Syzygy

Mars, Venus, and the crescent Moon

The crescent Moon, Venus, and fainter Mars in near alignment on February 20, 2015.

No, that’s not a typo. The word syzygy (pronounced “SIZ-i-jee”) is an obscure astronomical term that refers to an alignment in a straight line of three celestial bodies bound by gravity. I thought of this old term last week as I snapped the above image during the lovely conjunction of the crescent Moon, Mars, and Venus after sunset. While this alignment was not a syzygy in the proper sense, it was close.

A proper syzygy is not hard to find. A lunar or solar eclipse, when the Earth, Sun, and Moon fall in a line, are examples of syzygy. Same story with a planetary transit when Venus or Mercury cross the face of the Sun.

A syzygy can also occur from the point of view of other planets. Just last year, the Curiosity rover on Mars observed Mercury transiting the Sun, which meant Mars, Mercury, and the Sun were in syzygy.

The word syzygy comes from the ancient Greek suzugos, which means “yoked together”.

When the Moon is in syzygy with the Earth and Sun, there can be noticeable gravitational effects, especially “moonquakes” triggered by the gravity of the Earth on the innards of the Moon. Some believe earthquakes may occur during Earth-Moon-Sun alignments, but the Sun is sufficiently distant and the Moon sufficiently small that such effects are hard to prove.

A syzygy, or at least a near Earth-Moon-Sun alignment, does cause high tides on Earth. At full or new Moon, when the Earth, Moon, and Sun line up, the tides are some 20% higher than normal tides. So a syzygy is a practical matter, even if most people have neither heard of it and can’t pronounce it.

The video below shows the most dramatic visual example of syzygy when the Moon passes between the Earth and the Sun as a total solar eclipse: