re than a thousand years B.C. All of this shows
sufficiently that Tycho Brahe was not a great theorist. He was
essentially an observer, but in this regard he won a secure place in the
very first rank. Indeed, he was easily the greatest observing astronomer
since Hipparchus, between whom and himself there were many points of
resemblance. Hipparchus, it will be recalled, rejected the Aristarchian
conception of the universe just as Tycho rejected the conception of
Copernicus.

But if Tycho propounded no great generalizations, the list of specific
advances due to him is a long one, and some of these were to prove
important aids in the hands of later workers to the secure demonstration
of the Copernican idea. One of his most important series of studies had
to do with comets. Regarding these bodies there had been the greatest
uncertainty in the minds of astronomers. The greatest variety of
opinions regarding them prevailed; they were thought on the one hand to
be divine messengers, and on the other to be merely igneous phenomena
of the earth's atmosphere. Tycho Brahe declared that a comet which he
observed in the year 1577 had no parallax, proving its extreme distance.
The observed course of the comet intersected the planetary orbits,
which fact gave a quietus to the long-mooted question as to whether the
Ptolemaic spheres were transparent solids or merely imaginary; since the
comet was seen to intersect these alleged spheres, it was obvious that
they could not be the solid substance that they were commonly imagined
to be, and this fact in itself went far towards discrediting the
Ptolemaic system. It should be recalled, however, that this supposition
of tangible spheres for the various planetary and stellar orbits was
a mediaeval interpretation of Ptolemy's theory rather than an
interpretation of Ptolemy himself, there being nothing to show that the
Alexandrian astronomer regarded his cycles and epicycles as other than
theoretical.

An interesting practical discovery made by Tycho was his method of
determining the latitude of a place by means of two observations made at
an interval of twelve hours. Hitherto it had been necessary to observe
the sun's angle on the equinoctial days, a period of six months being
therefore required. Tycho measured the angle of elevation of some star
situated near the pole, when on the meridian, and then, twelve hours
later, measured the angle of elevation of the same star when it again
came to t