What is a syzygy?

Apart from being a wonderful word to use in the game of Scrabble, this astronomical term is an event in which one astronomical object is lined-up with another. This leads to the pithy aphorism: all eclipses are syzygys but not all syzygys are eclipses. For example, Full moon and New Moon are syzygys involving the lining up of the Sun, Earth and Moon, therefore, lunar and solar eclipses are syzygys.  When a planetary moon passes across the face of another body but does not eclipse it, this is called a transit. From Earth, the small disks of Venus and Mercury can be seen passing across the face of the sun during transits of Venus and Mercury. These also involve the straight-line alignment of the Sun, Earth and each planet. 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. Previously, the Curiosity rover has captured images of the Martian moons Phobos and Deimos transiting the sun.