What does sidereally mean?
Sidereally means in relation to the stars or their apparent motion. The adverb is used mainly in astronomy and precise measurement when an event or interval is referenced to the apparent positions of distant stars rather than to the Sun. A sidereal day, for example, is the time Earth takes to rotate once relative to the stars; it is slightly shorter than a solar day because Earth advances in its orbit while rotating. The word is closely associated with sidereal time, periods, and years, all of which use the stars as their reference. Although related to the familiar adjective sidereal, sidereally is less common and strongly technical. It is most likely to appear in scientific writing about celestial mechanics, observational astronomy, satellite tracking, and systems of astronomical time rather than in general conversation.
adverbIn relation to the apparent movement of the stars, or according to sidereal time.
- With reference to the stars or their apparent positions, especially in astronomical measurement.
"The orbital period was calculated sidereally rather than with reference to the Sun."
"The spacecraft's position was tracked sidereally using a catalogue of fixed stars."
"Astronomers distinguish the sidereal and solar forms of the Earth's rotation."
A sidereal day is about four minutes shorter than a solar day because Earth moves along its orbit while the stars appear to circle it.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does sidereally come from?
Sidereally derives from the English adjective sidereal, which came from Latin sidereus, meaning “of or relating to stars.” The Latin adjective in turn developed from sidus, meaning “star.” Related forms such as sidereal time and sidereal year retain the same Latin root and refer to measurement relative to the stars.
How sidereally is actually used
This is a technical term used chiefly in astronomy, navigation, and precise timekeeping. It is uncommon in everyday speech.
Easily confused with sidereally
Sidereal measurement uses the stars as a reference, whereas synodic measurement uses the Sun or another orbiting body.