If a fly at rest at the middle
of a clock face watched the ends of the two hands, they would seem to
go round him in circles; but if, instead, he was on the end of one of
the hands (and was not knocked off as the other passed), the end of
this other hand would not move round the fly in the same simple way.
When the two hands were together it would be near, when they were
opposite it would be far away, and, without entering into any
particular description of the way in which it would seem to move, you
can easily see that the motion would seem much more complicated
than if the fly watched it from the middle of the clock face. Now,
Copernicus _did_ enter into particulars, and showed by mathematical
reasoning that nearly all the peculiarities of the planets' motions
could be explained by supposing that the sun, not the earth, was the
body round which the planets move, and that they go round him nearly
in circles.
[Illustration: FIG. 3. THE PATHS OF MARS, THE EARTH, VENUS, AND
MERCURY.]
But Copernicus could not explain _all_ the motions.
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