Lunar Eclipse Phenomena

The Moon, when full, may enter the shadow of Earth. The motion of the Moon around Earth is from west to east. For an observer facing south, the shadowing of the Moon begins at its left edge (if the Moon were north of the observer, as, for example, in parts of the Southern Hemisphere, the opposite would be true). If the eclipse is a total one and circumstances are favourable, the Moon will pass through the umbra, the darkest part of the shadow, in about two hours. During this time the Moon is usually not completely dark. A part of the sunlight, especially the redder light, penetrates Earth's atmosphere, is refracted into the shadow cone, and reaches the Moon. Meteorological conditions on Earth strongly affect the amount and colour of light that can penetrate the atmosphere. Generally, the totally eclipsed Moon is clearly visible and has a reddish brown, coppery colour, but the brightness varies strongly from one eclipse to another.

Before the Moon enters the umbra and after it leaves the umbra, it must pass through the penumbra, or partial shadow. When the border between umbra and penumbra is visible on the Moon, the border is seen to be part of a circle, the projection of the circumference of Earth. This is a direct proof of the spherical shape of Earth, a discovery made by the ancient Greeks. Because of Earth's atmosphere, the edge of the umbra is rather diffuse, and the times of contact between the Moon and the umbra cannot be observed accurately.

During the eclipse the surface of the Moon cools at a rate dependent on the constitution of the lunar soil, which is not everywhere the same. Many spots on the Moon sometimes remain brighter than their surroundings during totality—par-ticularly in their output of infrared radiation—possibly because their heat conductivity is less, but the cause is not fully understood. An eclipse of the Moon can be seen under similar conditions at all places on Earth where the Moon is above the horizon.

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