Moon

This article is about Earth's moon. For moons in general, see Natural satellite.

Moon

A full moon as seen from Earth's northern hemisphere
Designations
Adjective	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)
Equatorial radius	1,738.14 km  (0.273 Earths)
Polar radius	1,735.97 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
Equatorial 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)

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 moon. (Other planets also have moons or "natural satellites.") The moon can be seen from Earth. The moon is actually very large, about a 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. It means that you will be 6 times lighter on the moon than on Earth. The moon is a rocky and dusty place.

Phases

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 gives out no light. People only see the parts that are reflecting light from the Sun. These different stages are called Lunar Phases. 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

Because the moon is tidally locked (always faces the same way) towards Earth, half of the moon cannot be seen from Earth. This 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, some of the big countries in the world, the United States and Russia (which used to be part of the USSR) sent robots to the moon. These robots would orbit, crawl, or in the case of Luna 2, crash into the moon. The robots were the first things that humans made that touched the Moon.

Humans finally landed on the Moon on 21 July, 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 to touch the Moon as he said, "That's one small step for a man, one giant leap for mankind." Famously, the "a" was lost on the way to Earth, and found many years after. Later, Buzz Aldrin hopped out of the Eagle, and together, they put the American flag near the spot where they landed. Even though their footprints were left on the moon a long time ago, it is likely that they are still there. If you leave footprints on Earth, wind will blow the dirt around, filling in the footprints and making them disappear. Rain could also smooth out the dirt. But on the moon there is no wind or rain or even air. Because of that, 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.

Characteristics

Because it's smaller in size, the Moon has less gravity than Earth (only 1/6 of the amount on Earth). So if a person weighs 60 pounds on Earth, the person would only weigh 10 pounds on the moon. But even though the moon's gravity is weaker than the Earth's gravity, it is still there. If person threw a ball while standing on the moon, it would still fall back down. However, it would fall much more slowly. And if a person jumped up as high as possible on the moon, he would jump higher than on Earth, but still fall back to the ground. A person could jump and do lots of tricks on the moon and not hurt himself at all, even if he did not land on his feet. That's because the moon's gravity would not pull as hard as the Earth's gravity does.

As was said before, the moon has no rain or wind, so footprints would never wear away naturally.

The Moon also has no atmosphere. Without an atmosphere, people are not protected from heat or cold. Because of this, astronauts have to wear very heavy spacesuits to protect themselves and to 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 big dents called craters. The moon has thousands of these craters.

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.

Water on the Moon

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

Other pages

• Late Heavy Bombardment

Other Websites

• The Dark Side of the Moon