*sidereal*

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 ]