urth period
of nine years. Throughout a considerable portion of this period the
great work of Copernicus was in manuscript, but it was not published
until the year of his death. The reasons for the delay are not very
fully established. Copernicus undoubtedly taught his system throughout
the later decades of his life. He himself tells us that he had even
questioned whether it were not better for him to confine himself to such
verbal teaching, following thus the example of Pythagoras. Just as his
life was drawing to a close, he decided to pursue the opposite course,
and the first copy of his work is said to have been placed in his hands
as he lay on his deathbed.

The violent opposition which the new system met from ecclesiastical
sources led subsequent commentators to suppose that Copernicus had
delayed publication of his work through fear of the church authorities.
There seems, however, to be no direct evidence for this opinion. It has
been thought significant that Copernicus addressed his work to the pope.
It is, of course, quite conceivable that the aged astronomer might wish
by this means to demonstrate that he wrote in no spirit of hostility
to the church. His address to the pope might have been considered as a
desirable shield precisely because the author recognized that his
work must needs meet with ecclesiastical criticism. Be that as it
may, Copernicus was removed by death from the danger of attack, and it
remained for his disciples of a later generation to run the gauntlet of
criticism and suffer the charges of heresy.

The work of Copernicus, published thus in the year 1543 at Nuremberg,
bears the title De Orbium Coelestium Revolutionibus.

It is not necessary to go into details as to the cosmological system
which Copernicus advocated, since it is familiar to every one. In a
word, he supposed the sun to be the centre of all the planetary motions,
the earth taking its place among the other planets, the list of which,
as known at that time, comprised Mercury, Venus, the Earth, Mars,
Jupiter, and Saturn. The fixed stars were alleged to be stationary, and
it was necessary to suppose that they are almost infinitely distant,
inasmuch as they showed to the observers of that time no parallax; that
is to say, they preserved the same apparent position when viewed from
the opposite points of the earth's orbit.

But let us allow Copernicus to speak for himself regarding his system,
His exposition is full of interest.