Syzygy (astronomy)

This article is about a straight line configuration of three celestial bodies. For other uses, see Syzygy.

In astronomy, a syzygy (/ˈsɪzdʒi/) (from the Ancient Greek syzygos (σύζυγος) meaning, “yoked together”.) 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 often applied when the Sun and Moon are in conjunction (new moon) or opposition (full moon).

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.

Effects

The gravitational effects of syzygies on planets, especially the Earth, are still being studied. It is known that the gravitational stress on the Moon during a Sun-Earth-Moon syzygy can trigger a Moonquake, a seismic event on the Moon similar in some ways to an Earthquake. So far, no evidence has been found that the Sun-Earth-Moon syzygy can trigger earthquakes, and it is considered highly unlikely that any correlation between syzygy and earthquakes exist since the earth is 82 times more massive than the moon, and thus the gravitational force on the earth from the moon is trivial compared to its mass.

There is no controversy about the effect of a syzygy on ocean tides. The syzygy produces the more powerful spring tide due to the enhanced gravitational effect of the Sun added to the Moon's gravitational pull.