An eclipse occurs when one astronomical object is temporarily obscured by passing into the shadow of another, an alignment of three celestial bodies known as a syzygy. The term most commonly refers to solar eclipses (when the Moon's shadow crosses Earth) and lunar eclipses (when the Moon moves into Earth's shadow), but it can also describe events like a moon passing into its planet's shadow or eclipsing binary star systems. These events only happen during an eclipse season—twice a year when the orbital planes of the Earth and Moon align—and the type (total, annular, or partial) depends on the apparent sizes of the Sun and Moon, which is why they don't occur monthly. The shadow itself is divided into the umbra (total eclipse), antumbra (annular eclipse), and penumbra (partial eclipse). While lunar eclipses are visible from the entire nightside of Earth, total solar eclipses at any given location are rare, sometimes decades apart. The word derives from the ancient Greek ékleipsis, meaning "abandonment" or "darkening of a heavenly body."