Syzygy (astronomy)
In astronomy, a syzygy ( ; ) is a roughly straight-line configuration of three or more celestial bodies in a gravitational system. Overview. 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 used to describe interesting configurations of astronomical objects in general. For example, one  such case occurred on March 21, 1894, around 23:00 GMT, when Mercury transited the Sun as would 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 on Mars observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides  Earth. Occultations, transits, and eclipses. Syzygy sometimes results in an occultation, transit, or eclipse. 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. Einstein ring. As electromagnetic rays are somewhat bent by gravitation, when they pass by a heavy mass they are bent. Thus, the heavy mass acts as a form of gravitational lens. If the light source, the gravitating mass and the observer stand in a line, one sees what is termed an Einstein ring. Tidal variation. Syzygy causes the bimonthly phenomena of spring and neap tides. At the new and full moon, the Sun and Moon are in syzygy. Their  tidal forces act to reinforce each other, and the ocean both rises higher and falls lower than the average. Conversely, at the first and third quarter, the Sun and Moon are at right angles, their tidal forces counteract each other, and the tidal range is smaller than average. Tidal variations can also be measured in the Earth's crust, and these tidal influences may affect the frequency of earthquakes.