dies. Copernicus had devoted much time to observation of the
planets with reference to measuring their distance, and his efforts had
been attended with considerable success. He did not, indeed, know the
actual distance of the sun, and, therefore, was quite unable to fix
the distance of any planet; but, on the other hand, he determined the
relative distance of all the planets then known, as measured in terms of
the sun's distance, with remarkable accuracy.

With these measurements as a guide, Kepler was led to a very fanciful
theory, according to which the orbits of the five principal planets
sustain a peculiar relation to the five regular solids of geometry.
His theory was this: "Around the orbit of the earth describe a
dodecahedron--the circle comprising it will be that of Mars; around
Mars describe a tetrahedron--the circle comprising it will be that of
Jupiter; around Jupiter describe a cube--the circle comprising it
will be that of Saturn; now within the earth's orbit inscribe an
icosahedron--the inscribed circle will be that of Venus; in the orbit
of Venus inscribe an octahedron--the circle inscribed will be that of
Mercury."(3)

Though this arrangement was a fanciful one, which no one would
now recall had not the theorizer obtained subsequent fame on more
substantial grounds, yet it evidenced a philosophical spirit on the
part of the astronomer which, misdirected as it was in this instance,
promised well for the future. Tycho Brahe, to whom a copy of the
work was sent, had the acumen to recognize it as a work of genius. He
summoned the young astronomer to be his assistant at Prague, and no
doubt the association thus begun was instrumental in determining the
character of Kepler's future work. It was precisely the training
in minute observation that could avail most for a mind which, like
Kepler's, tended instinctively to the formulation of theories. When
Tycho Brahe died, in 1601, Kepler became his successor. In due time
he secured access to all the unpublished observations of his great
predecessor, and these were of inestimable value to him in the progress
of his own studies.

Kepler was not only an ardent worker and an enthusiastic theorizer, but
he was an indefatigable writer, and it pleased him to take the public
fully into his confidence, not merely as to his successes, but as to
his failures. Thus his works elaborate false theories as well as correct
ones, and detail the observations through which the inco