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Essential Science for Teachers: Earth and Space Science

Our Nearest Neighbor: The Moon Our Nearest Neighbor: The Moon | A Closer Look

A Closer Look

Look for the following topics in the video, indicated by the onscreen icon, and click below to learn more.


Moon Motions

Features of the Moon

Angular Size


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?

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.

A Closer Look: Features of the Moon

How can we describe the geology of the Moon?

The face of the Moon — its “near side” to us — is divided into light areas called the lunar highlands and darker areas called maria. The maria are lower in altitude than the highlands and their appearance comes from the dark lava flows from earlier periods of lunar volcanism. Both the maria and the highlands have many craters, which are the result of meteor impacts. The impact destroys the meteor and displaces part of the moon’s surface. A bowl is created, with the edges higher than the surrounding surface, the interior much lower than the surrounding surface, and sometimes a raised bump in the center. During impact, rock is often ejected outward, and the material is scattered across the Moon’s surface in streaks radiating from the crater. There are many more impact craters in the highlands than in the maria.

Satellite pictures show that the far side of the Moon — the side that we can’t see — is almost completely covered by craters with virtually no maria. Why might this be? Scientists believe that the near side of the Moon has been “shielded” by its interaction with the Earth. The far side is thought to have been impacted much more frequently by meteors.

What do we know about the Moon’s geological history?

Scientists believe that when the Moon formed and its surface cooled, its interior was still mostly liquid. The energy from continual bombardment by meteors kept the Moon’s interior liquefied. When very large meteors struck the Moon, the Moon’s surface would crack. The lava from the Moon’s molten mantle was able to escape through the holes created by the meteor. The lava filled in the crater, creating the dark, smooth maria. When more lava escaped than could be contained in the crater, it would overflow, and cover more of the Moon’s surface. Because there are more craters in the highlands than in the maria, the highlands are thought to be significantly older than the maria.

Are there other landforms on the Moon?

Unlike the Earth, the Moon does not have active erosion by water or wind. Similarly, there is no shifting of tectonic plates to create mountains or subduct existing features into a mantle. Because of this, there are few landforms compared to those on Earth. However, the accumulation of volcanic processes and impact cratering is readily visible, and one can learn a lot about the Moon’s history by studying its surface.

A Closer Look: Angular Size

Kathy Price’s class uses angular size to measure
the relative size of the Moon.

Unlike most sciences, astronomers are generally unable to travel or to directly interact with the objects that they study, so it can be difficult to determine the distance to those sources. Among other tools, astronomers use geometry to estimate these distances.

Objects in the sky can be described by their angular size, which is how big they look, or their physical size, which is how big they are. How big an object looks depends on both how big it is and how far away it is. For instance, when looking at the Moon in the sky, it has an angular diameter of half a degree. This angular size is determined by the physical size of the Moon, and the distance to the Moon. If the Moon were twice as far away, it would look half as big, and if the Moon were twice as large, it would look twice as big. Once astronomers know an object’s physical size — which they can determine through a variety of techniques — they can easily measure its angular size. Those two measurements allow them to determine how far away an object is. For instance, in this video, the students learn that the Moon appears half as wide as their pinky when held at arm’s length. They also learned that when standing on the Moon, Earth appears as wide as two fingers held at arm’s length. Using this knowledge, plus knowing the diameter of the Earth, they can calculate the physical size of the Moon and the distance between Earth and the Moon.

Astronomers can use parallax to determine the distance to objects in the sky. As Earth orbits the Sun, we observe nearby stars from slightly different angles. By measuring how much the position of a star appears to shift (the parallax angle), astronomers can determine the distance to that star. To understand this, close one eye and hold a finger in front of your face. Now, alternate which eye is open and which eye is closed, and you will see that it appears as if your finger is changing position. As you move your finger closer to your face, the amount of change becomes larger. Just as you can estimate the distance to your hand by the amount by which your finger appears to move, astronomers can estimate the distance to objects in the sky by measuring the apparent change in position of the objects they study during different parts of Earth’s orbit.

A Closer Look: Anorthosite


Anorthosite is a kind of igneous rock that is composed mostly of the mineral plagioclase. Plagioclase is made of relatively lightweight elements like silicon, calcium, and aluminum. As the Moon was forming, it was heated not only from within, but by the continual bombardment of asteroids and planetary bodies. The outer several hundred kilometers of the lunar mantle was molten, yielding a magma ocean that began to cool and separate. When the magma started to crystallize, the light plagioclase crystals floated up toward the surface, while denser minerals sank to form the dense interior of the moon. Anorthosite formed the early lunar crust, of which the highland regions are made.

The Earth has much less anorthosite than the Moon, although it is found in some of the Earth’s oldest rock formations. When the Earth was first forming, anorthosite was probably produced the same way it was on the Moon. The Earth, however, is much bigger, and its crust was more dynamic. Because of plate tectonics, which continually destroy and create the Earth’s surface, most of the anorthosite that existed has probably been processed into the Earth ’s interior.

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Essential Science for Teachers: Earth and Space Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2004.
  • Closed Captioning
  • ISBN: 1-57680-742-8