In astronomy, a syzygy // (from the Ancient Greek suzugos (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 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.
Occultations, transits, and eclipses
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An occultation occurs when an apparently larger body passes in front of an apparently smaller one. A transit occurs when an apparently smaller body passes in front of an apparently larger one. In the combined case where the smaller body regularly transits the larger object, 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, occultations and eclipses in general are all effects of a syzygy.
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 here, 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, this entry from NASA's Astronomy Picture of the Day refers to the Moon eclipsing and occulting Saturn interchangeably.
The gravitational effects of syzygies on planets, especially 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. It is considered highly unlikely that any correlation between syzygy and earthquakes exists. This is because Earth is 82 times more massive than the Moon, and thus the gravitational force on Earth from the Moon is trivial compared to the mass of Earth.
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. 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 normal tides.
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