The total or partial disappearance from view of an astronomical object when it passes directly behind, or into the shadow of, another object. In the case of our Sun, a solar eclipse can occur only at new Moon, when the Moon is directly between the Earth and the Sun. Although the Moon is much nearer the Earth than the Sun, a coincidence of nature makes both appear nearly the same size in our sky. A total eclipse, when the whole disc is obscured, lasts a maximum of
5 min, often less; during such an eclipse, the chromosphere and corona are seen. A partial eclipse of much longer duration occurs before, after, and to each side of the path of totality. Sometimes the apparent size of the lunar disc is just too small for a total eclipse, and an annular eclipse results, in which a bright ring of sunlight surrounds the Moon. A lunar eclipse occurs when the Moon passes into the shadow of the Earth, which can happen only at full moon. Then the Moon is a dim coppery hue. Moons and satellites of other bodies in the Solar System are eclipsed when they pass through the shadow of their primary bodies. In binary star systems it is also possible for one star to eclipse another (an eclipsing binary star).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. 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.
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.
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.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.Phases of a lunar eclipse
There are three types of lunar eclipses: penumbral, when the Moon crosses only the Earth's penumbra;
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. 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. Lastly, fourth contact (also called second exterior contact) is the instant when the Moon clears the Earth's umbra completely.Eclipses elsewhere in the solar system
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.
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.
Pluto, with its large moon Charon, is also the site of many eclipses.
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