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Lunar Eclipse Overview

Introduction

A lunar eclipse occurs when the Earth comes between the Sun and the Moon and the Earth's shadow passes over the Moon. This phenomenon can be seen by any observer on Earth for whom the Moon is above the horizon. Lunar eclipses occur at the time of a Full Moon, but not every Full Moon, because the Moon has to be near one of the nodes of intersection between its orbit and the ecliptic plane.

Total Lunar Eclipse

The Earth umbra is larger than the whole Moon. So, one will observe either a total eclipse by the umbra (which can be well observed), a partial eclipse by the umbra and penumbra, or a total or partial eclipse by the penumbra only, depending on where the centre of the earth's shadow is with respect to the disk of the Moon. The duration of a lunar eclipse is much longer than a solar eclipse, and can take as much as six hours.

In practice, the lunar eclipse conditions are modified due to the refraction of the Sun's rays by the Earth's atmosphere. This refraction (of 35 minutes of arc) allows some light to penetrate the cone of the geometric umbra. So even during total lunar eclipse, the lunar disk is not completely dark. This grazing light is more absorbed by the Earth's atmosphere in the blue and yellow portions of the spectrum, giving a particular reddish light during total lunar eclipse.


Last Update: 21 September 2006

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