 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.
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The Moon
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 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.
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).
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.
The Ask an Astronomer team's favorite links about The Moon:
- Wikipedia - The Moon Great information about the moon, as well as related links.
- Google Moon Photographic map of the moon with pan and zoom capability - check out the Apollo landing sites!
- Nine Planets: The Moon Information about the moon from the Nine Planets Solar System Tour.
- NASA's Vision for Space Exploration Latest information on NASA's return to the moon.
- U.S. Naval Observatory Data Services: Easy to use web forms which provide data on the position of the Moon, rise and set times, etc.
- Information about space missions to the Moon.
- Full Moon Names and their meanings from the Farmer's Almanac.
Previously asked questions about The Moon:
General questions:
- Where does the name "Moon" come from? (Beginner)
- What are the names of the earth, moon, sun, and solar system? (Beginner)
- What percent of the time can astronomers observe without interference from the Moon? (Beginner)
- How do we measure the size of the Moon and of the Sun? (Beginner)
- Are Earth and the Moon "twin planets"? (Beginner)
- How long is twilight on the Moon? (Beginner)
- Does the Moon rotate? (Beginner)
- On which days can I see the Moon in the evening / morning and why is this so? (Beginner)
- Where can I find information on the moons of other planets? (Intermediate)
- Is the Moon hollow? (Intermediate)
- Is the Moon moving away from the Earth? When was this discovered? (Intermediate)
- Does the Moon rotate? Are there other moons that always keep one face toward their planet? (Intermediate)
- How does the Moon stay "suspended" in the air? (Intermediate)
- Can I buy land on the Moon? (Intermediate)
- What is Earthrise over the Moon? (Intermediate)
- Can you fire a gun on the Moon? (Intermediate)
- Are there any new craters on the moon? (Intermediate)
- Have astronomers discovered Earth's second moon? (Intermediate)
- What happens to a bullet fired on the moon? (Intermediate)
- Can moons have moons? (Intermediate)
- How close was the Moon to the Earth when it formed? (Intermediate)
- Can we prove that the Moon isn't hollow? (Intermediate)
- Is the moon spherical? (Intermediate)
- How do we know the mass of the Earth and the Moon? (Advanced)
The Moon landings:
- Did astronauts really go to the Moon, or is it a hoax? (Beginner)
- Are there telescopes that can see the flag and lunar rover on the Moon? (Beginner)
- Why hasn't NASA gone back to the moon? (Beginner)
- Who has landed on the moon? (Beginner)
Observing the Moon:
- Why is the Moon so bright? (Beginner)
- What is that ring (or rainbow) around the moon? (Beginner)
- Why is the shadow on the Moon the shape it is? (Beginner)
- Why are the Moon and Sun sometimes orange or red? (Beginner)
- Why is the moon in a different place every night? (Beginner)
- Will the Moon be invisible in 500 million years? (Beginner)
- How can you tell if a crescent moon is preceding or following a new-moon phase? (Beginner)
- How do I explain to my children why the Moon has phases? (Beginner)
- On which days can I see the Moon in the evening / morning and why is this so? (Beginner)
- Why does the Earth only see one side of the Moon? (Intermediate)
- Why does the Moon look big on the horizon? (Intermediate)
- Is it possible to see the full Moon and the Sun simultaneously in high northern latitudes when the Sun doesn't set? (Intermediate)
- How can I find out if the new moon has been sighted? (Intermediate)
- Why do the size and brightness of the full moon change? (Intermediate)
- Are there any new craters on the moon? (Intermediate)
- Does Mt. Everest cast a shadow on the moon? (Intermediate)
- How does the position of Moonrise and Moonset change? (Intermediate)
- When can I see the Moon through the hole in the Pantheon? (Intermediate)
- How does libration allow us to see more than 50% of the Moon? (Advanced)
- Why is the time between two successive full moons different from the lunar synodic month? (Advanced)
- Is the Moon seen as a crescent (and not a "boat") all over the world? Is the same phase of the moon visible from the Northern and Southern hemispheres? (Advanced)
Interaction with the Earth:
- Does lunar gardening really work? (Beginner)
- Are there solar tides? (Beginner)
- What would happen to the Earth if an asteroid hit the Moon? (Intermediate)
- What would happen if we did not have a Moon? (Intermediate)
- Why are there both high and low tides? (Intermediate)
- Why does the Earth have only one moon? (Intermediate)
- What would happen if Earth had more than one moon? (Intermediate)
- Will we ever stop having solar eclipses because of the moon's motion away from the Earth? (Intermediate)
- What would happen if the Moon fell out of its orbit around Earth? (Intermediate)
- Does the Mediterranean sea have tides? (Intermediate)
- How close was the Moon to the Earth when it formed? (Intermediate)
- The Moon slows the Earth's rotation, but how fast was it spinning billions of years ago? (Intermediate)
- Is Earth-moon tidal friction causing global warming? (Intermediate)
- Can other terrestrial moons be (or have been) stable? (Advanced)
Geology:
- Did I find a rock that came from the Moon? (Beginner)
- What kind of rock is the moon made of? (Intermediate)
- Are there any new craters on the moon? (Intermediate)
- Why are there more maria on the near side of the Moon? (Intermediate)
- Can we prove that the Moon isn't hollow? (Intermediate)
How to ask a question:
If you have a question about The Moon which isn't answered above, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.
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