syzygy

In astronomy, a syzygy is a straight line configuration of three celestial bodies in a gravitational system. The word is usually used in reference to the Sun, the Earth and either 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 occultations. The term is also applied to each instance of new moon or full moon when the Sun and Moon are in conjunction or opposition, even though they are not precisely on one line with the Earth.