n a tailless comet,
or possibly the newly found planet.

The next thing to do was to calculate backward, to see whether by any
possibility the planet could have been in that place at that time.
Examined in this way the tabulated observations of Flamsteed showed that
he had unwittingly observed Uranus five distinct times; the first time
in 1690, nearly a century before Herschel discovered its true nature.
But more remarkable still, Le Monnier, of Paris, had observed it eight
times in one month, cataloguing it each time as a different star. If
only he had reduced and compared his observations, he would have
anticipated Herschel by twelve years. As it was, he missed it. It was
seen once by Bradley also. Altogether it had been seen twenty times.

These old observations of Flamsteed and those of Le Monnier, combined
with those made after Herschel's discovery, were very useful in
determining an exact orbit for the new planet, and its motion was
considered thoroughly known. For a time Uranus seemed to travel
regularly, and as expected, in the orbit which had been calculated for
it; but early in the present century it began to be slightly refractory,
and by 1820 its actual place showed quite a distinct discrepancy from
its position as calculated with the aid of the old observations. 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 take