ly motion or its counterpart invisible to us."(1)

The fact that the stars show no parallax had been regarded as an
important argument against the motion of the earth, and it was still so
considered by the opponents of the system of Copernicus. It had, indeed,
been necessary for Aristarchus to explain the fact as due to the extreme
distance of the stars; a perfectly correct explanation, but one that
implies distances that are altogether inconceivable. It remained for
nineteenth-century astronomers to show, with the aid of instruments of
greater precision, that certain of the stars have a parallax. But
long before this demonstration had been brought forward, the system of
Copernicus had been accepted as a part of common knowledge.

While Copernicus postulated a cosmical scheme that was correct as to its
main features, he did not altogether break away from certain defects of
the Ptolemaic hypothesis. Indeed, he seems to have retained as much of
this as practicable, in deference to the prejudice of his time. Thus
he records the planetary orbits as circular, and explains their
eccentricities by resorting to the theory of epicycles, quite after
the Ptolemaic method. But now, of course, a much more simple mechanism
sufficed to explain the planetary motions, since the orbits were
correctly referred to the central sun and not to the earth.

Needless to say, the revolutionary conception of Copernicus did not meet
with immediate acceptance. A number of prominent astronomers, however,
took it up almost at once, among these being Rhaeticus, who wrote
a commentary on the evolutions; Erasmus Reinhold, the author of the
Prutenic tables; Rothmann, astronomer to the Landgrave of Hesse, and
Maestlin, the 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 wh