# Moon

Moon | The Moon as seen from Earth. This is a nearly full moon |
| Designations |
| Alternative names | Luna |
| Adjectives | lunar, selenic |
| Orbital characteristics |
| Perigee | 363,104 km  (0.002 4 AU) |
| Apogee | 405,696 km  (0.002 7 AU) |
| Semi-major axis | 384,399 km  (0.002 57 AU) |
| Eccentricity | 0.054 9 |
| Orbital period | 27.321 582 d  (27 d 7 h 43.1 min) |
| Synodic period | 29.530 589 d  (29 d 12 h 44 min 2.9 s) |
| Average orbital speed | 1.022 km/s |
| Inclination | 5.145° to the ecliptic (between 18.29° and 28.58° to Earth's equator) |
| Longitude of ascending node | regressing by one revolution in 18.6 years |
| Argument of perigee | progressing by one revolution in 8.85 years |
| Satellite of | Earth |
| Physical characteristics |
| Mean radius | 1,737.10 km  (0.273 Earths) |
| Flattening | 0.001 25 |
| Circumference | 10,921 km (equatorial) |
| Surface area | 3.793 × 107 km²  (0.074 Earths) |
| Volume | 2.195 8 × 1010 km³  (0.020 Earths) |
| Mass | 7.347 7 × 1022 kg  (0.012 3 Earths) |
| Mean density | 3,346.4 kg/m³ |
| Surface gravity | 1.622 m/s² (0.165 4 g) |
| Escape velocity | 2.38 km/s |
| Sidereal rotation period | 27.321 582 d (synchronous) |
| Equatorial rotation velocity | 4.627 m/s |
| Axial tilt | 1.542 4° (to ecliptic) 6.687° (to orbit plane) |
| Albedo | 0.12 |
|

| Surface temp. | min | mean | max |
| --- | --- | --- | --- |
| equator | 100 K | 220 K |  |
| 85°N | 70 K | 130 K | 230 K |

 |
| Apparent magnitude | −2.5 to −12.9 −12.74 (mean full moon) |
| Angular diameter | 29.3 to 34.1 arcminutes |
| Surface pressure | 2.25  × 10-12 torr  |
|  |

The **Moon**, also known as **Luna**, is Earth's only natural satellite (the only object which orbits the Earth and is not man-made). It is usually visible in the night sky, but is sometimes seen during the day.

The Moon is about one-fourth of the width of Earth. Because it is so far away it looks small in the sky, about half a degree wide.

The gravity on the Moon is one-sixth of the Earth's gravity. It means that an object will be one-sixth as heavy on the Moon compared to Earth.

The Moon is a rocky and dusty place. It moves slowly away from the Earth at a rate of 3.8 centimeters per year due to the effect of tidal dissipation.

Some other planets also have moons or natural satellites. The giant impact hypothesis is a common explanation for how the Moon formed.

## Origin

The giant impact hypothesis is the most common theory among scientists to explain how the Moon formed. According to this hypothesis, Earth collided with a Mars-sized proto-planet around 4.5 billion years ago, and the debris from the collision created the Moon.

Recent research has challenged this hypothesis. According to one article:

> [N]ew findings suggest that the Moon was primarily created from material ejected from Earth’s mantle, with minimal contribution from [the proto-planet].
>
>

## Characteristics

### Gravity

Because it is smaller, the Moon has only 1/6 of the gravity on Earth. So if a person weighs 60 kilograms on Earth, the person would only weigh 10 kilograms on the Moon.

Even though the Moon's gravity is weaker than the Earth's gravity, it is still there. If a person dropped a ball while standing on the Moon, it would still fall down. However, it would fall *much* more slowly. A person who jumped as high as possible on the Moon would jump higher than on Earth, but still fall back to the ground.

Because the Moon has no atmosphere, there is no air resistance, so a feather will fall as fast as a hammer.

### Atmosphere

#### Heat & cold

Without an atmosphere, the Moon's environment is not protected from heat or cold. Astronauts wore spacesuits to survive and carried oxygen to breathe. These suits weighed about as much as the astronauts (though they were not as heavy as on Earth because of the Moon's weak gravity). The astronauts never took their gloves or spacesuits off on the Moon.

#### Light

On the Earth, the sky is blue because the Sun's blue rays bounce off the gases in the atmosphere. This makes it look like blue light is coming from the sky. On the Moon, there is no atmosphere for light to bounce off. This means the sky looks black, even in the daytime.

#### Craters

Earth's atmosphere causes most meteorites (space rocks) to burn up. The Moon does not have this protection. Meteorites crash into it and create wide, shallow holes called craters. The Moon has thousands of them. Newer craters overlie older ones.

