When Copernicus propounded the theory that the earth moved around
the sun, and not the sun around the earth, he was able to fix the
relative distances of the several planets, and thus make a map of
the solar system. But he knew nothing about the scale of this map.
He knew, for example, that Venus was a little more than two-thirds
the distance of the earth from the sun, and that Mars was about
half as far again as the earth, Jupiter about five times, and
Saturn about ten times; but he knew nothing about the distance of
any one of them from the sun. He had his map all right, but he
could not give any scale of miles or any other measurements upon
it. The astronomers who first succeeded him found that the
distance was very much greater than had formerly been supposed;
that it was, in fact, for them immeasurably great, and that was
all they could say about it.

The proofs which Copernicus gave that the earth revolved around
the sun were so strong that none could well doubt them. And yet
there was a difficulty in accepting the theory which seemed
insuperable. If the earth really moved in so immense an orbit as
it must, then the stars would seem to move in the opposite
direction, just as, if you were in a train that is shunting off
cars one after another, as the train moves back and forth you see
its motion in the opposite motion of every object around you. If
then the earth at one side of its orbit was exactly between two
stars, when it moved to the other side of its orbit it would not
be in a line between them, but each star would have seemed to move
in the opposite direction.

For centuries astronomers made the most exact observations that