Afterward, Newton showed _how_ it happens that the planets obey these
laws, but as his part of the work had no particular reference to Mars,
I say no more about it in this place.
Here, in Fig. 3, are the real paths of Mars and the Earth, and also of
Venus and Mercury. No loops, you see, in any of them, simply because
we have set the sun in the middle. Set the earth in the middle, and
each planet would have its own set of loops, each set enormously
complicated, and all three sets mixed together in the most confusing
way. It is well to remember this when you see, as in many books of
astronomy, the old theory illustrated with a set of circles looking
almost as neat and compact as the set truly representing the modern
theory. For the idea is suggested by this simple picture of the old
theory that the theory itself was simple, whereas it had become so
confusing that not merely young learners, but the most profound
mathematicians, were baffled when they tried to unravel the motions of
the planets.
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