### Water

In 2009 Chandrayaan-1, India's first lunar exploration mission, found a lot of water on the Moon. However, the water is not liquid; it is in the form of hydrates and hydroxides. Liquid water cannot exist on the Moon because photodissociation quickly breaks down the molecules. However, based on the data received from Chandrayaan-1, liquid surface water may have once existed on the Moon.

## Phases

Because the Moon is round, half of it is lit up by the Sun. As it goes around (or orbits) the Earth, sometimes the side that people on Earth can see is all lit brightly. Other times only a small part of the side we see is lit.

This is because the Moon does not send out its own light. People only see the parts that are being lit the eclipse by the sunlight. These different stages are called Phases of the Moon.

It takes the Moon about a month (29 days, 12 hours, and 44 minutes) to complete the cycle, from big and bright to small and dim and back to big and bright. People throughout history have used the Moon's cycles to measure time.

The phases of the Moon are, in order:

1. The "new Moon" (when the Moon passes between the Earth and Sun)
2. The "waxing crescent"
3. The "first quarter"
4. The "waxing gibbous"
5. A "full Moon" (when the Moon and Sun are on opposite sides of the Earth)
6. The "waning gibbous"
7. The "third quarter"
8. The "waning crescent",
9. A new Moon again

Each of the four middle phases of the Moon lasts about seven days on average (around 7.38 days). This time can change by about ±11% because the Moon moves closer to or farther from Earth from its farthest point (apogee) to its closest point (perigee).

The number of days since the last new moon is called the Moon’s age. A full set of moon phases is called a lunation.

To find the Moon’s age and phase for any date, you can count how many days have passed since a known new moon (for example, January 1, 1900, or August 11, 1999). Then divide that number by the average length of a moon month (29.53059 days). The number left over after dividing is the Moon’s age.

This simple method is good for general or fun uses, like moon phase clocks. It is not exact because it does not use the real shape of the Moon’s orbit or the exact time of the new moon, so it can be off by a few hours. The farther you go from the reference date, the less accurate it becomes.

Because the Moon wobbles a little as it orbits Earth (a motion called libration), we can sometimes see a tiny bit more than the full moon (up to about 101%) or a small part of its far side (up to 5%).

The phases of the Moon
### Tidal locking

The Moon always shows the same side to Earth. Astronomers call this phenomenon tidal locking. This means that half of it can never be seen from Earth. The side facing away from Earth is called the far side or **dark side of the Moon** even though the Sun does shine on it—we just never see it lit.

## Exploration

Buzz Aldrin standing on the moon in 1969
### Robots

Before people stood on the Moon, the United States and the USSR sent robots to the Moon. Some of these robots orbited the Moon; others landed on its surface. The robots were the first man-made objects to touch the Moon.

### The first humans

Humans finally landed on the Moon on July 21, 1969. Astronauts Neil Armstrong and Buzz Aldrin landed their lunar ship (the *Eagle*) on the surface of the Moon. Then, as half the world watched him on television, Armstrong climbed down the ladder of the *Eagle* and was the first human to touch the Moon as he said, "That's one small step for a man, one giant leap for mankind."

Even though their footprints were left on the Moon a long time ago, they could remain there for millions of years. There is no wind or rain on the Moon, so erosion happens extremely slowly. This means the footprints do not get filled in or smoothed out like they would on Earth.

### Future landings

Ten more astronauts landed on the moon between 1969 and 1972. All were part of the United States' Apollo program.

The last spaceship to land on the Moon was Apollo 17. Eugene Cernan was the last person to touch its surface.

## Legal status

During the Cold War, the United States Army thought about making a military base on the Moon so they could attack targets on Earth. They also considered testing a nuclear weapon on the Moon. The United States Air Force had similar plans. However, both plans were brushed off as NASA moved from a military agency to a civilian-based one.

Even though the Soviet Union left remains on the Moon, and the United States left a few flags, no country has control over the Moon. The U.S., the Soviet Union, and many other countries signed the Outer Space Treaty, which calls the Moon and all of outer space the "province of all mankind". This treaty also bans all use of the military on the Moon, including nuclear weapons tests and military bases.

## Colonization

Some governments have ideas about Colonization of the Moon. (Related page, Colonization). A U.S. agency has ideas or plans about putting a nuclear reactor, on the Moon, according to media.

## Timekeeping

As of 2025's first quarter, there is no time standard on the Moon. The U.S. government has suggested Coordinated Lunar Time.

## Other information

A lunar night (or night on the moon), is each c. 14 Earth days.

## Related pages

* Late Heavy Bombardment

## More reading

* Iggulden, Hal; Iggulden, Conn (2007). "The Moon". *The Dangerous Book for Boys*. New York: HarperCollins. pp. 167–170. ISBN 978-0061243585.

## Other websites

 Media related to Moon at Wikimedia Commons

* The Dark Side of the Moon Archived May 20, 2011, at the Wayback Machine