What is "line of syzygies"

Syzygy \Syz"y*gy\ (s[i^]z"[i^]*j[y^]), n.; pl. Syzygies (-j[i^]z). [L. syzygia a joining together, conjunction, Gr. syzygi`a; sy`n with + zeygny`nai to join, zygo`n yoke: cf. F. syzygie. See Yoke, n.]

1. (Astron.) The point of an orbit, as of the moon or a planet, at which it is in conjunction or opposition; -- commonly used in the plural.

2. (Gr. & L. Pros.) The coupling together of different feet; as, in Greek verse, an iambic syzygy.

3. (Zo["o]l.)

   1. Any one of the segments of an arm of a crinoid composed of two joints so closely united that the line of union is obliterated on the outer, though visible on the inner, side.

   2. The immovable union of two joints of a crinoidal arm.

4. The intimately united and apparently fused condition of certain low organisms during conjugation.

   Line of syzygies (Astron.), the straight line connecting the earth, the sun, and the moon or a planet, when the latter is in conjunction or opposition; -- used chiefly of the moon.