Inastronomy, asyzygy (/ˈsɪzədʒi/SIZ-ə-jee; from Ancient Greek συζυγία (suzugía) 'union, yoking', expressing the sense of σύν (syn- "together") and ζυγ- (zug- "a yoke")[1][2])is a roughly straight-line configuration of three or morecelestial bodies in a gravitational system.[3]
The word is often used in reference to theSun,Earth, and either theMoon or aplanet, where the latter is inconjunction oropposition.Solar andlunareclipses occur at times of syzygy, as dotransits andoccultations.[4]
A syzygy sometimes results in an occultation, transit, or an eclipse.
As electromagnetic rays are affected by gravitation, when they pass by a heavy mass they are bent. As a result, 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.
A syzygy causes the fortnightly phenomena ofspring tides. At the new and full moon, the Sun and Moon are in syzygy. Theirtidal forces act to reinforce each other, and the ocean both rises higher and falls lower than the average.[6] Tidal variations can also be measured in theEarth's crust, and theseEarth tide influencesmay affect the frequency of earthquakes.
The wordsyzygy is often used to describe interesting configurations of astronomical objects in general. For example, one such case occurred on March 21, 1894, around 23:00GMT, whenMercury transited the Sun as would have been seen from Venus, and Mercury andVenus both simultaneously transited the Sun as seen fromSaturn.
On June 3, 2014, theCuriosity rover on Mars observed the planetMercury transiting the Sun, marking the first time aplanetary transit has been observed from a celestial body besidesEarth.[7]
The term 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.[8]
Because the orbits of all the planets in the Solar System (as well as the Moon) are inclined by only a few degrees, they always appear very near theecliptic in our sky. Therefore, although an apparent planetary alignment known as aplanetary parade may appear as a line (actually, agreat arc), the planets are not necessarily aligned in space.
