Chapter 52
[T]he faith that was pledged in chapte 6 and in many other places in this work, I have redeemed from all tincture of self-justification, and have shown that the eccentric of Mars cannot be referred to anything but the sun itself; and that, in addition, it is not only reason that stands with me, but the observations themselves, in my releasing the observations of Mars from the sun's mean motion and measuring them out by the apparent motion of the sun. (p.528)
Recall that in chapter 51, the distances from the sun to Mars on opposite sides of the line of apsides were equal. On this diagram from p. 527, A is the true sun, B the mean sun (the center of the earth's orbit), and C the center of Mars's orbit. It is clear that if AE and AD were repeatedly found to be equal all along the orbit that line AC is indeed the line of apsides, and the mean sun is not on it!
