What does precession mean?
Precession is the gradual change in the orientation of the axis of a rotating body. For Earth, it means that the planet's rotational axis traces a slow circle in space, altering the positions of the celestial poles and causing the equinoxes to move westward along the celestial sphere. One complete cycle takes approximately 26,000 years. The word is most often encountered in astronomy, where it helps explain changes in star positions, the relationship between seasons and the zodiac, and discrepancies between ancient and modern coordinates. More generally, precession can describe a gradual shift in direction, such as the slow precession of a trend or sequence. Its strongest connotation is therefore one of measured, continuous change rather than abrupt movement. In technical writing, the specific phrases “axial precession” and “precession of the equinoxes” make the intended sense clear.
nounPrecession is the gradual change in the orientation of the axis of a rotating body, especially Earth. In astronomy, it refers to the slow westward movement of the equinoxes relative to the stars.
- The slow, continuous change in the orientation of the axis of a rotating astronomical body.
- The gradual westward displacement of the equinoxes along the celestial sphere, caused by Earth's axial precession.
"The Earth's axial precession slowly alters the relationship between the celestial poles and the stars."
"Measurements of ancient alignments can help astronomers reconstruct the effects of precession."
"The precession of the equinoxes changes the seasonal relationship between the calendar and the constellations."
Used when referring to multiple instances of the phenomenon or to distinct historical or astronomical cycles.
"Researchers have modelled how earlier precessions affected ancient alignments."
Earth's equinoxes are slowly drifting westward, completing one precessional cycle over roughly 26,000 years.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does precession come from?
The word comes from Latin praecessio, a noun formed from praecedere, meaning “to go before.” The Latin term originally referred to a going before or an advance and was later applied to the movement of the equinoxes. English borrowed the word in the 17th century for the astronomical phenomenon now called precession of the equinoxes. The same Latin root also underlies precede, precedent, procedure, and process.
Related word forms
How precession is actually used
The term is primarily technical, especially in astronomy, mechanics, and geodesy. It is sometimes used more broadly for the gradual displacement or reversal of a dominant trend, though such extended uses can sound formal or metaphorical.
Easily confused with precession
Precession is the gradual rotational shifting of a body's axis, whereas a procession is an organized group moving in a line.