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.
- Where does the name "Moon" come from? (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)
- How long is twilight on the Moon? (Beginner)
- Does the Moon rotate? (Beginner)
- Is the Moon hollow? (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)
- Can you fire a gun on the Moon? (Intermediate)
- What happens to a bullet fired on the Moon? (Intermediate)
- Can moons have moons? (Intermediate)
- Can we prove that the Moon isn't hollow? (Intermediate)
- Is the Moon spherical? (Intermediate)
- Why is the Moon so bright? (Beginner)
- What is that ring (or rainbow) around the moon? (Beginner)
- Are there telescopes that can see the flag and lunar rover on 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)
- The Moon and Sun are the same apparent size in the sky. The Moon is moving away from us at 3.8 centimeters/year. Isn't it improbable that we live in an epoch in which they appear the same size in the sky? (Beginner)
- Does the direction of the moon's crescent change through the night? (Beginner)
- Does the Moon look different in the northern and southern hemispheres? (Beginner)
- Why is the Moon higher in the sky in winter and lower in the summer? (Beginner)
- 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)
- How does the position of Moonrise and Moonset change? (Intermediate)
- When can I see the Moon through the hole in the Pantheon? (Intermediate)
- Is the Moon always visible during winter on the North Pole? (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)
- A line drawn perpendicular to a line through the tips of the horns of the crescent moon doesn't point to the Sun! Why not? (Advanced)
- Are Earth and the Moon "twin planets"? (Beginner)
- How much does the Earth "wobble" on its orbit due to the Moon's gravity? (Intermediate)
- What will happen to Earth's tides as the moon moves away from us? (Intermediate)
- Why are there high tides during a Full Moon? (Intermediate)
- Did the Moon or planets form in a manner similar to that of Earth? (Beginner)
- Are there solar tides? (Beginner)
- Does lunar gardening really work? (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)
- Is the gravitational force exerted by the Earth on the Moon equal to the centripetal force acting on the Moon? (Intermediate)
- The Moon slows the Earth's rotation, but how fast was it spinning billions of years ago? (Intermediate)
- What happens to the Earth as the moon gets farther away and will it ever go away entirely? (Intermediate)
- Why does the Earth only see one side of the Moon? (Intermediate)
- Why are there both high and low tides? (Intermediate)
- Why does the Earth have only one moon? (Intermediate)
- Is the Moon moving away from the Earth? When was this discovered? (Intermediate)
- What would happen if Earth had more than one moon? (Intermediate)
- What would happen if the Moon fell out of its orbit around Earth? (Intermediate)
- What is Earthrise over the Moon? (Intermediate)
- Have astronomers discovered Earth's second moon? (Intermediate)
- Does Mt. Everest cast a shadow on the moon? (Intermediate)
- Does the Mediterranean sea have tides? (Intermediate)
- How close was the Moon to the Earth when it formed? (Intermediate)
- Is Earth-moon tidal friction causing global warming? (Intermediate)
- How do we know the mass of the Earth and the Moon? (Advanced)
- Can other terrestrial moons be (or have been) stable? (Advanced)
- What is the three-body problem? (Advanced)
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.
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