Eclipse
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An eclipse (Greek verb: ekleipô, "to vanish", though it derives from the prefix 'ex-', "away from", and Greek 'leipein', "to leave") is an astronomical event that occurs when one celestial object moves into the shadow of another. The term is most often used to describe either a solar eclipse, when the Moon's shadow crosses Earth's surface, or a lunar eclipse, when the Moon moves into the shadow of Earth. However, it can also refer to such events beyond the Earth-Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its parent planet, or a moon passing into the shadow of another moon. An eclipse is a type of syzygy, as are transits and occultations. Eclipses are impossible on Mercury and Venus, which have no moons.
[edit] Earth-Moon system
An eclipse involving the Sun, Earth and Moon can occur only when they are nearly in a straight line. Because the orbital plane of the Moon is tilted with respect to the orbital plane of the Earth (the ecliptic), eclipses can occur only when the Moon is close to the intersection of these two planes (the nodes). The Sun, Earth and nodes are aligned twice a year, and eclipses can occur during a period of about two months around these times. There can be from four to seven eclipses in a calendar year, which repeat according to various eclipse cycles, such as the Saros cycle.
During a solar eclipse, the Moon can sometimes perfectly cover the Sun because its apparent size is nearly the same as the Sun when viewed from the Earth. This was not the case 100 million years ago (when the Moon was closer to the Earth), and it will cease to be the case in the distant future. A solar eclipse is actually a misnomer; the phenomenon is more correctly described as an occultation.
Lunar eclipses occur when the Moon passes through the Earth's shadow. There are three types of lunar eclipses: penumbral, when the Moon crosses only the Earth's penumbra; partial, when the Moon crosses partially into the Earth's umbra; and total, when the Moon crosses entirely within the Earth's umbra.
[edit] Mars
On Mars, only partial solar eclipses are possible, because neither of its moons is large enough to cover the Sun's disc as seen from the surface of the planet. (Lunar eclipses are not only possible, but common.) Martian eclipses have been photographed from both the surface of Mars and from orbit.
[edit] Gas giants
The gas giant planets (Jupiter, Saturn, Uranus and Neptune) have many moons and thus frequently display eclipses. The most striking involve Jupiter, which has four large moons and a low axial tilt, making eclipses more frequent: it is common to see the larger moons casting circular shadows upon Jupiter's cloudtops. On the other three giants, eclipses only occur at certain periods during the planet's orbit, due to their higher axial tilts.
[edit] Pluto
Pluto, with its large moon Charon, is also the site of many eclipses.
[edit] See also
- Solar eclipse
- Lunar eclipse
- Eclipse cycle
- Eclipsing binary
- Saros cycle
- Syzygy
- List of solar eclipses
- Solar eclipse on 29 March 2006
- Solar eclipses in fiction
[edit] External links
- A Catalogue of Eclipse Cycles
- Search 5,000 years of eclipses (notice: loads slowly)
- NASA eclipse home page
- International Astronomical Union's Working Group on Solar Eclipses
- Solar and Lunar Eclipse Image Gallery
- Interactive eclipse maps site
- Prof. Druckmüller's eclipse photography site
- Williams College eclipse collection of images
- Why do Hindus believe that the mythological demons Rahu and Ketu cause solar eclipses?
