A Syzygy Is A Rare Event In What Two Distinct Categories?
Answer: Poetry and Astronomy
Syzygy is a term used in both poetry and astronomy. In poetry it denotes the combination of two metrical feet into a single unit. Phonetic syzygy is not particularly rare as it is essentially a type of structured alliteration. A metrical syzygy, however, is significantly more rare and only occurs when you have an extra metrical foot in a line of poetry that is, for the sake of scansion, treated as one. Examples of this in English and most other languages are few and far between.
In the field of astronomy, the term syzygy is also used and refers to a (slightly less) rare event. In studies of the night sky, a syzygy is the very rare alignment of three celestial bodies (all from the same solar system) in a straight-line configuration. If you catch a blurb on the evening news about how you should stay up late and make a trek to your local observatory to witness Jupiter, Venus, and Mercury in alignment, the anchorman is talking about catching a syzygy.
The wordy syzygy comes to us from Ancient Greek and means “yoked together”.
Image courtesy of the ESO.a straight-line configurationa straight-line configuration