GNU Collaborative International Dictionary of English

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1.       
   Sidereal , a. [L. sidereus, from sidus, sideris, a constellation, a star.  Cf. Sideral, Consider, Desire.]
   Relating to the stars; starry; astral; as, sidereal astronomy.
   1913 Webster
   (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