equant point: a point in Ptolemy's model of the solar system around which the sun and the planets were supposed to rotate.
Equant
Ptolemy placed the earth offcenter and the opposite point from the deferent on this off center circle is called the equant.
Equatorial Mounting ...
equant: The point off-center in the deferent from which the center of the epicycle appears to move uniformly.
equatorial mounting: A telescope mounting that allows motion parallel to and perpendicular to the celestial equator.
Equant - In the Ptolemaic system, the point from which the motion of the epicycle around the deferent is uniform ...
This prompted him to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed.
Nasir al-Din al-Tusi (b. 1201) resolved significant problems in the Ptolemaic system by developing the Tusi-couple as an alternative to the physically problematic equant introduced by Ptolemy.
His model used devices such as the the deferent and epicycle of Hipparchus and his own idea, the equant. An equant is sort of like an eccentric, but a bit more off center (this is shown in Figure 7).
He supposed that the Earth was located a short distance from the center of the deferent for each planet and that the center of the planet's deferent and the epicycle described uniform circular motion around what he called the equant, ...
Ptolemy also had to introduce, however, another mathematical device known as the equant: an imaginary point halfway between the center of the deferent and the eccentric point representing the earth's position.
Granular - most of the grains are equant.
Granoblastic - typical of non-foliated metamorphic rocks without porphyroblasts. Grains have sutured boundaries, are approximately equidimensional, and meet at ~120° triple junctions.
He then assumed that the motion of the center of the epicycle along the deferent appeared uniform, not from the center of the deferent or from the eccentric, but from a third point symmetrically displaced from the eccentric, called the equant.
equant (astronomy)
geocentric system (astronomy)
heliocentric system (astronomy)
interplanetary dust particle (IDP) (astronomy)
interplanetary exploration
interplanetary medium (astronomy)
main-belt asteroid (astronomy) ...
He added some refinements to explain the details of the observations: an ``eccentric'' for each planet that was the true center of its motion (not the Earth!) and an ``equant'' for each planet moved uniformily in relation to (not the Earth!).
Copernicus therefore had no choice but to use the eccentrics, epicycles, and equants of Ptolemaic cosmology, to which he added three kinds of motion he used to describe the observed behaviour of the Earth: ...
spheres): one sphere is the deferent, with a center offset somewhat from the Earth; the other sphere is an epicycle embedded in the deferent, with the planet embedded in the spherical epicycle. Through the use of the epicycle, eccentric, and equant, ...
Although Ptolemy relied mainly on the work of Hipparchus, he introduced at least one idea, the equant, which appears to be his own, and which greatly improved the accuracy of the predicted positions of the planets.
 See also: Sun, Earth, Astronomy, Planet, Moon
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