5. The Moon: stepping stone to the planets
Once or twice in a typical year, the Moon's orbital motion carries it through the Earth's shadow. This is an eclipse of the Moon, and it can be seen from half of the Earth. There are two regions in the Earth's shadow at the time of a lunar eclipse: the umbral region where there is no direct sunlight, and the penumbral region where the Sun's light is partially shadowed. The umbral shadow is darker, and it is in the shape of a narrow cone pointing away from the Earth. The full Moon turns a deep red when in the umbral shadow of the Earth.
The outer atmosphere of the Sun, known as the corona, becomes momentarily visible to the unaided eye when the Sun's visible disk is bloked out by the Moon and it becomes dark during the day. The corona is then seen at the limb, or apparent edge, of the Sun, against the blackened sky as a faint, shimmering halo of pearl-white light. But be careful if you go watch an eclipse, for the light of the corona is still very hazardous to human eyes and should not be viewed directly. The million-degree corona can be seen all across the Sun's disk, and at any time, when viewing the Sun in X-rays with telescopes aboard satellites such as Yohkoh.
Since the Moon and the Earth move along different orbits whose planes are inclined to each other, a total eclipse of the Sun does not happen very often. The Moon only passes between the Earth and the Sun about three times every decade on average. Even then, a total eclipse occurs along a relatively narrow region of the Earth's surface, where the tip of the Moon's shadow touches the Earth. At other nearby places on the Earth, the Sun will be partially eclipsed, and at more remote locations you cannot see any eclipse of the Sun.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University