Models of Mars

Over at Starts With a Bang, Ethan Siegel has, under the title “Suck it, Ptolemy!”, a rather nice presentation  of the different explanatory models of retrograde planetary motion by Ptolemaeus and Copernicus , complete with lots of pretty pictures and animations but then he goes and spoils it all with the following sentence, “By time Copernicus came along, Mars’ orbit had been so carefully studied that geocentric modelers of the Solar System had placed seventy-eight epicycles on Mars’ orbit!“

I’m sorry, there is no polite way to say it, this statement is pure and utter crap! As one of his commentators points out Copernicus continued to use the same system of deferents and epicycles that Ptolemaeus had used and at the time of publication of Copernicus’ De Revolutionibus in 1547 1543, the current Ptolemaic system in use, that of Peuerbach, used a total of 40 circles to explain the motion of all the planets (not just Mars!) whereas Copernicus’ system required 48.

One commentator, Russell, remarks, “The remarkable thing is that so little progress was made between Ptolemy and the Renaissance.” In fact shortly after Ptolemaeus published his Syntaxis Mathematiké mathematical astronomy disappeared in Europe in the general political and social decline. On the level of Ptolemaeus’ work it first reappeared in Europe in the middle of the 15th century due to the efforts of Peuerbach and Regiomontanus, whose books Copernicus used to learn astronomy. So in reality, if one ignores the astronomy-free break caused by circumstances beyond the control of the astronomers, it was not actually very long between Ptolemaeus’ model and that of Copernicus. The interesting question is why the Islamic mathematical astronomers who revitalised the Ptolemaic astronomy already in the 8th century and also developed all of the mathematics that Copernicus used in his reform of the Ptolemaic model never developed a heliocentric system.

Another commentator Didac wonders, “The strange thing for me, is that almost nobody in the Pre-Copernican times assumed that Mercury and Venus orbit really the Sun and not the Earth. Even, when they were completely aware of the fact that Mercury and Venus do not depart the Sun away more than a maximum elongation.”

Actually they did. The so called ‘Egyptian’ system of Herakleides in which mercury and venus circle the sun, which circles the earth was well known in antiquity and in the works of Martianus Capella and Macrobius was also well known in the Middle Ages and Renaissance. Of course Tycho Brahe went one better and had all of the planets, except the moon, circling the sun, which circled the earth thus utilising Copernicus’ explanation for retrograde motion within a geo-centric system.


Filed under History of Astronomy, Myths of Science, Renaissance Science

2 responses to “Models of Mars

  1. Thanks for setting the record straight! Looks like you got the number of epicycles much more accurately than I did, so thanks!

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