Eclipse

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This article discusses astronomical eclipses. For other meanings, see Eclipse (disambiguation).

Image:Solar eclips 1999 4.jpg

An eclipse (Greek verb: ekleipô, "to vanish") 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. A solar eclipse is actually a misnomer; the phenomenon is actually an occultation.

An eclipse is a type of syzygy, as are transits and occultations.

1 Eclipses in the Earth-Moon system
2 Eclipses elsewhere in the solar system
3 See also
4 External links

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Eclipses in the Earth-Moon system

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. The Sun passes either node once a year, and eclipses occur in a period of about two draconic months around these times. There can be from four to seven eclipses in a calendar year. They repeat according to eclipse cycles.

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Types of eclipse

Image:Eclipse99mir.jpg The most dramatic eclipses visible from Earth are:

Lunar eclipses - the Earth obscures the Sun, from the Moon's point of view. The Moon moves through the shadow cast by the Earth. This can only happen at full moon.
Solar eclipses - the Moon occults the Sun, from the Earth's point of view. The Moon casts a shadow that touches the surface of the Earth. This can only happen at new moon.

Eclipses can be divided into different types:

Total eclipses, in which the light source is totally blocked off by the eclipsing body. For total solar eclipses, the viewer is in the umbra part of the Moon's shadow. The total phase of a total eclipse can have a duration of up to 7 min 31 s.
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). For solar eclipses, the viewer is in the penumbra part of the Moon's shadow.
Annular eclipse, which are a total eclipse of luminary where a thin ring of light is visible around the intervening object. For solar eclipses, the viewer is in the antumbra part of the Moon's shadow. It is pure coincidence that the Moon and Sun have nearly equal apparent sizes, making annular eclipses possible. Annular eclipses are ideal times for observing solar prominences.
Hybrid solar eclipses, which consists of three phases: the eclipse starts as an annular one, then turns into a total and by the end it returns to the annular phase. Total phase of hybrid eclipses a typically very short. Maximum possible duration is 1 min 49 s.

The ratio between the apparent sizes of the eclipsing body and that of the luminary is called the magnitude of the eclipse. For solar eclipses, the ratio varies around 1, being sometimes more than 1, sometimes less. For lunar eclipses, the magnitude is much larger than 1; they never appear annular (viewed from the Moon).

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Eclipse phases

Image:Solar eclips 1999 5.jpgThese were used in occult ceremonies.

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General phases of a solar eclipse

The general eclipse begins when the Moon's penumbra cone starts to sweep across the Earth's disc.
The total or annular eclipse begins when the Moon's umbra starts to sweep across the Earth's disc.
The centrality begins when the axis of the Moon's shadow cone starts to sweep across the Earth's disc.
The eclipse's maximum occurs when the terrestrial surface within the umbra reaches its largest area.
The centrality ends when the axis of the Moon's shadow finishes its sweep across the Earth's disc.
The total or annular eclipse ends when the Moon's shadow finishes its sweep across the Earth's disc.
The general eclipse ends when the Moon's penumbra finishes its sweep across the Earth's disc.

Image:Film eclipse soleil 1999.jpg

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Local phases of a solar eclipse

First contact (also called first exterior contact) is the instant when the Moon's disc starts to cover the Sun's.
Second contact (also called first interior contact) is the instant when the Moon's disc is entirely surrounded by the Sun's (for an annular eclipse) or the instant when the Sun's disc disappears completely behind the Moon's (for a total eclipse).
Third contact (also called second interior contact) is the instant when the Moon's disc starts to come out of the Sun's (for an annular eclipse) or the instant when the Sun's disc reappears from behind the Moon's (for a total eclipse).
Lastly, fourth contact (also called second exterior contact) is the instant when the Moon's disc clears the Sun's.

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Phases of a lunar eclipse

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.

Image:Eclipse lune.jpg

First contact (also called first exterior contact) is the instant when the Moon starts to enter into the Earth's umbra.
Second contact (also called first interior contact) is the instant when the Moon enters completely into the Earth's umbra. This is the beginning of totality.
The maximum of the eclipse occurs when the angular distance between the centre of the Moon's disc and the centre of the shadow cone is at its smallest value.
Third contact (also called second interior contact) is the instant when the Moon starts to come out of the Earth's umbra. This is the end of totality.
Lastly, fourth contact (also called second exterior contact) is the instant when the Moon clears the Earth's umbra completely.

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The eclipse in mythology

Image:EclipseCarving.jpg

Before modern astronomy arose there were long-standing explanations for eclipses in many cultures. These would typically involve conflicts between mythic forces. For example, in Hindu mythology, the two demons Rahuand Ketu were believed to be the cause of eclipses. Similarly in China, at the Imperial observatory in Beijing, is a carved stone with the following explanation:

"This carved stone chart explained the cause of solar eclipses. The center of the golden bird (the symbol of the sun) was covered by the toad (the symbol of the moon). The people of the Han Dynasty called the phenomenon a good combination of the sun and the moon."

In this explanation we see a recognition of the celestial realities and a cheerful outlook regarding the event. In other cultures an eclipse could be both a surprising and a terrifying event.

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Eclipses elsewhere in the solar system

Image:JupiterandIo.jpg Eclipses are impossible on Mercury and Venus, which have no moons.

On Mars, only partial eclipses are possible, because neither of its moons is large enough to cover the Sun's disc. Martian eclipses have been photographed from both the surface of Mars and from orbit. See Transit of Phobos from Mars and Shadow of Phobos on Mars.

The gas giants, which have many moons, frequently display eclipses. The most striking involve Jupiter, which has four large moons, and which has a low axial tilt, making eclipses more frequent. It is common to see the larger moons casting circular shadows upon Jupiter's cloudtops.

Pluto, with its large moon Charon, is also the site of many eclipses.

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