system, instead of tinkering with it. The old geocentric system once
shaken, the way was gradually smoothed for the heliocentric system,
which Copernicus, still hampered by tradition, did not quite reach. He
was hardly a practical astronomer in the observational sense. His first
recorded observation, of an occultation of Aldebaran, was made in 1497,
and he is not known to have made as many as fifty astronomical
observations, while, of the few he did make and use, at least one was
more than half a degree in error, which would have been intolerable to
such an observer as Hipparchus. Copernicus in fact seems to have
considered accurate observations unattainable with the instruments at
hand. He refused to give any opinion on the projected reform of the
calendar, on the ground that the motions of the sun and moon were not
known with sufficient accuracy. It is possible that with better data he
might have made much more progress. He was in no hurry to publish
anything, perhaps on account of possible opposition. Certainly Luther,
with his obstinate conviction of the verbal accuracy of the Scriptures,
rejected as mere folly the idea of a moving earth, and Melanchthon
thought such opinions should be prohibited, but Rheticus, a professor at
the Protestant University of Wittenberg and an enthusiastic pupil of
Copernicus, urged publication, and undertook to see the work through the
press. This, however, he was unable to complete and another Lutheran,
Osiander, to whom he entrusted it, wrote a preface, with the apparent
intention of disarming opposition, in which he stated that the
principles laid down were only abstract hypotheses convenient for
purposes of calculation. This unauthorised interpolation may have had
its share in postponing the prohibition of the book by the Church of
Rome.

According to Copernicus the earth is only a planet like the others, and
not even the biggest one, while the sun is the most important body in