accompanied by his wife, and for some time lived entirely at Tycho's
expense, writing by way of return essays against Reymers and another
man, who had claimed the credit of the Tychonic system. This Kepler
could do with a clear conscience, as it was only a question of priority
and did not involve any support of the system, which he deemed far
inferior to that of Copernicus. The following year saw friction between
the two astronomers, and we learn from Kepler's abject letter of apology
that he was entirely in the wrong. It was about money matters, which in
one way or another embittered the rest of Kepler's life, and it arose
during his absence from Prague. On his return in September, 1601, Tycho
presented him to the Emperor, who gave him the title of Imperial
Mathematician, on condition of assisting Tycho in his calculations, the
very thing Kepler was most anxious to be allowed to do: for nowhere else
in the world was there such a collection of good observations sufficient
for his purpose of reforming the whole theory of astronomy. The
Emperor's interest was still mainly with astrology, but he liked to
think that his name would be handed down to posterity in connection
with the new Planetary Tables in the same way as that of Alphonso of
Castile, and he made liberal promises to pay the expenses. Tycho's other
principal assistant, Longomontanus, did not stay long after giving up
the Mars observations to Kepler, but instead of working at the new lunar
theory, suddenly left to take up a professorship of astronomy in his
native Denmark. Very shortly afterwards Tycho himself died of acute
distemper; Kepler began to prepare the mass of manuscripts for
publication, but, as everything was claimed by the Brahe family, he was
not allowed to finish the work. He succeeded to Tycho's post of
principal mathematician to the Emperor, at a reduced official salary,
which owing to the emptiness of the Imperial treasury was almost always
in arrear. In order to meet his expenses he had recourse to the casting
of nativities, for which he gained considerable reputation and received