If you look up at the sky and see the full moon, you’re witnessing an example of syzygy.
From our point of view the sun is then on the opposite side of the sky to the moon, and so is said to be in opposition to it. The three are also in syzygy at new moon, this time with the moon and the sun apparently next to each other in the sky — a state called conjunction. Halfway between these states, when the moon is half full, the sun and moon are in quadrature.
Spring tides form during syzygy (full and new moon), whereas neap tides form during quadrature (the waxing and waning phases of the moon).
Science, 5 Jul. 1996.
The word appeared in English in the seventeenth century, and at first could apply only to conjunctions. It comes via late Latin from the Greek suzugia, which derives from suzugos, yoked or paired. It was not until a century later that its meaning was extended to cover astronomical opposition, in defiance of its etymology.
Syzygy has a variety of other meanings in mathematics, poetry and zoology, all of them developed from the idea of things being joined together. For example:
As a soldier studies his guns, and a dentist his tools, so a writer must study the laws of rhythm, accent, phrasing, alliteration, phonetic syzygy, run-on and double-ending lines, rhyme, and, last but not least, the melodies of common speech.
The World’s Best Books, by Frank Parsons, 1893.
Phonetic syzygy is a little like alliteration, in that a consonant is repeated throughout a passage. Unlike alliteration, the sound doesn't have to be at the beginning of successive words. The most famous example of phonetic syzygy is probably the sequence of m sounds in Tennyson’s
The moan of doves in immemorial elms And murmuring of innumerable bees.
Lovers of wordplay may know syzygy as the shortest word in the language containing three ys.