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From The Collaborative International Dictionary of English v.0.48:
Sidereal \Si*de"re*al\, a. [L. sidereus, from sidus, sideris, a constellation, a star. Cf. Sideral, Consider, Desire.] 1. Relating to the stars; starry; astral; as, sidereal astronomy. [1913 Webster] 2. (Astron.) Measuring by the apparent motion of the stars; designated, marked out, or accompanied, by a return to the same position in respect to the stars; as, the sidereal revolution of a planet; a sidereal day. [1913 Webster] Sidereal clock, day, month, year. See under Clock, Day, etc. Sideral time, time as reckoned by sideral days, or, taking the sidereal day as the unit, the time elapsed since a transit of the vernal equinox, reckoned in parts of a sidereal day. This is, strictly, apparent sidereal time, mean sidereal time being reckoned from the transit, not of the true, but of the mean, equinoctial point. [1913 Webster]