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sidereal time

sidereal time (ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian. Because the earth moves in its orbit about the sun, the sidereal day is about 4 min shorter than the solar day (see solar time). Thus, a given star will appear to rise 4 min earlier each night, so that different stars are visible at different times of the year. The local sidereal time of an observer is equal to the hour angle of the vernal equinox.

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sidereal period

sidereal period Orbital period of a planet or other celestial body with respect to a background star. It is the true orbital period. Sidereal time is local time reckoned according to the rotation of the Earth with respect to the stars. The sidereal day is 23 hours, 56 minutes and 4 seconds of mean solar time, nearly 4 minutes shorter than the mean solar day. The sidereal year is equal to 365.25636 mean solar days.

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sidereal day

sidereal day The time it takes for the Earth to complete one rotation on its axis such that a particular point on the surface returns to its former position in relation to the position of the fixed stars. The sidereal day is 4.09 minutes shorter than the mean solar day, because the movement of the Earth in its solar orbit is imposed on its rotational motion.

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sidereal year

sidereal year, time required for the earth to complete an orbit of the sun relative to the stars. The sidereal year is 365 days, 6 hr, 9 min, 9.5 sec of mean solar time (see solar time). It is 20 min longer than the tropical year because of the precession of the equinoxes; for this reason, the sidereal year does not stay in step with the seasons.

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sidereal day

si·de·re·al day • n. Astron. the time between two consecutive transits of the First Point of Aries. It represents the time taken by the earth to rotate on its axis relative to the stars, and is almost four minutes shorter than the solar day because of the earth's orbital motion.

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year

year Length of time taken by the Earth to circle once round the Sun in its orbit. It is defined in various ways, such as the sidereal year, which is timed with reference to the fixed stars.

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sidereal period

sidereal period, in astronomy, length of time a body takes to complete an orbit relative to the fixed stars. See sidereal time.

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sidereal day

sidereal day: see sidereal time.

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