It was
thought at first that this discrepancy must be due to inaccuracies in
the older observations, and they were accordingly rejected, and tables
prepared for the planet based on the newer and more accurate
observations only. But by 1830 it became apparent that it did not
coincide with even these. The error amounted to about 20". By 1840 it
was as much as 90", or a minute and a half. This discrepancy is quite
distinct, but still it is very small; and had two objects been in the
heavens at once, the actual Uranus and the theoretical Uranus, no
unaided eye could possibly have distinguished them or detected that they
were other than a single star.
The errors of Uranus, though small, were enormously greater than other
things which had certainly been observed; there was an unmistakable
discrepancy between theory and observation. Some cause was evidently at
work on this distant planet, causing it to disagree with its motion as
calculated according to the law of gravitation. If the law of
gravitation held exactly at so great a distance from the sun, there must
be some perturbing force acting on it besides all the known forces that
had been fully taken into account.
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