The since prehistoric times. It is the second

The Moon is the only natural satellite of Earth: orbit: 384,400 km from Earthdiameter: 3476 km mass: 7.

35e22 kg Called Luna by the Romans, Selene and Artemisby the Greeks, and many other names in other mythologies. The Moon, of course,has been known since prehistoric times. It is the second brightest object in thesky after the Sun. As the Moon orbits around the Earth once per month, the anglebetween the Earth, the Moon and the Sun changes; we see this as the cycle of theMoon’s phases. The time between successive new moons is 29.5 days (709 hours),slightly different from the Moon’s orbital period (measured against the stars)since the Earth moves a significant distance in its orbit around the Sun in thattime. Due to its size and composition, the Moon is sometimes classified as aterrestrial “planet” along with Mercury, Venus, Earth and Mars.

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TheMoon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the onlyextraterrestrial body to have been visited by humans. The first landing was onJuly 20, 1969 (do you remember where you were?); the last was in December 1972.The Moon is also the only body from which samples have been returned to Earth.

In the summer of 1994, the Moon was very extensively mapped by the littlespacecraft Clementine and again in 1999 by Lunar Prospector. The gravitationalforces between the Earth and the Moon cause some interesting effects. The mostobvious is the tides. The Moon’s gravitational attraction is stronger on theside of the Earth nearest to the Moon and weaker on the opposite side. Since theEarth, and particularly the oceans, is not perfectly rigid it is stretched outalong the line toward the Moon. From our perspective on the Earth’s surface wesee two small bulges, one in the direction of the Moon and one directlyopposite.

The effect is much stronger in the ocean water than in the solid crustso the water bulges are higher. And because the Earth rotates much faster thanthe Moon moves in its orbit, the bulges move around the Earth about once a daygiving two high tides per day. But the Earth is not completely fluid, either.The Earth’s rotation carries the Earth’s bulges get slightly ahead of the pointdirectly beneath the Moon.

This means that the force between the Earth and theMoon is not exactly along the line between their centers producing a torque onthe Earth and an accelerating force on the Moon. This causes a net transfer ofrotational energy from the Earth to the Moon, slowing down the Earth’s rotationby about 1.5 milliseconds/century and raising the Moon into a higher orbit byabout 3.8 centimeters per year. (The opposite effect happens to satellites withunusual orbits such as Phobos and Triton). The asymmetric nature of thisgravitational interaction is also responsible for the fact that the Moon rotatessynchronously, i.e. it is locked in phase with its orbit so that the same sideis always facing toward the Earth.

Just as the Earth’s rotation is now beingslowed by the Moon’s influence so in the distant past the Moon’s rotation wasslowed by the action of the Earth, but in that case the effect was muchstronger. When the Moon’s rotation rate was slowed to match its orbital period(such that the bulge always faced toward the Earth) there was no longer anoff-center torque on the Moon and a stable situation was achieved. The samething has happened to most of the other satellites in the solar system.Eventually, the Earth’s rotation will be slowed to match the Moon’s period, too,as is the case with Pluto and Charon. Actually, the Moon appears to wobble a bit(due to its slightly non-circular orbit) so that a few degrees of the far sidecan be seen from time to time, but the majority of the far side (left) wascompletely unknown until the Soviet spacecraft Luna 3 photographed it in 1959.(Note: there is no “dark side” of the Moon; all parts of the Moon getsunlight half the time. Some uses of the term “dark side” in the pastmay have referred to the far side as “dark” in the sense of”unknown” (eg “darkest Africa; but even that meaning is no longervalid today!) The Moon has no atmosphere.

But evidence from Clementine suggestedthat there may be water ice in some deep craters near the


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