ck
by the plague, Tycho settled temporarily at Wandsbeck, in Holstein, but
towards the end of 1598 set out to meet the Emperor at Prague. Once
more plague intervened and he spent some time at Dresden, afterwards
going to Wittenberg for the winter. He ultimately reached Prague in
June, 1599. Rudolph granted him a salary of at least 3000 florins,
promising also to settle on him the first hereditary estate that should
lapse to the Crown. He offered, moreover, the choice between three
castles outside Prague, of which Tycho chose Benatek. There he set about
altering the buildings in readiness for his instruments, for which he
sent to Uraniborg. Before they reached him, after many vexatious delays,
he had given up waiting for the funds promised for his building
expenses, and removed from Benatek to Prague. It was during this
interval that after considerable negotiation, Kepler, who had been in
correspondence with Tycho, consented to join him as an assistant.
Another assistant, Longomontanus, who had been with Tycho at Uraniborg,
was finding difficulty with the long series of Mars observations, and it
was arranged that he should transfer his energies to the lunar
observations, leaving those of Mars for Kepler. Before very much could
be done with them, however, Tycho died at the end of October, 1601. He
may have regretted the peaceful island of Hveen, considering the
troubles in which Bohemia was rapidly becoming involved, but there is
little doubt that had it not been for his self-imposed exile, his
observations would not have come into Kepler's hands, and their great
value might have been lost. In any case it was at Uraniborg that the
mass of observations was produced upon which the fame of Tycho Brahe
rests. His own discoveries, though in themselves the most important made
in astronomy for many centuries, are far less valuable than those for
which his observations furnished the material. He discovered the third
and fourth inequalities of the moon in longitude, called respectively
the variation and the annual equation, also the variability of the
motion of the moon's nodes and the inclination of its orbit to the
ecliptic. He obtained an improved value of the constant of precession,
and did good service by rejecting the idea that it was variable, an idea
which, under the name of trepidation, had for many centuries been
accepted. He discovered the effect of refraction, though only
approximately its amount, and determined improve