Early astronomers had very practical reasons for studying the sky. Some stars (such as Polaris) served as navigational guides. (Sec. P.1) Others served as primitive calendars to predict planting and harvesting seasons. (Sec. P.2) In a real sense, then, human survival depended on knowledge of the heavens. The ability to predict and even explain astronomical events was undoubtedly a highly prized, and perhaps jealously guarded, skill.

The Moon also played an important role in ancient astronomy. Calendars and religious observances were often tied to its phases and cycles. Even today the calendars of most of the world’s major religions are still based wholly or partly on the lunar orbit. The Moon’s regularly changing appearance (as well as its less regular, but much more spectacular, eclipses) was an integral part of the framework within which ancient astronomers sought to understand the heavens. We will study the Moon’s physical properties in more detail in Chapter 5. Here we complete our inventory of the ancient sky with a brief description of the motion of our nearest neighbor in space.

Lunar Phases

Apart from the Sun, the Moon is by far the brightest object in the sky. Like the Sun, the Moon appears to move relative to the background stars. Unlike the Sun, however, the explanation for this motion is the obvious one—the Moon really does revolve around Earth. The Moon’s appearance undergoes a regular cycle of changes, or phases, taking a little more than 29 days to complete. (The word month is derived from the word Moon.) Figure 1.1 illustrates the appearance of the Moon at different times in this monthly cycle. Starting from the new Moon, which is all but invisible in the sky, the Moon appears to wax (grow) a little each night and is visible as a growing crescent (panel 1 of Figure 1.1). One week after new Moon, half of the lunar disk can be seen (panel 2). This phase is known as a quarter Moon. During the next week, the Moon continues to wax, passing through the gibbous phase (more than half of the lunar disk visible, panel 3) until, two weeks after new Moon, the full Moon (panel 4) is visible. During the next two weeks, the Moon wanes (shrinks), passing in turn through the gibbous, quarter, and crescent phases (panels 5–7), eventually becoming new again.

The location of the Moon in the sky, as seen from Earth, depends on its phase. For example, the full Moon rises in the east as the Sun sets in the west, while the first quarter Moon actually rises at noon, but often only becomes visible late in the day as the Sun’s light fades. By this time the Moon is already high in the sky. Some connections between lunar phase and rising/setting times are indicated on Figure 1.1.

Unlike the Sun and the other stars, the Moon emits no light of its own. Instead, it shines by reflected sunlight, giving rise to the phases we see. As illustrated in Figure 1.1, half of the Moon’s surface is illuminated by the Sun at any instant. However, not all of the Moon’s sunlit face can be seen because of the Moon’s position with respect to Earth and the Sun. When the Moon is full, we see the entire “daylit” face because the Sun and the Moon are in opposite directions from Earth in the sky. In the case of a new Moon, the Moon and the Sun are in almost the same part of the sky, and the sunlit side of the Moon is oriented away from us. At new Moon the Sun is almost behind the Moon, from our perspective. Notice, by the way, that the Moon always keeps the same face toward Earth—as indicated on the figure, it rotates on its axis in exactly the same time it takes to orbit Earth. We will discuss the reason for the Moon’s synchronous rotation in Chapter 5.

As it revolves around Earth, the Moon’s position in the sky changes with respect to the stars. In one sidereal month (27.3 days), the Moon completes one revolution and returns to its starting point on the celestial sphere, having traced out a great circle in the sky. The time required for the Moon to complete a full cycle of phases, one synodic month, is a little longer—about 29.5 days. The synodic month is a little longer than the sidereal month for the same basic reason that a solar day is slightly longer than a sidereal day (Figure P.5): Because of Earth’s motion around the Sun, the Moon must complete slightly more than one full revolution to return to the same phase in its orbit.