limb and far side of the Moon. The lower left part of the image shows a portion of the moon visible from Earth. The dark area at the 8:00 position on the edge is Mare Crisium. To the right of that is Mare Smythii. The upper right area shows the heavily cratered lunar far side. The Moon is 3475 km in diameter and North is at 10:30 in this image. (Credit: Apollo 16 Crew, NASA)Credit: Apollo 16 Crew, NASA

The Far Side of the Moon. Apollo 16 metric camera image of the Moon's eastern limb and far side. The lower left part of the image shows a portion of the moon visible from Earth. The dark area at the 8:00 position on the edge is Mare Crisium. To the right of that is Mare Smythii. The upper right area shows the heavily cratered lunar far side. The Moon is 3475 km in diameter and North is at 10:30 in this image.
Other effects of the lunar tides are to slow the rotation of the Earth and to increase the size of the lunar orbit. The Moon therefore formed much closer to the Earth than its current position. It is now believed that the Moon was created by re-accretion of fragments resulting from a collision between the Earth and one body the size of Mars, in the last phases of the formation of the Solar System. That could explain why the average density of the Moon is closer to the density of the Earth's mantle.

The new moon is the position when the Earth, the Sun, and the Moon are roughly aligned. New moons are invisible to us on Earth. The first quarter is when half of the visible side is illuminated, which occurs about a week after the new moon. Between the time the Moon is new and full, it is said to be waxing. During the time between the quarter moons and the full moon, when we see the Moon as rounded on both sides, it is gibbous. About a week after the full moon is the last quarter face, when the other half of the visible side is illuminated. As the phase changes from full back to new, the moon is waning. Often when the Moon is rising or setting, it appears larger than at times when it is high in the sky. This apparent increase in size is an optical illusion.

Questions About The Moon

  • General Questions
  • The Moon Landings
  • Observing the Moon
  • Lunar Eclipses
  • Geology
  • The Moon and the Earth
  • The Ask an Astronomer team's favorite links about The Moon:

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