instructor of Kepler. The Prutenic tables, just referred to, so
called because of their Prussian origin, were considered an
improvement on the tables of Copernicus, and were highly esteemed
by the astronomers of the time. The commentary of Rhaeticus gives
us the interesting information that it was the observation of the
orbit of Mars and of the very great difference between his
apparent diameters at different times which first led Copernicus
to conceive the heliocentric idea. Of Reinhold it is recorded
that he considered the orbit of Mercury elliptical, and that he
advocated a theory of the moon, according to which her epicycle
revolved on an elliptical orbit, thus in a measure anticipating
one of the great discoveries of Kepler to which we shall refer
presently. The Landgrave of Hesse was a practical astronomer, who
produced a catalogue of fixed stars which has been compared with
that of Tycho Brahe. He was assisted by Rothmann and by Justus
Byrgius. Maestlin, the preceptor of Kepler, is reputed to have
been the first modern observer to give a correct explanation of
the light seen on portions of the moon not directly illumined by
the sun. He explained this as not due to any proper light of the
moon itself, but as light reflected from the earth. Certain of
the Greek philosophers, however, are said to have given the same
explanation, and it is alleged also that Leonardo da Vinci
anticipated Maestlin in this regard.[2]

While, various astronomers of some eminence thus gave support to
the Copernican system, almost from the beginning, it
unfortunately chanced that by far the most famous of the
immediate successors of Copernicus declined to accept the theory
of the earth's motion. This was Tycho Brahe, one of the greatest
observing astronomers of any age. Tycho Brahe was a Dane, born at