In that apparent movement of the heavens about the earth, which men now
know to be caused by the mere rotation of the earth itself, a slight
change is observed to be continually taking place. The stars, indeed,
are always found to be gradually drawing westward, _i.e._ towards the
sun, and losing themselves one after the other in the blaze of his
light, only to reappear, however, on the other side of him after a
certain lapse of time. This is equivalent to saying that the sun itself
seems always creeping slowly _eastward_ in the heaven. The rate at which
this appears to take place is such that the sun finds itself back again
to its original position, with regard to the starry background, at the
end of a year's time. In other words, the sun seems to make a complete
tour of the heavens in the course of a year. Here, however, we have
another illusion, just as the daily movement of the sky around the earth
was an illusion. The truth indeed is, that this apparent movement of the
sun eastward among the stars during a year, arises merely from a
_continuous displacement of his position_ caused by an actual motion of
the earth itself around him in that very time. In a word, it is the
earth which really moves around the sun, and not the sun around the
earth.

The stress laid upon this fundamental point by Copernicus, marks the
separation of the modern from the ancient view. Not that Copernicus,
indeed, had obtained any real proof that the earth is merely a planet
revolving around the sun; but it seemed to his profound intellect that a
movement of this kind on the part of our globe was the more likely
explanation of the celestial riddle. The idea was not new; for, as we
have already seen, certain of the ancient Greeks (Aristarchus of Samos,
for example) had held such a view; but their notions on the subject were
very fanciful, and unsupported by any good argument.

What Copernicus, however, really seems to have done was to _insist_ upon
the idea that the sun occupied the _centre_, as being more consonant
with common sense. No doubt, he was led to take up this position by the
fact that the sun appeared entirely of a different character from the
other members of the system. The one body in the scheme, which performed
the important function of dispenser of light and heat, would indeed be
more likely to occupy a position apart from the rest; and what position
more appropriate for its purposes than the centre!

But here Copernicus o