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Lunar Eclipse

This model illustrates the nature of lunar eclipses.

During a total lunar eclipse, the Moon completely disappears in the Earth's umbra. The phase of a total lunar eclipse lasts much longer than the phase of a solar eclipse. The shape of the umbra during lunar eclipses served as strong evidence of the Earth's spherical shape for the ancient Greek philosopher and scholar Aristotle. Ancient Greeks have established that the Earth is approximately three times larger than the Moon, basing their calculations on the periods of eclipses (the precise value of the Earth to Moon size coefficient is actually 3.66).

During a lunar eclipse, the Moon is deprived of sunlight. Consequently, a total lunar eclipse can be observed from any point of the Earth's hemisphere. The eclipse begins and ends simultaneously for all geographical locations. (Note that the local times will still be different.) The left edge of the Moon will be the first to enter the Earth's umbra, since the Moon is moving eastward.

The eclipse may be of total or partial nature, depending on whether the Moon completely penetrates the umbra or travels in the vicinity of the umbra's edge. The closer to the lunar mode the lunar eclipse occurs, the larger will be its phase. Penumbra eclipses take place when the lunar disk encounters the penumbra rather than umbra. Penumbra eclipses cannot be observed by the naked eye.

During a lunar eclipse, the Earth's shadow covers the Moon. It would seem that the Moon should disappear out of sight completely each time an eclipse occurs, since the Earth is obviously non-transparent. However, as the Earth's atmosphere disperses solar rays that are falling on the shadow-covered lunar surface and bypass the Earth. This accounts for the fact that the lunar disk appears reddish during the eclipses: red and orange rays are the best at passing through the atmosphere.

Lunar eclipses occur several times a year.

Press "Run" button to activate the model. "Stop" and "Reset" buttons suspend the animation, and return the model to its initial state. Observe the time that has elapsed since the beginning of the eclipse in the model's bottom-left side.

© OpenTeach Software, 2007