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Earth & Space Science: Session 7

A Closer Look: Moon Motions

What’s the difference between the Moon’s rotation and its revolution?

The rotation of a planetary body refers to the length of time it takes it to turn 360¾ on its axis, which is called “one day.” The length of one day on the Moon is 27.3 Earth days — that is, it takes 27.3 of our days for the Moon to rotate once. Revolution refers to the time it takes an orbiting planetary body — called a satellite in the Moon’s case — to orbit its planet. The time it takes the Moon to orbit Earth is also 27.3 days. The length of the lunar day is thus identical to the length of its revolution around Earth. As a result, the same side of the Moon always faces the Earth. Therefore, relative to Earth, the Moon has a "near side" and a "far side."

There is evidence that the length of the lunar day has not always been the same as the duration of the lunar orbit — in fact, for a moon to rotate on its axis with the same frequency as it rotates around its planet is quite rare. When the Moon first formed, scientists believe it rotated more rapidly, so that its day was shorter. Just as the Moon creates tides on Earth by exerting gravitational pull on the water, Earth created “tides” on the Moon, although the tidal forces acted on rock, not water. As the Moon turned on its axis, the tidal bulge closer to Earth was pulled on by the Earth's gravity in the opposite direction of the Moon's rotation — this would be like trying to pull someone who was turning away from you back toward you. Over time, scientists believe that this constant gravitational pull on the Moon's tidal bulge by the Earth's gravity slowed down the Moon's rotation until the length of the lunar day became equal to the length of the Moon's orbit around Earth.

What causes the phases of the Moon?

half moon

Your observations of the Moon will confirm that the Moon has a different appearance from night to night during its orbit around Earth. Why is this so? Because the Moon doesn't make any of its own light, the only light we see from it is reflected from the Sun toward Earth. At any moment during the Moon's 27.3 day orbit around Earth, half of the Moon is lit by the Sun and half is in darkness, just as half of Earth is lit by the Sun at any point during the day. The only part of the Moon we can observe from Earth, however, is its "near side," which is the side of the Moon that faces Earth. The different appearances, or phases, of the Moon occur because, during its orbit, the relative positions of the Moon, Earth, and Sun change, causing the amount of illumination that can be seen from Earth to change.

During a new Moon, for example, we are unable to see the Moon at all. When this happens, the Moon is between Earth and the Sun, and the entire far side of the Moon, which we can’t see, is illuminated by light from the Sun. During these periods, the near side of the Moon dark and the Moon is positioned over the illuminated side of the Earth. During a full Moon, the entire disk of the Moon is visible and the Moon is positioned over the dark side of the Earth, which is why it’s visible at night. When this happens, Earth is between the Sun and the Moon. The rest of the time we are able to see only a portion of the Moon's “face.” Between the new moon and the full moon, some portion of the near side of the Moon is illuminated, and the remaining portion of the near side is dark.

As a new cycle of the Moon begins, the new Moon goes through a crescent phase (less than half of the Moon is illuminated) to the first-quarter stage (one-half of its face is illuminated). The Moon continues through a gibbous phase (more than half of the Moon is illuminated) until the full moon. The second half of the cycle proceeds from full moon through a gibbous phase to the third-quarter stage. Finally, a crescent phase occurs before the next new Moon occurs.

It is important to realize that the phases of the Moon are not in any way caused by the shadow of the Earth.

What causes an eclipse?

There are two types of eclipses: solar and lunar. During a solar eclipse, the Moon's orbit around Earth brings it directly between and on the same plane with the Earth and the Sun. The apparent size of the Moon in the sky is the same as the apparent size of the Sun in the sky, so the Moon is able to completely block the Sun. Because the Sun and Moon have to be on the same side of Earth for a solar eclipse to occur, there are only solar eclipses during a new Moon. Most new moons do not result in solar eclipses, however, because the Moon does not orbit the Earth in the same plane as the Earth orbits the Sun, so the alignment necessary for a solar eclipse doesn't happen often.

During a lunar eclipse, the Moon's orbit around Earth brings Earth directly between the Moon and the Sun. Earth blocks the Sun's light from the Moon, which casts a shadow over it. Because the Sun and the Moon have to be on opposite sides of the Earth for a lunar eclipse to occur, there are only lunar eclipses during a full Moon. Most full moons do not result in a lunar eclipse because the Moon does not orbit Earth in the same plane as Earth orbits the Sun, so the alignment necessary for a lunar eclipse doesn't happen often.

If the orbit of the Moon were on the ecliptic (plane of the orbit of Earth around the Sun), each lunar orbit would result in a solar eclipse during the new Moon and a lunar eclipse during the full Moon. The plane of the Moon's orbit is at a 5° angle to the ecliptic, which usually causes the Moon to pass above or below the Sun as seen from the Earth, without an eclipse taking place.

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