2.3 The Moon’s Appearance Changes as It Orbits Earth

The Changing Phases of the Moon

The Moon and its changing aspects have been the frequent subject of mythology, art, literature, and music. In mythology, the Moon was the Greek goddess Artemis, the Roman goddess Diana, and the Inuit god Igaluk. We speak of the “man in the Moon,” the “harvest Moon,” and sometimes a “blue Moon.”

Unlike the Sun, the Moon has no light source of its own; instead, the Moon shines by reflected sunlight. As the Moon orbits Earth, our view of the illuminated portion of the Moon constantly changes. Those different appearances of the Moon are called phases. During a new Moon, when the Moon is between Earth and the Sun, the side facing away from us is illuminated, and during a full Moon, when Earth is between the Sun and the Moon, the side facing toward us is illuminated. The rest of the time, we can see only part of the illuminated portion from Earth. Sometimes the Moon appears as a circular disk in the sky. Other times it is nothing more than a thin sliver, or its face appears dark.

To help you visualize the changing phases of the Moon, use a sphere (such as an orange or a softball), a lamp, and your head (Figure 2.19). Your head is Earth, the sphere is the Moon, and the lamp is the Sun. Turn off all the other lights in the room, and step back as far from the lamp as you can. Hold up the sphere slightly above your head so that the lamp illuminates it from one side. Move the sphere clockwise around your head and watch how the appearance of the sphere changes. When you are between the sphere and the lamp, the face of the sphere that is toward you is fully illuminated. The sphere appears to be a bright, circular disk. As the sphere moves around its circle, you will see a progression of lighted shapes, depending on how much of the bright side and how much of the dark side of the sphere you can see. That progression of shapes mimics the changing phases of the Moon.

Figure 2.20 shows the changing phases of the Moon, from two perspectives. The orbit is shown as viewed from a point above Earth’s North Pole; the outer circle of Moon images then shows how the Moon appears from Earth at that phase. The new Moon occurs when the Moon is between Earth and the Sun. The far side of the Moon is illuminated, but the near side is in darkness and we cannot see it. At new phase, the Moon is close to the Sun in the sky, so it is up in the daytime with the Sun: it rises in the east at sunrise, crosses the meridian near noon, and sets in the west near sunset. A new Moon is never above the horizon in the nighttime sky.

AstroTour: The Moon’s Orbit: Eclipses and Phases

A few days after a new Moon, the Moon has moved farther along its orbit of Earth. At this point, a sliver of its illuminated half, called a waxing crescent Moon, becomes visible. Waxing here means “growing in size and brilliance”; the name refers to the fact that the Moon appears to be “filling out” from night to night at that time. From our perspective, the Moon has also moved away from the Sun in the sky. Because the Moon travels around Earth in the same direction in which Earth rotates, we now see the Moon following the Sun, so the Moon is east of the Sun in the sky. A waxing crescent Moon is visible in the western sky in the evening, near the setting Sun but remaining above the horizon after the Sun sets. The “horns” of a crescent Moon always point directly away from the Sun.

As the Moon moves even farther along in its orbit, the angle between the Sun and Moon grows larger, so more and more of the Moon’s near side becomes illuminated. About a week after the new Moon, half of the near side of the Moon is illuminated and half is in darkness. This phase is called a first quarter Moon because the Moon has moved a quarter of the way around Earth and has completed the first quarter of its cycle from new Moon to new Moon (Figure 2.20). The first quarter Moon rises at noon, crosses the meridian at sunset, and sets at midnight.

As the Moon moves beyond first quarter, more than half of its near side is illuminated. This phase is called a waxing gibbous Moon, from the Latin gibbus, meaning “hump.” The waxing gibbous Moon continues nightly to “grow” until finally we see the entire near side of the Moon illuminated—a full Moon. Earth is now between the Sun and the Moon, so they appear opposite each other in the sky when viewed from Earth. As a result, the full Moon rises as the Sun sets, crosses the meridian at midnight, and sets in the morning as the Sun rises.

what if . . .

