tool to the heavens. Sometimes they tried to guess how far it
probably was from the vault of heaven to the earth, and they had a myth
as to the time it took Vulcan to fall. Ptolemy knew that the moon was
about thirty diameters of the earth distant from us, and he knew that
the sun was many times farther than the moon; he thought it about
twenty times as far, but could not be sure. We know that it is nearly
four hundred times as far.

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 they
were able without having succeeded in detecting any such apparent
motion among the stars. Here was a mystery which they could not solve.
Either the Copernican system was not true, after all, and the earth did
not move in an orbit, or the stars were at such immense distances that
the whole immeasurable orbit of the earth is a mere point in
comparison.