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An eclipse is an astronomical event that occurs when one celestial object moves into the shadow of another. When an eclipse occurs within a stellar system, such as the Solar System, it forms the alignment of three or more celestial bodies in the same gravitational system along a straight line.^[1]

The term eclipse is most often 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, 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 host planet, or a moon passing into the shadow of another moon. A binary star system can also produce eclipses if the plane of their orbit intersects the position of the observer.

The term is derived from the ancient Greek noun ἔκλειψις (ékleipsis), which is derived from the verb ἐκλείπω (ekleípō), "to cease to exist,"^[2] a combination of prefix εκ- (ek-), from preposition εκ, εξ (ek, ex), "out," and of verb λείπω (leípō), "to be absent".^[3][4]

A syzygy is the alignment of three or more celestial bodies in the same gravitational system along a straight line. The word is usually used in context with the Sun, Earth, and the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of syzygy, as do transits and occultation there is a syzygy between a star and two celestial bodies, such as a planet and a moon. The shadow cast by the object closest to the star intersects the more distant body, lowering the amount of luminosity reaching the latter's surface. The region of shadow cast by the occulting body is divided into an umbra, where the radiation from the star's radiation-emitting photosphere is completely blocked, and a penumbra, where only a portion of the radiation is blocked.^[5]

A total eclipse will occur when the observer is located within the umbra of the occulting object. Totality occurs at the point of maximum phase during a total eclipse, when the occulted object is completely covered. When the star and a smaller occulting object are nearly spherical, the umbra forms a cone-shaped region of shadow in space.

Beyond the end of the umbra is a region called the antumbra, where a planet or moon will be seen transiting across the star but not completely covering it. For an observer inside the antumbra of a solar eclipse, for example, the Moon appears smaller than the Sun, resulting in an annular eclipse.^[5] The remaining volume of shadowed space, where only a fraction of the occulting object overlaps the star, is called the penumbra. An eclipse that does not reach totality, such as when the observer is in the penumbra, is called a partial eclipse.

For spherical bodies, when the occluding object is smaller than the star, the length (L) of the Umbra's cone-shaped shadow is given by:

where R_s is the radius of the star, R_o is the occulting object's radius, and r is the distance from the star to the occulting object. For Earth, on average L is equal to 1.384 × 10⁶ km, which is much larger than the Moon's semimajor axis of 3.844 × 10⁵ km. Hence the umbral cone of the Earth can completely envelop the Moon during a lunar eclipse.^[6] If the occulting object has an atmosphere, however, some of the luminosity of the star can be refracted into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth—producing a faint, ruddy illumination of the Moon even at totality.

An astronomical transit is also a type of syzygy, but is used to describe the situation where the nearer object is considerably smaller in apparent size than the more distant object. Likewise, an occultation is a syzygy where the apparent size of the nearer object appears much larger than the distant object, and the distant object becomes completely hidden during the event.

An eclipse cycle takes place when a series of eclipses are separated by a certain interval of time. This happens when the orbital motions of the bodies form repeating harmonic patterns. A particular instance is the Saros cycle, which results in a repetition of a solar or lunar eclipse every 6,585.3 days, or a little over 18 years. However, because this cycle has an odd number of days, a successive eclipse is viewed from a different part of the world.^[7]

An eclipse involving the Sun, Earth and Moon can occur only when they are nearly in a straight line, allowing one to be hidden behind another, viewed from the third. 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.

Between 1901 and 2100 there are the maximum of 7 eclipses in: ^[8]

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