Syzygy (astronomy)

Above the round domes of La Silla Observatory, three astronomical objects in the Solar System — Jupiter (top), Venus (lower left), and Mercury (lower right).

In astronomy, a syzygy /ˈsɪzᵻdʒi/ (from the Ancient Greek suzugos (σύζυγος) meaning, "yoked together") is a straight-line configuration of three celestial bodies in a gravitational system.

Mercury transiting the Sun as viewed by the Curiosity rover on Mars (June 3, 2014).

The word is often used in reference to the Sun, 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 around 23:00 GMT, when Mercury transited the Sun (as could have been 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.

On June 3, 2014, the Curiosity rover observed the planet Mercury transiting the Sun (from Mars), marking the first time a planetary transit has been observed from a celestial body besides Earth.

Occultations, transits, and eclipses

Syzygy sometimes results in a transit, an occultation or an eclipse.

• An occultation occurs when an apparently larger body passes in front of an apparently smaller one.
• A transit occurs when a smaller body passes in front of a larger one.
  • In the combined case where the smaller body regularly transits the larger, an occultation is also termed a secondary eclipse.
• An eclipse occurs when a body disappears or partially disappears from view, either by an occultation, as with a solar eclipse, or by passing into the shadow of another body, as with a lunar eclipse (thus both are listed on NASA's eclipse page).

Transits and occultations of the Sun by Earth's Moon are called solar eclipses regardless of whether the Sun is completely or partially covered. By extension, transits of the Sun by a satellite of a planet may also be called eclipses, as with the transits of Phobos and Deimos shown on NASA's JPL photojournal, as may the passage of a satellite into the planet's shadow, as with this eclipse of Phobos. The term eclipse is also used more generally for bodies passing in front of one another. For example, a NASA Astronomy Picture of the Day refers to the Moon eclipsing and occulting Saturn interchangeably.

Effects

There is no controversy about the effect of a syzygy on ocean tides. The syzygy produces the more powerful spring tide due to the combined gravitational effect of the Sun with that of the Moon. The spring tides (highest tides) occur at full moon and new moon times while the neap tides (lowest tides) occur when the Sun and Moon are at right angles in the first and third quarters of the Moon's cycle. Spring and neap tides are about 20% higher and lower, respectively, than the average tide. Syzygy also produces stronger earth tides, although there is controversy about the tidal triggering of earthquakes.