urse of a year.

It would be impossible, in a sketch like the present, to enter into any
detail as to the geometrical propositions on which this beautiful
investigation of Copernicus depended. We can only mention a few of the
leading principles. It may be laid down in general that, if an observer
is in movement, he will, if unconscious of the fact, attribute to the
fixed objects around him a movement equal and opposite to that which he
actually possesses. A passenger on a canal-boat sees the objects on the
banks apparently moving backward with a speed equal to that by which he
himself is advancing forward. By an application of this principle, we
can account for all the phenomena of the movements of the planets, which
Ptolemy had so ingeniously represented by his circles. Let us take, for
instance, the most characteristic feature in the irregularities of the
outer planets. Mars, though generally advancing from west to east among
the stars, occasionally pauses, retraces his steps for a while, again
pauses, and then resumes his ordinary onward progress. Copernicus showed
clearly how this effect was produced by the real motion of the earth,
combined with the real motion of Mars. When the earth comes directly
between Mars and the sun, the retrograde movement of Mars is at its
highest. Mars and the earth are then advancing in the same direction.
We, on the earth, however, being unconscious of our own motion,
attribute, by the principle I have already explained, an equal and
opposite motion to Mars. The visible effect upon the planet is that Mars
has two movements, a real onward movement in one direction, and an
apparent movement in the opposite direction. If it so happened that the
earth was moving with the same speed as Mars, then the apparent movement
would exactly neutralize the real movement, and Mars would seem to be at
rest relatively to the surrounding stars. Under the actual circumstances
considered, however, the earth is moving faster than Mars, and the
consequence is that the apparent movement of the planet backward exceeds
the real movement forward, the net result being an apparent retrograde
movement.

With consummate skill, Copernicus showed how the applications of the
same principles could account for the characteristic movements of the
planets. His reasoning in due time bore down all opposition. The supreme
importance of the earth in the system vanished. It had now merely to
take rank as one of the planets.