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An eclipse, also known as an occultation, is the temporary blockage of either sun or moon as seen from earth. Although an occultation occurs any time three celestial bodies move into direct alignment, we commonly use the term "eclipse" to refer only to occultations of the sun and moon with respect to the earth.

Thus an eclipse occurs only when the earth, moon, and sun are in direct alignment. However, not every new and full moon becomes an eclipse. The moon's orbit is at a 5 degree angle relative to that of the earth around the sun; and so the moon usually passes slightly above or below the penumbral cone. Only when the earth is exactly between the sun and the moon does the moon move into the earth's shadow, causing a lunar eclipse. Similarly, only when the moon is exactly between the sun and the earth does part of the earth move into the moon's shadow, causing a solar eclipse. Because of this movement into shadow, a solar eclipse can only occur while the moon is new; while a lunar eclipse can only occur while the moon is full.

There are three types of eclipse: total, partial, and annular. The distinction refers to whether the sun or moon is completely or partly obscured, which in turn depends on which part of the shadow the eclipsed object intersects, and also on the relative size of sun and moon at the time of eclipse. Although the moon is much smaller than the sun, it is also proportionately close to the earth: so that its face usually seems to be roughly the same size as the sun. The shadow itself has two parts: the penumbra, which is the wide cone where some of the sun's rays are blocked; and the umbra, which is the narrow cone where all of the sun's rays are blocked. Only within the umbra is a total eclipse possible.

Total eclipses refer to complete obscurement of the main body of the sun or moon. Total lunar eclipses occur when the moon moves into the earth's umbral shadow: but the moon does not go completely dark. Rather, after the sliver of remaining moon becomes thinner and thinner and finally disappears altogether, the full moon suddenly changes colour to a deep, dark red: radiating the reflected light of the earth. Total solar eclipses occur when the moon moves between the sun and the earth and completely blocks off the central solar disc, leaving only the stunning corona visible. At the moment of entering or breaking totality comes the classic diamond ring effect. As well, "Bailey's beads" may often be seen at the edge of the moon's disc, where the

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