Lunar Phases and Eclipses

Many centuries ago ancient astronomers recognized that the Moon goes through phases because it reflects sunlight as it revolves about the Earth. Pythagoras even argued that the Moon had to be a sphere, based on his observation that the region between illumination and darkness, the terminator, on the Moon’s face was curved rather than a straight line.

Figure 7.1 is drawn with the parallel rays of the distant Sun coming from the right. For an observer on the Earth, the Sun is just setting. The outer circles indicate the observed positions and phases of the Moon around sunset through one cycle of phases. Notice that near new Moon the Moon sets around sunset while at full Moon the Moon rises at sunset. The inner circles show the Moon seen by an observer in space looking back at the Moon as it revolves about the Earth. Viewed from space the Moon does not go through phases and is always half illuminated.

The Moon’s true period of revolution about the Earth is known as the sidereal period and is 27.3 days. The time between two new moons, called the synodic period, is 29.5 days.

Figure 7.1 Lunar Phases

If the angular separation between two celestial objects is small and they therefore appear close together in the sky, they are said to be in conjunction. Figure 7.2 shows conjunctions between the full Moon and Earth’s shadow and between the new Moon and Sun. In the first instance a lunar eclipse may occur and in the second, a solar eclipse may occur.

Figure 7.2 Conjunctions

When the Moon is in a full phase, it sometimes passes through the Earth’s shadow. When this happens, a lunar eclipse occurs (see Figure 7.3).

Figure 7.3  Lunar Eclipse Geometry

But although the Moon goes through the full phase once each month there is not an eclipse each month. Generally, the Moon passes either above or below the shadow. Hence, the Moon being in a full phase is a necessary but not a sufficient condition for an eclipse.

The other required condition is, of course, that the Moon must pass through the Earth’s shadow, which can happen only if the Moon is in or close to the Earth’s orbital plane. This plane, called the ecliptic, derives its name from the requirement that the Moon be near the plane for an eclipse to occur. The reason that the Moon is not always in the ecliptic is that the Moon’s orbital plane is inclined to the ecliptic plane by about 5 degrees.

The Earth’s shadow has two parts. The inner, darker region is called the umbra and the outer, lighter region is called the penumbra. Neither the umbra nor the penumbra is completely dark because the Earth’s atmosphere scatters sunlight into the shadow. During an eclipse the Moon darkens and becomes redder, but does not completely disappear.

Figure 7.4 shows the possible types of lunar eclipses. When the Moon is in the penumbra, only a very small darkening occurs, so penumbral eclipses are difficult to detect with the naked eye. It is partial and total umbral eclipses that are most easily seen.

Figure 7.4 Types of Lunar Eclipses

As the Moon revolves about the Earth, it sometimes passes between the Earth and the Sun. When this happens a solar eclipse results. As you can see from the Figure 7.5, a necessary condition for a solar eclipse is that the Moon be in a new phase.

Figure 7.5 Solar Eclipse Geometry

But although the Moon goes through the new phase once each month, there is not a solar eclipse each month.

Usually at new Moon, the Moon passes either above or below the Sun, but for a total solar eclipse to occur the Moon must pass directly between the Earth and Sun. This can happen only if the Moon is in the ecliptic plane. The reason that the Moon is not always in the ecliptic is that the Moon’s orbital plane is inclined to the ecliptic plane by about 5 degrees.

Just as the Earth’s shadow has two parts, so does the shadow of the Moon. The inner, darker region is called the umbra and the outer, lighter region is called the penumbra.

Figure 7.6 shows the possible types and duration of solar eclipses. Notice, that the Moon’s shadow covers a relatively small area on the Earth’s surface. Only those few people who are in the shadow region will actually see an eclipse. Individuals in the penumbra will see a partial eclipse while those in the umbra will view a total eclipse. The Moon’s shadow sweeps across the Earth’s surface from west to east, and an eclipse is seen at different times by people at different locations. The path traveled by the Moon’s umbra is known as the eclipse path.

Figure 7.6 Eclipse Types and Duration

Because the Moon revolves about the Earth in an elliptical orbit, its distance from the Earth varies. When the Moon is close to the Earth, its apparent size is larger than when it is far from the Earth. If an eclipse occurs when the Moon is close, its apparent size will be somewhat larger than that of the Sun, the eclipse path will be wider, and the duration of the eclipse will be longer (Figure 7.6, A). The maximum possible duration of totality is around eight minutes. The reverse is true when the Moon is far from the Earth (Figure 7.6, B). From time to time, the Moon can be so far away that its apparent size is less than the Sun’s. When this happens the Moon does not completely cover up the Sun, and a ring (annulus) of sunlight remains visible around the Moon. This type of eclipse is called an annular eclipse (Figure 7.6, C).

The outermost part of the Sun’s atmosphere, the corona, is a very faint, tenuous gas that is difficult to observe with ground-based telescopes except during a total solar eclipse. For this reason astronomers have historically gone to great lengths and great distances to observe solar eclipses. Now that orbiting instruments have made it possible to produce artificial eclipses with some ease, the scientific importance of natural eclipses is not quite as great. However, the esthetic aspects of solar eclipses still make them one of the most striking and exciting of astronomical events.