we call it a total eclipse. In a total eclipse, the darkest part of the Moon's shadow, the umbra strikes the Earth. Otherwise it is a partial eclipse, and the lighter part of the shadow, or penumbra, passes over the Earth. When the Earth is between the Moon and the Sun, it can block the sunlight reaching the Moon. This is called an eclipse of the Moon or a lunar eclipse. There are also total and partial lunar eclipses, depending on whether the Moon passes through the Earth's umbra or penumbra. The relative placements of the Earth, Sun and Moon for both of these are shown in Fig. 22.13.

You might think that there would be a lunar eclipse at each full moon and a solar eclipse at each new moon. If the Moon's orbit were in the same plane as the Earth's orbit, this would be the case. However, the Moon's orbit is tilted by about 5° relative to the plane of the Earth's orbit. This is also shown in Fig. 22.13. Therefore, during most new moons, the Moon's shadow passes above or



below the Earth, and during most full moons the Moon passes above or below the Earth's shadow. We can only have an eclipse when the new or full moon occurs while the Moon is crossing the plane of the Earth's orbit, the ecliptic. The plane of the Earth's orbit intersects that of the Moon's orbit in a straight line, called the line of nodes. We can only have an eclipse when the Moon is close to a node. Also, we can only have an eclipse when the Moon is full or new.

This favorable arrangement occurs two times a year. These are called eclipse seasons. Each season is approximately 38 days long. Each new or full moon during the eclipse seasons results in at least a partial eclipse. The eclipse season is sufficiently long that any total eclipse of one object (Moon or Sun) must mean a partial eclipse of the other, either two weeks before or after. The direction of the plane of the Moon's orbit shifts around, going through a full cycle every 18.6 years. The amount of tilt doesn't change, but the direction of the tilt does. This results in an eclipse year that is actually 346.6 days long. Therefore, there may be up to seven (lunar plus solar) eclipses in a calendar year, of which two to five will be solar.

In a total lunar eclipse, the whole Moon passes through the darkest part of the Earth's shadow, the umbra. In some eclipses, the Moon only passes through the lighter (outer) part of the Earth's shadow, the penumbra. Penumbral eclipses are hardly noticeable. If the Moon is partly in the umbra, we see a partial eclipse. The Moon is never completely dark, even for a total lunar eclipse. Some sunlight is refracted (bent) by the Earth's atmosphere and illuminates the Moon. Since the atmosphere filters out the blue light better than the red, the Moon appears red. Particles in the Earth's atmosphere sometimes block sunlight more than at other times, and different eclipses have different amounts of light reaching the Moon. Eclipses just after major volcanic eruptions are particularly dark.

Lunar eclipses can be seen from any point on the nighttime side of the Earth. They are currently of limited scientific value. They used to provide astronomers with information on the thermal properties of the lunar soil. Astronomers could use radio and infrared observations to see how fast that soil cooled when the sunlight was

I Eclipse geometry. Since the Moon's orbit is inclined by 5°, whether or not there is an eclipse depends on whether the Earth is in a part of its orbit where the new and full moon are in the ecliptic. (a) The Moon is out of the ecliptic at the new and full moon and no eclipse takes place. (b) The Moon is in the ecliptic at new and full moon and eclipses can take place.The times of year when eclipses are possible are called eclipse seasons.


removed. However, we now have samples of lunar material in the laboratory. As the Earth's shadow passes across the Moon, its shape serves as a reminder of the Earth's roundness.

Solar eclipses are of continuing scientific value, as was discussed in Chapter 6, when we talked about the Sun. As viewed from the Earth, the Sun and Moon cover almost exactly the same angle on the sky. This is just a coincidence. Since the Moon's orbit is elliptical, it is sometimes closer and sometimes farther from Earth. This means that usually the Moon covers a slightly larger region than the Sun, but occasionally the Moon is directly in line, but doesn't cover the Sun completely. A ring (or annulus) of the Sun shines around the edge of the Moon, so these are called annular eclipses.

When a total solar eclipse does occur, it is quite spectacular. First the Moon begins to cover the Sun slowly, creating a partial eclipse that engulfs more and more of the Sun. During any stage of a partial eclipse, there is still enough sunlight present to damage your eyes. You should not look directly at the partial stages of a solar eclipse. The easiest way to follow the progress is to use a small telescope to project an image of the Sun on a screen. Special solar filters can also be used, but sunglasses, or exposed film, do not filter out the harmful radiation. During totality it is safe to look at the Sun. With the bright disk of the Sun blocked, you can see the outer atmosphere, or corona, of the Sun. During totality, the sky becomes almost as dark as at night, and you can briefly see stars.

Since the Moon barely covers the Sun, at any instant, a total eclipse can only be seen over a small section of the Earth. As the Moon's shadow moves across the Earth, it traces out a thin band in which a total eclipse can be seen. Along the band, totality progresses from one end to the other. Since solar eclipses are only visible in limited areas, we generally have to plan to travel to see them. Therefore, it is important to know when and where you can see one.

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