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Every 29 ½ days the Moon goes through all its phases, from new to full and then back again to new. It is no wonder that ancient peoples used the dependable phases of the Moon as a basis for their calendar. By merely looking at the Moon, they were able to determine within a day or two the time of the month. The cycle of lunar phases continues steadily, year after year with one major exception – the lunar eclipse. During a lunar eclipse, the face of a full Moon darkens and returns to full brightness in a matter of hours.
The sudden darkening of the lunar surface during an eclipse is quite different than the gradually monthly darkening as the Moon goes from its full phase to a new Moon and back again. The rays of light coming from the Sun light up usually precisely half of the Moon's surface. Because of the different positions of the Moon, we see a different part of the Moon lit up from night to night. A lunar eclipse occurs when the shadow created by the Earth falls onto the Moon. An eclipse of the Moon only occurs during a full Moon, during which time you will recall that the Earth is placed between the Sun and the Moon. Our Earth, like any object placed before a strong source of light, creates a shadow. The shadow consists of two parts, the penumbra and umbra. The penumbra is the area of space in which only some of the Sun's rays reach. If you were sitting in the penumbra, you would see some but not all of the Sun. This area does not interest us much because the small difference of light reaching the Moon when it is within the penumbra is not easily detectable. The other part of the shadow is the umbra. No light reaches umbra. If you were looking towards the direction of the Sun from a spot on the Moon within the umbra of the Earth’s shadow, you would be unable to see the Sun at all and you would be in total darkness. Whenever the umbra intersects with the Moon, we see the Moon’s surface darken, as the surface of any object appears dark when it is in the shade, and an umbra eclipse of the Moon occurs.
The Earth's shadow always exists and the Moon passes around the Earth once a month; why is there not an eclipse of the Moon every month? If the Earth, Moon and Sun were all in the same plane, the Moon would pass exactly through the center of the Earth's shadow and we would see a monthly lunar eclipse. But the Moon is not exactly in the same plane as the Sun and Earth. Its path has a tilt of five degrees. Every month the Moon twice crosses the Sun-Earth plane, first in one direction and then two weeks later, in the opposite direction. When this crossing over takes place during a full Moon, there is a lunar eclipse. At that moment, the Sun, Earth and Moon are in a straight line. During a regular full Moon, the Earth’s shadow exists and reaches out to some point above or below the Moon as viewed from Earth.
The fortunate circumstances to produce an umbra eclipse occur from zero to three times a year. Unlike the movement of the stars which is unchanging (almost) from year to year, there is no simple formula for predicting eclipses. The "perfect" eclipse is called a central lunar eclipse in which the Moon passes through the exact center of the Earth’s shadow. This type of eclipse lasts four hours. The Moon’s surface steadily darkens during the first hour as the Earth's shadow moves across its surface. It remains dark for another two hours and during the fourth hour the shadow gradually passes from the Moon’s surface. The Moon is not completely black even when it is fully within the umbra of the Earth’s shadow. Nor does it have the glow produced by earthshine. Instead, the Moon’s surface during an eclipse has a reddish tint. This color is produced by the light rays coming form the Sun, skirting through the Earth's atmosphere and then bouncing off the Moon. The Earth’s atmosphere does two things. First, it scatters the blue component of the Sun’s light as part of the same phenomenon that gives us blue skies. Second, the atmosphere bends the light rays so that they can intersect with the Moon. Since the blue rays of light are scattered into space, the red rays reach the Moon's surface and give it a red appearance.