World Library  
Flag as Inappropriate
Email this Article


The basic elements of Ptolemaic astronomy, showing a planet on an epicycle with a deferent and an equant point.

Equant (or punctum aequans) is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of the planets. The equant is used to explain the observed speed change in planetary orbit during different stages of the orbit. This planetary concept allowed Ptolemy to keep the theory of uniform circular motion alive by stating that the path of heavenly bodies was uniform around one point and circular around another point.


  • Placement 1
  • Equation 2
  • Discovery and use 3
  • Criticism 4
  • References 5
  • External links 6
  • See also 7


The equant point, indicated in the diagram by the large • , is placed so that it is directly opposite the Earth from the center of the deferent (known as the "eccentric"), indicated by the X. A planet or the center of an epicycle (a smaller circle carrying the planet) was conceived to move with a uniform angular speed with respect to the equant. In other words, to a hypothetical observer placed at the equant point, the center of the epicycle would appear to move at a steady angular speed. However, the center of the epicycle will not move at a uniform speed along its deferent.[1]

The reason for the implementation of the equant was to maintain a semblance of uniform circular motion of heavenly bodies, a long-standing article of faith originated by Aristotle for philosophical reasons, while also allowing for the best match of the computations of the observed movements of the body, particularly in the size of the apparent retrograde motion of all solar system bodies except the sun and the moon.


The angle α at the earth between the planet and the equant is a function of time t:

\alpha(t) = \Omega t - \arcsin\left(\frac{E}{R} \sin(\Omega t) \right)

where Ω is the constant angular speed seen from the equant which is situated at a distance E when the radius of the deferent is R.[2]

The equant model has a body in motion on a circular path that does not share a center with Earth. The moving object's speed will actually vary during its orbit around the outer circle (dashed line), faster in the bottom half and slower in the top half. The motion is considered uniform only because the planet sweeps around equal angles in equal times from the equant point. The speed of the object is non-uniform when viewed from any other point within the orbit.

Discovery and use

Ptolemy introduced the equant in "Almagest". The evidence that the equant was a required adjustment to Aristotelian physics relied on observations made by himself and a certain "Theon" (perhaps, Theon of Smyrna).[1]

In models of the universe that precede Ptolemy, generally attributed to Hipparchus, the eccentric and epicycles were already a feature. The Roman Pliny in the 1st century CE, who apparently had access to writings of late Greek astronomers, and not being an astronomer himself, still correctly identified the lines of apsides for the five known planets and where they pointed in the zodiac.[3] Such data requires the concept of eccentric centers of motion. Most of what we know about Hipparchus comes to us through mentions of his works by Ptolemy in the Almagest. Hipparchus' models' features explained differences in the length of the seasons on Earth (known as the "first anomaly"), and the appearance of retrograde motion in the planets (known as the "second anomaly"). But Hipparchus was unable to make the predictions about the location and duration of retrograde motions of the planets match observations; he could match location, or he could match duration, but not both simultaneously.[4] Ptolemy's introduction of the equant resolved that contradiction: the location was determined by the deferent and epicycle, while the duration was determined by uniform motion around the equant.

Ptolemy's model of astronomy was used as a technical method that could answer questions regarding astrology and predicting planets positions for almost 1500 years, even though the equant and eccentric were violations of pure Uraniborg.

It wasn't until

  • Equidimensional: This is a synonym for equant when it is used as an adjective.

See also

  • Ptolemaic System – at Rice University's Galileo Project
  • Java simulation of the Ptolemaic System – at Paul Stoddard's Animated Virtual Planetarium, Northern Illinois University

External links

  1. ^ a b Evans, James (April 18, 1984). "On the function and probable origin of Ptolemy's equant" (PDF).  
  2. ^ Eccentrics, deferents, epicycles and equants (Mathpages)
  3. ^ Pliny the Elder. The Natural History, Book 2: An account of the world and the elements, Chapter 13: Why the same stars appear at some times more lofty and some times more near. Retrieved August 7, 2014. 
  4. ^ "The New Astronomy - Equants, from Part 1 of Kepler's Astronomia Nova". Retrieved August 1, 2014.  An excellent video on the effects of the equant
  5. ^ Perryman, Michael (2012-09-17). "History of Astrometry". Retrieved August 7, 2014. 
  6. ^ Bracco; Provost (2009). "Had the planet Mars not existed: Kepler's equant model and its physical consequences" (PDF). European Journal of Physics 30: 1085–92.  
  7. ^ Van Helden. "Ptolemaic System". Retrieved 20 March 2014. 
  8. ^ Craig G. Fraser, 'The cosmos: a historical perspective', Greenwood Publishing Group, 2006 p.39
  9. ^   (copyright renewed 1985)
  10. ^ Koestler A. (1959), The Sleepwalkers, Harmondsworth: Penguin Books, p. 322; see also p. 206 and refs therein. [2]


The equant solved the last major problem of accounting for the anomalistic motion of the planets but was believed by some to compromise the principles of the ancient Greek philosopher/astronomers, namely uniform circular motion about the Earth.[7] The uniformity was generally assumed to be observed from the center of the deferent, and since that happens at only one point, only non-uniform motion is observed from any other point. Ptolemy moved the observation point explicitly off the center of the deferent to the equant. This can be seen as breaking part of the uniform circular motion rules. Noted critics of the equant include the Persian astronomer Nasir al-Din Tusi who developed the Tusi-couple as an alternative explanation,[8] and Nicolaus Copernicus, whose alternative was a new pair of epicycles for each deferent. Dislike of the equant was a major motivation for Copernicus to construct his heliocentric system.[9][10] This violation of perfect circular motion around the center of the deferent bothered many thinkers, especially Copernicus who mentions the equant as a monstrous construction in De Revolutionibus. Copernicus' movement of the Earth away from the center of the universe obviated the primary need for Ptolemy's epicycles by explaining retrograde movement as an optical illusion, but he re-instituted two smaller epicycles into each planet's motion in order to replace the equant.



This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.