and not 4′.5) was found by Horrox, but he applied it in the same manner as Tycho had done.
It is very interesting to see that Kepler had independently discovered the annual equation about the same time as Tycho did. In the calendar for 1598, which he had to prepare as provincial mathematician for Styria, Kepler had in detail described the solar eclipse of the 7th March (25th February) 1598, making use of Magini's tables.[1] But the phenomenon turned out very different from what he expected, as the eclipse not only was very far from being total (or nearly so), but occurred an hour and a half later than expected. As the only reservation taken by Kepler had been that the eclipse might possibly occur half an hour earlier, he had to say something about the cause of this error in the calendar for 1599. In this he therefore stated that the solar eclipse, as well as the lunar eclipse in February and the Paschal full moon, had been more than an hour late; but the lunar eclipse in August had been too early, and it appeared to him that one would have to assume "that a natural month or period of the moon with regard to the sun in winter, ceteris paribus, is a little longer and slower than in summer, and the fault is the moon's and not the sun's, as nothing can be reformed as to the latter without great confusion; but whether the inequality is to be applied to the moon itself or to the length of the day, and what cause it may have in nature and the Copernican philosophy, cannot be explained in a few words."[2] A letter to Mästlin of December 1598 shows that Kepler had not thought further about the matter, and
- ↑ Opera, i. p. 396.
- ↑ Ibid., i. p. 409.
causas, primo propter inaequales partium signiferi ascensiones rectas, deinde propter motus Solis diurnos inaequales. Hanc posteriorem Tycho negligit, causam afferens experientiam, qua deprehendatur in collatione eclipsium aequalitatis rationem iniri non posse, nisi aut haec negligatur aequatio, aut annuus circellus tot epicyclis Lunæ insuper adjiciatur." In 1603 Kepler had also to explain to Fabricius that experience had shown Tycho the necessity of omitting part of the equation of time in the lunar motion (ii. p. 96).
