That was a useful and rather easy first lesson about the motions of
the bodies called planets.
We have now to consider a rather less simple case, but one a great
deal more interesting. Two planets intrude among our evening stars,
each following a looped track, but the tracks are unlike; the two
planets are unlike in appearance, and they are also very unlike in
reality.
I hope many of my young readers have already found out for themselves
that these intrusive bodies have been wandering among our fixed stars.
I purposely said nothing about the visitors last August, so that those
who try to learn the star-groups from my maps may have had a chance of
discovering the two planets for themselves. If they have done so, they
have in fact repeated a discovery which was made many, many years ago.
Ages before astronomy began to be a science, men found out that some
of the stars move about among the rest, and they also noticed the kind
of path traveled in the sky by each of those moving bodies.
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