﻿

# sidereal day

a unit of time used in astronomy, equal to the period of time in which the earth makes one rotation relative to the stars. If we could view the earth from outside the Solar System, we would see that it actually completes 366.242 rotations during one year (one revolution around the sun). We only count 365.242 because one rotation is cancelled out for us by our tour around the sun. Thus the sidereal day, the average interval between two successive risings of the same star, is shorter than the mean solar day (see day) by 1/366.242. The sidereal day equals 23 hours 56 minutes 4.090 54 seconds, or 86 164.090 54 seconds. Like the regular day it is divided into 24 sidereal hours, each sidereal hour being divided into 60 sidereal minutes and each sidereal minute into 60 sidereal seconds. The sidereal hour equals 59 minutes 50.17 seconds; the sidereal minute equals 59.8362 seconds, and the sidereal second equals 0.997 270 second. Traditionally, observatories had clocks set to this sidereal cycle, and astronomers still use sidereal time in making telescope settings.

Dictionary of units of measurement. 2015.

### Look at other dictionaries:

• Sidereal day — Day Day (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the …   The Collaborative International Dictionary of English

• sidereal day — n. the time between two successive passages of the vernal equinox across the meridian: it measures one rotation of the earth and equals 23 hours, 56 minutes, 4.1 seconds of mean solar time …   English World dictionary

• sidereal day — The period between two successive transits of a particular meridian on the earth by the first point of Aries or successive transits of the vernal equinox. Because the first point of Aries is not fixed in space and it precesses along the… …   Aviation dictionary

• sidereal day — A day measured by the transit of certain stars. A day which begins at the noon or middle of the day, in common parlance. 52 Am J1st Time § 14. A mean sidereal day is 23 hours, 56 minutes, 4.091 seconds of mean solar time. Rochester German Ins. Co …   Ballentine's law dictionary

• sidereal day — noun the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day • Syn: ↑day • Hypernyms: ↑time unit, ↑unit of time, ↑sidereal time …   Useful english dictionary

• Sidereal Day —    The true rotation period of a planet, determined by the time between appearances of a star in the same place in the sky. Since the rotation of the Earth is gradually slowing down, a day in the past was shorter than now, and in the future it… …   The writer's dictionary of science fiction, fantasy, horror and mythology

• sidereal day — side′real day′ n. astron. the interval between two successive passages of the vernal equinox over the meridian, being about four minutes shorter than a mean solar day • Etymology: 1785–95 …   From formal English to slang

• sidereal day — /saɪˌdɪəriəl ˈdeɪ/ (say suy.dearreeuhl day) noun See day (def. 3e) …   Australian English dictionary

• sidereal day — Astron. the interval between two successive passages of the vernal equinox over the meridian, being about four minutes shorter than a mean solar day. [1785 95] * * * …   Universalium

• sidereal day — noun Astronomy the time between consecutive meridional transits of the First Point of Aries, almost four minutes shorter than the solar day …   English new terms dictionary