In astronomy, a syzygy is a straight line configuration of three celestial bodies in a gravitational system. The word is often 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.
The word syzygy is often loosely used to describe interesting configurations of planets in general. For example, one such case occurred on March 21, 1894 at around 23:00 GMT, when Mercury transited the Sun as seen from Venus, and Mercury and Venus both simultaneously transited the Sun as seen from Saturn. It is also used to describe situations when all the planets are on the same side of the Sun although they are not necessarily in a straight line, such as on March 10, 1982.[1]