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Moon Moon symbol
Full moon
A full moon as seen from Earth's northern hemisphere
Adjective lunar, selenic
Shortest distance from what it orbits around 363,104 km  (0.002 4 AU)
Longest distance from what it orbits around 405,696 km  (0.002 7 AU)
Longest distance from the center of its orbital path
("semi-major axis")
384,399 km  (0.002 57 AU[1])
How egg-shaped its orbit is
0.054 9[1]
How long it takes to complete an orbit 27.321 582 d  (27 d 7 h 43.1 min[1])
How long an orbit seems to take
(from the central body)
29.530 589 d  (29 d 12 h 44 min 2.9 s)
Average speed 1.022 km/s
Angle above the reference plane
5.145° to the ecliptic[1]
(between 18.29° and 28.58° to Earth's equator)
Longitude of where it comes up through the reference plane regressing by one revolution in 18.6 years
Angle between its shortest distance from what it orbits around and where it comes up through the reference plane
("argument of periapsis")
progressing by one revolution in 8.85 years
What it orbits Earth
Size and Other Qualities
Average distance from its center to its surface 1,737.10 km  (0.273 Earths)[1][2]
Distance from its center to its surface at its equator 1,738.14 km  (0.273 Earths)[2]
Distance from its center to its surface at the poles 1,735.97 km  (0.273 Earths)[2]
Flatness at the poles 0.001 25
Distance around its equator 10,921 km (equatorial)
Area of its surface 3.793 × 107 km²  (0.074 Earths)
Volume inside it 2.195 8 × 1010 k  (0.020 Earths)
Mass 7.347 7 × 1022 kg  (0.012 3 Earths[1])
Average density 3,346.4 kg/m³[1]
Gravity at its surface 1.622 m/s² (0.165 4 g)
Slowest speed able to escape into space
("escape velocity")
2.38 km/s
How long it takes to turn around one time
(in relation to the stars)
27.321 582 d (synchronous)
Turning speed 4.627 m/s
Angle at which it turns
(in relation to its orbit)
1.542 4° (to ecliptic)
6.687° (to orbit plane)
How much light it reflects 0.12
Surface temp. Min. Avg. Max.
equator 100 K 220 K
85°N[5] 70 K 130 K 230 K
Seeming brightness
("apparent magnitude")
−2.5 to −12.9[3]
−12.74 (mean full moon)[2]
Seeming size
("angular diameter")
29.3 to 34.1 arcminutes[2][4]
Pressure 2.25  × 10-12 torr [6]
The Moon as seen from Earth. This a nearly full moon

The Moon (Latin: luna) is what people generally say when talking about Earth's largest satellite. (Other planets also have moons or "natural satellites.") The Moon can be seen from Earth. This moon is about quarter the size of the earth, but because it is far away it looks very small. The gravity on the moon is one-sixth of the Earth's gravity.[7] It means that something will be six times lighter on the Moon than on Earth. The Moon is a rocky and dusty place. The Moon drifts away from Earth at the rate of four centimeters per year.[8]


The Moon is lit up by the sun as it goes around (orbits) the Earth. This means sometimes people on Earth can see the whole Moon and other times only small parts of it. This is because the Moon does not emit light. People only see the parts that are reflecting light from the Sun. These different stages are called Phases of the Moon. It takes the Moon about 29.53 days (29 days, 12 hours, 44 minutes) to complete the cycle, from big and bright to small and dim and back to big and bright. As the Moon passes between the Earth and Sun, this phase is called the New Moon. The next phase of the moon is called the "waxing crescent", followed by the "first quarter", "waxing gibbous", then to a full moon. A full Moon occurs when the moon and sun are on opposite sides of the Earth. As the Moon continues its orbit it becomes a "waning gibbous", "third quarter", "waning crescent", and finally back to a new moon. People used the moon to measure time. A month is approximately equal in time to a lunar cycle.

The phases of the Moon

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 dark side of the Moon even though the sun does shine on it—we just never see it lit.

