Eclipse

Understand the spatial relationships between the Sun, the Moon, and Earth during eclipsesSee all videos for this article

eclipse, in astronomy, complete or partial obscuring of a celestial body by another. An eclipse occurs when three celestial objects become aligned.

From the perspective of a person on Earth, the Sun is eclipsed when the Moon comes between it and Earth, and the Moon is eclipsed when it moves into the shadow of Earth cast by the Sun. Eclipses of natural satellites (moons) or of spacecraft orbiting or flying past a planet occur as the bodies move into the planet’s shadow. The two component stars of an eclipsing binary star move around each other in such a way that their orbital plane passes through or very near Earth, and each star periodically eclipses the other as seen from Earth.

When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation. Examples are the disappearance of a star, nebula, or planet behind the Moon or the vanishing of a natural satellite or spacecraft behind some body of the solar system.

A transit occurs when, as viewed from Earth or another point in space, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area. Mercury and Venus, for example, periodically transit the Sun, and a natural satellite may transit its planet. Extrasolar planets (e.g., HD 209458b) have been discovered when they perform a transit of their stars.