In "On the Revolutions of the Heavenly Spheres" Book 5. chapter 16, Copernicus appears to acknowledge a discrepancy between the old Ptolemy observation and the model. Here is a Google Translate from a German translation:

if one calculates back, it turns out that the location of the apogeum of the eccentric circle was in 119 ° 40 ′ of the fixed star sphere at this last observation. Ptolemy found this place at the time of Antoninus at 108 ° 50 ′; so it has moved forward by 10 ° 50 ′ except for us. We have found the distance between the centers to be 40 such parts smaller, of which 10,000 come to the radius of the eccentric circle. Not as if Ptolemy or we were wrong, but to prove that the center of the earth's orbit has approached the center of the orbit of Mars while the sun has remained immobile...

Copernicus seems to be saying neither he nor Ptolemy were in error, but that the "center of the Earth's orbit" has moved. Did Copernicus ever try to explain how the "center of the Earth's orbit" moved? or to investigate the nature of the movement?

  • 2
    $\begingroup$ Apparently, he does not. Mazer in Shifting the Earth only remarks that "Copernicus' conclusion must have been most disturbing as it presents a movement that his model does not explain". $\endgroup$
    – Conifold
    Jul 16, 2021 at 2:23
  • $\begingroup$ @Conifold, good find. thanks. $\endgroup$
    – d_e
    Jul 16, 2021 at 5:32


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