Eclipse

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This article is about astronomical eclipses. For other uses, see Eclipse (disambiguation).

"Total eclipse" redirects here. For other uses, see Total Eclipse.

The French 1999 eclipse

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. A solar eclipse is actually a misnomer; the phenomenon is actually an occultation.

The Earth happens to be in the midst of a cosmic coincidence. During a solar eclipse, the Moon perfectly covers the Sun. This is because the Moon moves in a particular orbit that gives it the same apparent size 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.

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

[edit] Eclipses in the Earth-Moon system

Sun eclipse visible from moon. Painting by Lucien Rudaux.

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.

[edit] Eclipse phases

These were used in occult ceremonies.^{[citation needed]}

[edit] General phases of a solar eclipse

The general eclipse begins when the Moon's penumbra cone starts to sweep across the Earth's surface.
The total or annular eclipse begins when the Moon's umbra starts to sweep across the Earth's surface.
The centrality begins when the axis of the Moon's shadow cone starts to sweep across the Earth's surface.
The eclipse's maximum occurs when the axis of the Moon's shadow comes closest to the centre of the Earth.
The centrality ends when the axis of the Moon's shadow finishes its sweep across the Earth's surface.
The total or annular eclipse ends when the Moon's shadow finishes its sweep across the Earth's surface.
The general eclipse ends when the Moon's penumbra finishes its sweep across the Earth's surface.

The French 1999 eclipse

[edit] 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.

[edit] 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.

The progression of a lunar eclipse

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.

[edit] The eclipse in mythology

Han Dynasty Carving

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.However Aryabhata gave an accurate explanation of the eclipse in his scientific treatise Aryabhatiya dated 499 AD .

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.

[edit] Eclipses elsewhere in the solar system

A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io's shadow.

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 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.