TheMoon, also known asLuna, isEarth's onlynatural satellite (the only object whichorbits 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 thewidth of Earth.[7] Because it is so far away it looks small in the sky, about half adegree wide.
Thegravity on the Moon is one-sixth of the Earth's gravity.[8] It means that an object will be one-sixth as heavy on the Moon compared to Earth.
The Moon is arocky and dusty place. It moves slowly away from the Earth at a rate of 3.8centimeters per year due to the effect oftidal dissipation.
Some other planets also have moons or natural satellites. Thegiant impact hypothesis is a common explanation for how the Moon formed.
Recent research has challenged this hypothesis. According to one article:[10]
[N]ew findings suggest that the Moon was primarily created from material ejected from Earth’smantle, with minimal contribution from [the proto-planet].
Because it is smaller, the Moon has only 1/6 of thegravity on Earth. So if a person weighs 60kilograms on Earth, the person would only weigh 10kilograms on the Moon.[nb 1]
Even though the Moon's gravity is weaker than the Earth's gravity, it is still there. If a person dropped aball while standing on the Moon, it would still fall down. However, it would fallmuch 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.
Without an atmosphere, the Moon's environment is not protected from heat or cold.Astronauts worespacesuits to survive and carriedoxygen 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 theirgloves or spacesuits off on the Moon.
On the Earth, thesky isblue because the Sun's bluerays bounce off thegases in theatmosphere. This makes it look likebluelight 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 thedaytime.
Earth's atmosphere causes mostmeteorites (space rocks) to burn up. The Moon does not have this protection. Meteorites crash into it and create wide, shallow holes calledcraters. The Moon has thousands of them. Newer craters overlie older ones.
In 2009Chandrayaan-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 ofhydrates andhydroxides. Liquid water cannot exist on the Moon becausephotodissociation quickly breaks down themolecules. However, based on the data received from Chandrayaan-1, liquid surface water may have once existed on the Moon.
Because the Moon is round, half of it is lit up by theSun. 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 thesunlight. These different stages are calledPhases of the Moon.
It takes the Moon about a month (29 days, 12hours, and 44minutes) to complete thecycle, 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:
The "new Moon" (when the Moon passes between the Earth and Sun)
The "waxing crescent"
The "first quarter"
The "waxing gibbous"
A "full Moon" (when the Moon and Sun are on opposite sides of the Earth)
The "waning gibbous"
The "third quarter"
The "waning crescent",
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.[12] A full set of moon phases is called a lunation.[13]
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).[14] 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%).[15]
The Moon always shows the same side to Earth.Astronomers call this phenomenontidal locking. This means that half of it can never be seen from Earth. The side facing away from Earth is called the far side ordark side of the Moon even though the Sun does shine on it—we just never see it lit.
Before people stood on the Moon, theUnited States and theUSSR sentrobots 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.
Humans finally landed on the Moon on July 21, 1969.[16]AstronautsNeil Armstrong andBuzz Aldrin landed their lunar ship (theEagle) on the surface of the Moon. Then, as half the world watched him on television, Armstrong climbed down the ladder of theEagle 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 theirfootprints were left on the Moon a long time ago, they could remain there for millions of years.[17] There is nowind orrain on the Moon, soerosion happens extremely slowly.[17] This means the footprints do not get filled in or smoothed out like they would on Earth.
During theCold War, theUnited States Army thought about making a military base on the Moon so they could attack targets on Earth. They also considered testing anuclear weapon on the Moon.[19] TheUnited States Air Force had similar plans.[20][21] However, both plans were brushed off asNASA 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.[22] The U.S., the Soviet Union, and many other countries signed theOuter Space Treaty,[23] which calls the Moon and all ofouter 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.
123Williams, Dr. David R. (February 2, 2006)."Moon Fact Sheet". NASA (National Space Science Data Center). Archived fromthe original on May 21, 2020. RetrievedDecember 31, 2008.
↑Themaximum value is given based on scaling of the brightness from the value of -12.74 given for an equator to Moon-centre distance of 378000km in the NASA factsheet reference to the minimum Earth-Moon distance given there, after the latter is corrected for the Earth's equatorial radius of 6378km, giving 350600km. Theminimum value (for a distantnew moon) is based on a similar scaling using the maximum Earth-Moon distance of 407000km (given in the factsheet) and by calculating the brightness of theearthshine onto such a new moon. The brightness of the earthshine is[ Earthalbedo ×(Earth radius / Radius ofMoon's orbit)²] relative to the direct solar illumination that occurs for a full moon. ({{{1}}};{{{1}}} radius× equatorial{{{1}}}).
↑The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378000km, theangular size is 1896arcseconds. The same fact sheet gives extreme Earth-Moon distances of 407000km and 357000km. For the maximum angular size, the minimum distance has to be corrected for the Earth's equatorial radius of 6378km, giving 350600km.
↑Kilograms are technically a unit ofmass, but they are often used as a unit ofweight. If a person goes to the Moon, their mass will not change, but their weight will change.
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