The Middle Course

For there, the distances computed according to the law of the circle were longer than the observed distances at the middle longitudes, while here, the distances resulting from the hypothesis that makes the planet's path is neither a circle nor such as to make as great an incursion from the circle at the sides as does the oval that arose from the opinion of chapter 45 and was described in chapter 46; but takes the middle course. (p.541)

Here, point M traces out the middle path between point D (which forms the red-dashed oval of chapter 45) and the black circle. Kepler does not actually propose that the planet move along M at this speed, but only that the overall path traced out by point M is the path traversed by the planet.

e = 0.4

e = 0.2

e = 0.1

It is thus argued rightly that the truth is in the middle, between the two. (p.542)