An eclipse occurs when a celestial body becomes totally or partly darkened. The sun is eclipsed when the moon comes between it and the earth. The moon is eclipsed when it passes into the shadow cast by the earth.
By a fortunate coincidence, unique in the solar system, the moon appears to be the same size as the sun, so it can completely obscure the sun when it passes directly between the earth > and the sun. There are three types of solar eclipses: total, partial, and annular eclipses. Which of these is seen depends on the position of the earth (and of the observer on earth) in relation to the moon’s shadow.
The moon’s orbit is elliptical, hence its apparent size varies as its distance from earth changes. Similarly, the apparent size of the sun varies as a result of the earth’s elliptical orbit. A total eclipse is possible only when the apparent sizes of the sun and moon are such that the moon is large enough to completely obscure the bright disk of the sun (called the photosphere). But the area of the main shadow cone (the umbra) cast by the moon is relatively small, so a total eclipse is visible from only a limited area of the earth. As the moon orbits the earth and the earth rotates on its axis, the umbra passes across the earth, sweeping out a narrow band (the belt of totality) from which the total eclipse is visible. The belt of totality is widest (up to a maximum of 170 miles (274 kilometers) across when the moon is closest to earth and the sun is at aphelion (its greatest distance from earth).
On either side of the belt of totality is a second region, from which a partial eclipse can be seen. This area is the zone swept out by the penumbra of the moon’s shadow and is considerably larger than the belt of totality. A partial eclipse can also occur without a total eclipse. This happens when the sun, moon, and earth are not exactly in line, so that only the moon’s penumbra passes across the earth, the umbra missing our planet entirely.
An annular eclipse occurs when the earth lies beyond the tip of the moon’s umbra. In this situation, the moon is too small to completely obscure the sun’s photosphere, which therefore appears as a bright ring around the moon.
Several impressive phenomena can be seen during solar eclipses. During totality (which lasts for a maximum of eight minutes, and is usually much shorter), the glare of the photosphere is concealed and the sun’s atmosphere—the corona, which is normally invisible—appears as a spectacular halo. Just before totality, the thin crescent of the photosphere suddenly becomes broken up by the irregular surface features at the “edge” of the moon, thereby making the sun’s crescent look like a series of bright beads—a phenomenon called Baily’s beads. And, as the sun slowly reappears from behind the moon, there is a brief, dramatic flare of light from one side— the “diamond ring” effect.
Lunar eclipses occur when the earth passes directly between the sun and the moon. The length of the earth’s umbra! shadow cone is more than three times the average distance between the moon and the earth, so the shadow is relatively wide at the point where the moon crosses it. As a result, lunar eclipses last a comparatively long time (totality may last up to a maximum of 1 hour 40 minutes).
Three types of lunar eclipses are possible, depending on the orientation of the moon’s orbit relative to the earth’s shadow. A total lunar eclipse occurs when all of the moon passes through the umbra (it also passes through the penumbra both before and after totality). In a partial eclipse, the entire moon passes through the penumbra, but only part of it passes through the umbra. And in a penumbral eclipse, the moon passes through only the penumbra.
The moon is not invisible during an eclipse, even during totality, because the earth’s atmosphere refracts light from the sun into the umbra and on to the lunar surface. Atmospheric conditions on earth affect the amount and color of the refracted light, but generally, the moon is clearly visible and is a red-brown color during totality.
Occurrence of eclipses
Solar eclipses are more common than lunar eclipses, there being at least two solar eclipses every year (and as many as five in an exceptional year). In contrast, there are usually between zero and three lunar eclipses (most years have two) each year. At a given location, however, lunar eclipses are more frequent because every lunar eclipse is visible from the entire hemisphere of the earth facing the moon, whereas solar eclipses can be seen from only a very limited area. From any one point, a solar eclipse is visible only about once every 360 years.
Eclipses can occur only at new moon (for solar eclipses) and full moon (for lunar eclipses), but they do not occur every month because the moon’s orbit is inclined to the ecliptic (the plane of the earth’s orbit). Thus, seen from the earth, the moon usually passes above or below the sun (which precludes a solar eclipse) and similarly, passes above or below the earth’s shadow (thereby precluding a lunar eclipse). Only when the moon is in the plane of the ecliptic, therefore, are eclipses possible.
It has been known since the time of the Babylonians that eclipses of the same type recur at an interval of 18 years 10 days (or 11,depending on the number of leap years) 8 hours. This is called the Saros cycle and can be used to predict both solar and lunar eclipses. The periodicity occurs because the sun, moon, and earth return to almost the same relative positions after every Saros period. But because the positions are not identical, every sequence of eclipse types ends after about 1,300 years, and a new sequence begins; at any one time, however, there are about 40 overlapping sequences. Because the Saros cycle is not an exact number of days, the successive eclipses in a sequence are not visible from the same place on earth.