All citations of Kepler's work are taken from individual volumes of the modern edition: KeplerJohannes, Gesammelte Werke (Munich, 1937–; hereafter KGW).
2.
In Astronomia nova, C.19, KGW, iii, p. 178 lines 1–12.
3.
Parallax is, in general, the change in direction that occurs when a body is observed from two different positions. Its practical manifestation is as the angular measurement from which one could obtain the distance of the body (here the Sun, but also the Moon) in terms of the radius (or diameter) of the Earth as baseline.
4.
Kepler himself perpetrated the mistake on a couple of occasions in this section — But in these cases, as it happened, the meaning was quite clear.
5.
In any numerical work relating to astronomy, it was Kepler's custom to retain precisely five decimal places — That is, he employed a unit of 100000, which means that one cannot in practice distinguish an angle ≤ 1/4° from its tangent.
6.
This suggests that these alternative angular measures must have been in use well before the equivalence of degrees and radians became formalized.
7.
KGW, vi: Kepler mistakenly gave the reference to chap. 6.
8.
Van HeldenA., Measuring the universe (Chicago, 1985).