What if the Moon were a cube rather than a sphere? How would this change the appearance of the Moon’s phases?

The second half of the Moon’s orbit is the reverse of the first half. The Moon continues in its orbit, again appearing gibbous but now becoming smaller each night. This phase is called a waning gibbous Moon, where waning means “becoming smaller.” When the Moon is waning, the left side—as viewed from the Northern Hemisphere—appears illuminated. A third quarter Moon occurs when half of the near side is illuminated by sunlight and half is in darkness. A third quarter Moon rises at midnight, crosses the meridian near sunrise, and sets at noon. The cycle continues with a waning crescent Moon in the morning sky, until the new Moon once again rises and sets with the Sun, and the cycle begins again. Notice that when the Moon is farther from the Sun than Earth is, it is in gibbous (or full) phases, whereas when the Moon is closer to the Sun than Earth is, it is in crescent (or new) phases.

Interactive Simulation: Phases of the Moon

You can always tell a waxing Moon from a waning Moon because the illuminated side is always the side facing the Sun. When the Moon is waxing, it appears in the evening sky, so its western side is illuminated. That is the right side as viewed from the Northern Hemisphere. Conversely, when the Moon is waning, it appears in the morning sky, so the eastern side appears bright. That is the left side as viewed from the Northern Hemisphere.

Figure 2.21 illustrates two types of lunar periods. The first one is based on the Moon’s orbit in space, and the second is based on the alignment of the moving Moon, Earth, and Sun. The Moon completes one orbit around Earth in 27.32 days. That sidereal period is how long the Moon takes to return to the same location in its orbit. However, the relationships among Earth, the Moon, and the Sun change as a result of Earth’s orbital motion, so going from one full Moon to the next takes 29.53 days. That cycle is known as the Moon’s synodic period and is the basis for our “month,” because it is what we can easily observe from Earth.

Do not try to memorize all possible combinations of where the Moon is in the sky at what phase and at what time of day. Instead, work on understanding the motion and phases of the Moon, and then use your understanding to figure out the specifics of any given case. To study the phases of the Moon, draw a picture like Figure 2.20, and use it to follow the Moon around its orbit. From your drawing, figure out what phase you would see and where it would appear in the sky at a given time of day. You might also try the simulations described in “Exploration: Phases of the Moon” at the end of the chapter.

The Moon’s Visible Face

Although the Moon’s illumination varies as it orbits, one aspect of the Moon’s appearance does not change: the face that we see. If we were to go outside next week, next month, 20 years from now, or 200 centuries from now, we would still see the same side of the Moon that we see tonight. That is because the Moon rotates on its axis exactly once for each revolution that it makes around Earth.

Imagine walking around a tree while always keeping your face toward the tree. By the time you complete one circle around the tree, your head has turned completely around once. When you were south of the tree, you were facing north; when you were east of the tree, you were facing west; and so on. But someone looking at you from the tree would always see your face. The Moon does the same thing, rotating on its axis once per revolution around Earth, always keeping the same face toward Earth (Figure 2.22). Objects whose revolution and rotation are synchronized (or “in sync”) with each other are said to be in synchronous rotation. It occurs as a result of the gravitational and tidal forces between Earth and the Moon (Section 4.4). In later chapters we discuss other examples of synchronous rotation.

The Moon’s far side, facing away from Earth, is often called the “dark side of the Moon.” In fact, no side of the Moon is always dark. At any given time, half of the Moon is in sunlight and half is in darkness—just as for Earth. The side of the Moon that faces away from Earth, the “far side,” spends just as much time in sunlight as the side of the Moon that faces Earth.

CHECK YOUR UNDERSTANDING 2.3

If you see the Moon rising just as the Sun is setting, what is the phase of the Moon? What is the phase if the Moon is setting at noon?

AnswerAnswer

full moon; third quarter