[sahy-deer-ee-uh l]

adjective Astronomy.

determined by or from the stars: sidereal time.
of or relating to the stars.

Origin of sidereal

1625–35; < Latin sīdere(us) of, belonging to the stars (sīder-, stem of sīdus star, constellation + -eus adj. suffix) + -al1
Related formssi·de·re·al·ly, adverbnon·si·de·re·al, adjectiveun·si·de·re·al, adjective Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for sidereal

Historical Examples of sidereal

  • And that one should consider the sidereal system and the maxims of Epictetus and be comforted.

  • These sidereal time-keepers mark the centuries and eras of other worlds for you.


    Camille Flammarion

  • So the constitution of the sidereal universe is just like that of the bodies which we call material.


    Camille Flammarion

  • Not only nebul are probably unstable, but also many of the sidereal systems.

    The Asteroids

    Daniel Kirkwood

  • See, for example, the January number, 1876: Sidereal Astronomy.

    Mysterious Psychic Forces

    Camille Flammarion

British Dictionary definitions for sidereal



of, relating to, or involving the stars
determined with reference to one or more starsthe sidereal day
Derived Formssidereally, adverb

Word Origin for sidereal

C17: from Latin sīdereus, from sīdus a star, a constellation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for sidereal

also siderial, 1630s, "star-like;" 1640s, "of or pertaining to the stars," earlier sideral (1590s), from French sidereal (16c.), from Latin sidereus "starry, astral, of the constellations," from sidus (genitive sideris) "star, group of stars, constellation," probably from PIE root *sweid- "to shine" (cf. Lithuanian svidus "shining, bright"). Sidereal time is measured by the apparent diurnal motion of the fixed stars. The sidereal day begins and ends with the passage of the vernal equinox over the meridian and is about four minutes shorter than the solar day, measured by the passage of the sun over the meridian.

Online Etymology Dictionary, © 2010 Douglas Harper

Science definitions for sidereal



Relating to the stars or constellations.
Measured with respect to the background of fixed stars instead of the Sun.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.