Talk:Equant

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Serious problem with the article "Equant"[edit]

This article is accompanied by a figure that shows two points, one designated by an "x" and one designated by a dot. The caption does not explicitly indicate what these two points signify. I believe that the dot is the equant and the "x" is the center of the deferent. However, I'm not an expert in this area, so I'm not sure this is the correct interpretation. this problem is compounded by the fact that the text contains the following:

"The equant point is placed so that it is directly opposite the Earth from the center of the deferent, indicated in the diagram by the • ."

A reasonable parsing of this sentence would conclude that the phrase "indicated in the diagram by the •" modifies the phrase "center of deferent" and not the word "equant." Thus, if my understanding of these terms is correct, the text refers to the wrong point in the figure as the equant.

I would correct his myself, BUT I'M NOT SURE.

I think that the sentence referenced to above should probably be re-written as follows:

"The equant point, indicated in the diagram by the [dot], is placed so that it is directly opposite the Earth from the center of the deferent." —The preceding unsigned comment was added by Xorkster (talkcontribs) 15:53, August 20, 2007 (UTC).


I believe you are correct (sitting in a History of Math class learning about Equants right now) -- there are animations of how an Equant with an eccentric deferen and epicycle actually work -- check out http://people.scs.fsu.edu/~dduke/mars.html

I support this change. Ceramufary 18:44, 11 September 2007 (UTC)

My understanding of the equant point was that the Earth is supposed to be at the center (as in the geocentric model) where the X is located. The epicycle moves about the equant along the deterent with uniform angular velocity. Ref.:
Hummel, Charles E. (1986). The Galileo connection: resolving conflicts between science & the Bible. InterVarsity Press. pp. 35–38. ISBN 087784500X. 
RJH (talk) 19:19, 6 January 2010 (UTC)


I support the above change as well. And, there is another problem further down the article. The author does not describe the placement of the angle α. He wrote: The angle α between the axis on which the equant and the Earth lie is a function of time t. He specifies an axis, a single line, but no second line between which is the angle α. The reader needs to know the second line, which must intersect the axis he specified and another point, presumably the center of the orange planet in the diagram. hfcamp (talk) 13:06 31 October 2013 (UTC)

Worse yet, how does the Equant account for the observed motion of heavenly bodies? The definition is silent on this! hfcamp (talk) 15:27 31 October 2013 (UTC)

History of the Equant[edit]

What about the history of the equant? I am looking for a project for my History of Science class and I noticed this article doesn't cover much on the history of the equant. I think this page may benefit from a survey of the key persons and developments behind the equant. What do you all think? Zoulogy (talk) 21:50, 22 May 2013 (UTC)

The equant ("center of uniform motion") was apparently originally invented by Claudius Ptolemy and first explained in his Almagest. I don't think it was further developed theoretically until Kepler threw it out altogether. Kepler destroyed the idea of uniform motion of planets with his laws, and thus obviated a need for a center for such. There is a note about Kepler actually using it early on in his calculations, and then abandoning it when its flaws became apparent. SkoreKeep (talk) 00:05, 3 August 2014 (UTC)

Relevant Equant Research Sources[edit]

James Evans, "On the Function and the Probable Origin of Ptolemy's Equant," American Journal of Physics 52 (1984): 1080-89.

Dennis Rawlings, "Ancient Heliocentrists, Ptolemy and the Equant," American Journal of Physics 55 (1987): 235-39.

C Bracco and JP Provost, "Had the planet Mars not existed: Kepler's equant model and its physical consequences," European Journal of Physics 30 (2009): 1085-92. — Preceding unsigned comment added by SeanMcMahon (talkcontribs) 00:56, 16 February 2014 (UTC)