What does equant mean?
An equant is a device of ancient mathematical astronomy. In the Ptolemaic system, which dominated Western thought for over a thousand years, planets were imagined to ride on small circles (epicycles) whose centres travelled around a larger circle called the deferent. To account for the observed fact that planets speed up and slow down across the sky, Ptolemy introduced the equant: a point offset from the deferent's centre, on the opposite side from the Earth, about which the epicycle's centre was stipulated to move at constant angular speed. This elegant fudge preserved the Greek commitment to uniform circular motion while fitting observation remarkably well — yet Copernicus later condemned it as a monstrosity, since motion could not truly be uniform about a point that was not the circle's centre. Today the word survives mainly in histories of astronomy.
In Ptolemaic astronomy, the point situated symmetrically opposite the Earth with respect to the centre of a planet's deferent, about which the centre of the planet's epicycle moves with uniform angular velocity.
"By shifting the equant away from the true centre of the orbit, Ptolemy could reproduce the planets' uneven speeds while keeping the motion mathematically uniform."
Used almost exclusively in historical treatments of pre-Copernican astronomy.
Having equal dimensions or being equidistant from a central point; equidimensional.
"Under the microscope, the quartz grains appeared roughly equant rather than elongated."
A technical sense found chiefly in geology and crystallography; far less common than the noun.
The plural is regular but rarely needed, as discussions typically concern a single equant per planetary model.
"Each planet in the Ptolemaic scheme required its own carefully positioned equants and deferents."
To save the appearance of the planets, Ptolemy cheated at his own game: he made the heavens move uniformly around a point — the equant — that wasn't even the centre of the circle.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of equant
Equant derives from Latin aequans, the present participle of aequare, meaning 'to make equal', itself from aequus ('equal, level'). The term entered scientific usage through medieval Latin astronomical texts and passed into English with the revival of classical learning, retaining the sense of 'that which makes equal' — the point ensuring uniform angular motion. It shares this Latin root with words such as equal, equate, adequate, and equator, all ultimately descended from the same Indo-European root meaning 'even' or 'level'.
How equant is actually used
Almost entirely confined to the history of science and discussions of pre-Copernican astronomy; outside that context the word will be unfamiliar to most readers. The geological sense describing equidimensional grains is technical and rare.
Easily confused with equant
The equator is the great circle dividing the Earth into hemispheres, while an equant is an off-centre point used in Ptolemaic astronomy to explain planetary motion.