The changes are
very slight; they can be detected only by the most laborious and
refined methods; yet they must have a cause, and we should like to
know what that cause is.
The moon shows a similar irregularity of motion. For half a
century, perhaps through a whole century, she will go around the
earth a little ahead of her regular rate, and then for another
half-century or more she will fall behind. The changes are very
small; they would never have been seen with the unaided eye, yet
they exist. What is their cause? Mathematicians have vainly spent
years of study in trying to answer this question.
The orbit of Mercury is found by observations to have a slight
motion which mathematicians have vainly tried to explain. For some
time it was supposed to be caused by the attraction of an unknown
planet between Mercury and the sun, and some were so sure of the
existence of this planet that they gave it a name, calling it
Vulcan. But of late years it has become reasonably certain that no
planet large enough to produce the effect observed can be there.
So thoroughly has every possible explanation been sifted out and
found wanting, that some astronomers are now inquiring whether the
law of gravitation itself may not be a little different from what
has always been supposed.
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