2. (Science: astronomy) 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. Sidereal clock, day, month, year. See 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.
Origin: L. Sidereus, from sidus, sideris, a constellation, a star. Cf. Sideral, Consider, Desire.
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... mentioned days in form of weeks and years and:  Astronomy For a given planet, there are two types of day defined in astronomy: 1 apparent sidereal day = a single rotation of a planet with respect to the distant stars (for Earth it is 23.934 solar hours or 24 sidereal hours) 1 solar day ...
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