Group B Eclipses

Eclipses, if they are solar or lunar, occur when the Earth, Sun and Moon are in a line. If the Moon is in-between the Earth and the Sun, it blocks the view of the Sun from some parts of the Earth, and this produces a solar eclipse. Or if it is the Earth that is in-between the Sun and Moon, then the earth will block the light from the Sun before it can get to the Moon. Since moonlight is just the light the Moon reflects from the Sun, this will darken the Moon, and we get a lunar eclipse.

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Since the Moon goes around the Earth every 28 days, shouldn't we expect a solar eclipse about every 28 days (when the Moon is new), and a lunar one in the same period, (when the Moon is full) ?

Well, this would be so if the orbit of the Moon were in the same plane as the orbit of the Earth around the Sun. But we know eclipses are rarer than that; and the Moon's orbit is not in the same plane. Instead, it is tilted with respect to it, and the Moon does not in general pass directly on the Earth-Sun line. Moreover, the Moon's orbit tilt varies slowly. To have an eclipse, then, it is not enough that the three bodies be in the right order; the Moon's orbit should also be at the right tilt.

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As with solar eclipses, there are partial and total lunar eclipses. If the Moon does not enter into the umbra, the darkest part of the Earth's shadow, then it does not darken completely, and we get a partial eclipse. These are hard to notice; the Moon just darkens a bit, but does not disappear completely into the night.

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