the system, and the stars probably too far away for any motion of the
earth to affect their apparent places. The earth in fact is very small
in comparison with the distance of the stars, as evidenced by the fact
that an observer anywhere on the earth appears to be in the middle of
the universe. He shows that the revolution of the earth will account for
the seasons, and for the stationary points and retrograde motions of the
planets. He corrects definitely the order of the planets outwards from
the sun, a matter which had been in dispute. A notable defect is due to
the idea that a body can only revolve about another body or a point, as
if rigidly connected with it, so that, in order to keep the earth's axis
in a constant direction in space, he has to invent a third motion. His
discussion of precession, which he rightly attributes to a slow motion
of the earth's axis, is marred by the idea that the precession is
variable. With all its defects, partly due to reliance on bad
observations, the work showed a great advance in the interpretation of
the motions of the planets; and his determinations of the periods both
in relation to the earth and to the stars were adopted by Reinhold,
Professor of Astronomy at Wittenberg, for the new Prutenic or Prussian
Tables, which were to supersede the obsolete Alphonsine Tables of the
thirteenth century.

In comparison with the question of the motion of the earth, no other
astronomical detail of the time seems to be of much consequence. Comets,
such as from time to time appeared, bright enough for naked eye
observation, were still regarded as atmospheric phenomena, and their
principal interest, as well as that of eclipses and planetary
conjunctions, was in relation to astrology. Reform, however, was
obviously in the air. The doctrine of Copernicus was destined very soon
to divide others besides the Lutheran leaders. The leaven of inquiry was
working, and not long after the death of Copernicus real advances were