History of exploring the Moon

Buzz Aldrin standing on the moon in 1969

Before people stood on the Moon, the United States and the USSR sent robots to the Moon. These robots would orbit the Moon or crawl on its surface. The robots were the first man-made objects to touch the Moon.

Humans finally landed on the Moon on 21 July, 1969.[9] 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, it is likely that they are still there, as there is no wind or rain, making erosion extremely slow. The footprints do not get filled in or smoothed out.

More people landed on the moon between 1969 and 1972, when the last spaceship, Apollo 17 visited. Eugene Cernan of Apollo 17 was the last person to touch the moon.


Because it is smaller, the Moon has less gravity than Earth (only 1/6 of the amount on Earth). So if a person weighs 120kg on Earth, the person would only weigh 20kg on the moon. But even though the Moon's gravity is weaker than the Earth's gravity, it is still there. If 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. A person could also fall on the Moon and not feel as much pain, because the Moon's gravity does not pull as hard as the Earth's gravity does.[Note 1]

The Moon also has no atmosphere. Without an atmosphere, the environment is not protected from heat or cold. Because of this, astronauts have to wear very heavy spacesuits for protection and carry oxygen to breathe. Of course, since the Moon does not have a lot of gravity, the spacesuit would not be as heavy as it would be on Earth, which is very heavy.

In the Earth, the sky is blue because the blue rays of the sun bounce off the gases in the atmosphere, making it look like blue light is coming from the sky. But on the moon, because there is no atmosphere, the sky looks black, even in the daytime. And because there is no atmosphere to protect the moon from the rocks that fall from outer space, they crash right into the moon and make wide, shallow holes called craters. The moon has thousands of them. Newer craters gradually wear away the older ones.

Origin of the Moon

The giant impact hypothesis is that the Moon was created out of the debris from a collision between the young Earth and a Mars-sized protoplanet. This is the favored scientific hypothesis for the formation of the Moon.[10]

Water on the Moon

On November 13, 2009, NASA said that they had found a lot of water on the moon.[11]

Other pages


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Wieczorek, M.; et al. (2006). "The constitution and structure of the lunar interior". Reviews in Mineralogy and Geochemistry 60: 221–364. doi:10.2138/rmg.2006.60.3 .
  2. 2.0 2.1 2.2 2.3 2.4 Williams, Dr. David R. (February 2, 2006). "Moon Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-31.
  3. The maximum value is given based on scaling of the brightness from the value of -12.74 given for an equator to Moon-centre distance of 378 000 km 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 6 378 km, giving 350 600 km. The minimum value (for a distant new moon) is based on a similar scaling using the maximum Earth-Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of the earthshine onto such a new moon. The brightness of the earthshine is [ Earth albedo × (Earth radius / Radius of Moon's orbit)² ] relative to the direct solar illumination that occurs for a full moon. ({{{1}}}; {{{1}}} radius × equatorial {{{1}}}).
  4. 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 378 000 km, the angular size is 1896 arcseconds. The same fact sheet gives extreme Earth-Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km.
  5. A.R. Vasavada, D.A. Paige, and S.E. Wood (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus 141: 179. doi:10.1006/icar.1999.6175 .
  6. "Encyclopædia Britannica". Encyclopædia Britannica Online. September 10, 2008. Retrieved July 14, 2009.
  7. The acceleration due to gravity on the Moon is 1.62 m/s2. This is approximately 1/6 of the acceleration due to gravity on Earth, which is 9.81 m/s2.
  8. "Interesting Facts about the Moon". 16 August 2013.
  9. "1969: Man takes first steps on the Moon". BBC. Retrieved 2009-09-05.
  10. Belbruno, E.; J. Richard Gott III (2005). "Where did the Moon come from?". The Astronomical Journal 129 (3): 1724–1745. doi:10.1086/427539 .
  11. "NASA: 'lots of water' on the moon". Toronto Star. Retrieved 2011-07-21.


  1. Because the Moon has no atmosphere, the friction with air does not slow down a thrown ball or a person jumping.

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