14-04-2007 Eclipses (Article 81)

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Simon Kenny
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14-04-2007 Eclipses (Article 81)

Post by Simon Kenny » Wed Feb 04, 2009 10:53 am

Eclipses (Article 81)

Without doubt one of the most spectacular astronomical events is an eclipse. An eclipse (which derives from the Greek work ekeipo “to vanish”) occurs when one celestial object moves into the shadow of another.
The term is most commonly used to describe either a solar eclipse (when the Moon’s shadow crosses the Earth's surface), or a lunar eclipse, (when the Moon moves into the shadow of Earth). However, an eclipse also refers to events beyond our limited Earth-Moon system. In the vast infinity that is the universe eclipses occur in a wide range of ways. For example, a planet moving into the shadow cast by one of its moons, or a moon passing into the shadow cast by its parent planet are valid eclipses.
An eclipse involving the Sun, Earth and Moon can only occur when they are in a line. Because the plane of the orbit of the Moon is tilted with respect to the plane of the orbit of the Earth (the ecliptic), eclipses occur only when the three bodies are near the intersection (the node) of these planes. There can be up to seven eclipses in a calendar year.
The most dramatic eclipses visible from Earth are the “Lunar Eclipses” when the Earth obscures the Sun, from the Moon’s point of view and the “Solar eclipses” — when the Moon eclipses the Sun.
Eclipses can be divided into different types: Total eclipses, in which the light source is totally blocked off by the eclipsing body. The total phase can have a duration of up to 7 min 31s. Partial eclipses, in which only part of the luminary is covered (solar eclipses), or when only part of a body is eclipsed by the shadow (lunar eclipses). Annular eclipses, are a total eclipse of luminary where a thin ring of light is visible around the intervening object. It is pure coincidence that the Moon and Sun have nearly equal apparent sizes, making annular eclipses possible.
Before modern astronomy there were long-standing explanations for eclipses in many cultures. Up to not so long ago, the disappearance of the sun during the day proved a terrifying experience and was interpreted as a terrible omen. Indeed many battles through history were abandoned due to the sudden disappearance of the sun from the sky!

Thankfully due to excellent work of astronomers through the ages we can now fully understand and appreciate this marvellous celestial dance. Clear Skies!

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