Ptolemy’s equant
E1025921
Ptolemy’s equant is a geometric device in his planetary theory that allows planets to move with uniform angular speed around an off-center point, introduced to better match observed non-uniform planetary motion.
Statements (44)
| Predicate | Object |
|---|---|
| instanceOf |
concept in astronomy
ⓘ
geometric device ⓘ mathematical construct ⓘ |
| addresses | non-uniform apparent speed of planets ⓘ |
| appliesTo |
Moon’s motion
ⓘ
Sun’s apparent motion ⓘ planetary motion ⓘ |
| assumes | Earth near but not at the center of planetary orbits ⓘ |
| category |
ancient Greek astronomy
ⓘ
history of science ⓘ |
| conceptualStatus | approximation to elliptical motion ⓘ |
| contrastsWith | uniform motion about the geometric center of the deferent ⓘ |
| criticizedBy |
Ibn al-Shatir
NERFINISHED
ⓘ
Nasir al-Din al-Tusi NERFINISHED ⓘ medieval Islamic astronomers ⓘ |
| definesMotionAs | uniform angular speed around an off-center point ⓘ |
| describedIn | Almagest NERFINISHED ⓘ |
| field |
ancient astronomy
ⓘ
celestial mechanics ⓘ |
| hasConsequence |
increased accuracy of Ptolemaic predictions
ⓘ
philosophical tension with uniform circular motion doctrine ⓘ |
| hasPurpose |
to model non-uniform planetary motion
ⓘ
to preserve uniform angular motion about a point ⓘ |
| historicalPeriod | 2nd century CE ⓘ |
| improves | fit to observed planetary positions ⓘ |
| influenced |
Copernican heliocentric theory
ⓘ
development of alternative planetary models in Islamic astronomy ⓘ |
| inspired | geometric devices such as the Tusi couple ⓘ |
| introducedBy | Claudius Ptolemy NERFINISHED ⓘ |
| locatedRelativeTo |
Earth
NERFINISHED
ⓘ
center of the deferent ⓘ |
| mathematicalRole | parameterization of orbital non-uniformity ⓘ |
| namedAfter | Claudius Ptolemy NERFINISHED ⓘ |
| partOf | Ptolemaic planetary theory ⓘ |
| relatedTo |
eccentric deferent
ⓘ
epicycle ⓘ |
| relativeTo | deferent circle ⓘ |
| replacedBy | elliptical orbits in Keplerian astronomy ⓘ |
| replacedIn | Copernican system NERFINISHED ⓘ |
| represents | point from which angular motion appears uniform ⓘ |
| usedFor |
predicting planetary longitudes
ⓘ
refining astronomical tables ⓘ |
| usedIn | Ptolemaic model of the Solar System NERFINISHED ⓘ |
| violates | Aristotelian principle of uniform circular motion about the center ⓘ |
Referenced by (1)
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