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Page 38
Suffice it to say that when Uranus was discovered by the elder
Herschel in 1781, that world was supposed to be the outside planet of
our system. Hitherto the splendid Saturn had marked the uttermost
excursion of astronomical knowledge as it respected our solar group.
For about a quarter of a century after Herschel's discovery the world
rested upon it as a finality. The orbit of Uranus was thought to
circumscribe the whole. But in the meantime, observations of this
orbit led to the knowledge that it did not conform in all respects to
astronomical and mathematical conditions. The orbit showed
irregularities, disturbances, perturbations, that could not be
accounted for when all of the known mathematical calculations were
applied thereto. Uranus was seen to get out of his path. At times he
would lag a little, and then at other times appear to be accelerated.
Each year, when the earth would swing around on the Uranian side of
the sun, the observations were renewed, but always with the result
that the planet did not seem to conform perfectly to the conditions of
his orbit. What could be the cause of this seeming disregard of
mathematical laws?
Astronomers could not accept the supposition that there was any actual
violation of the known conditions of gravitation. Certainly Uranus was
following his orbit under the centripetal and centrifugal laws in the
same manner as the other planets. There must, therefore, be some
undiscovered disturbing cause. It had already been noted that in the
case of the infra-Uranian planets they were swayed somewhat from their
paths by the mutual influence of one upon the other. This was
noticeable in particular in the movements of Jupiter, Saturn and
Uranus. When Saturn, for instance, would be on the same side of the
sun with Jupiter, it might be noted that the latter was drawn outward
and the former inward from their prescribed curves. The perturbation
was greatest when the planets were nearest, together. In like manner
Uranus did obeisance to both his huge neighbors on the sun's side of
his orbit. He, too, veered toward them as he passed, and they in turn
recognized the courtesy by going out of their orbits as they passed.
What, therefore, should be said of the outswinging movement of Uranus
from his orbit in that part of his course where no disturbing
influence was known to exist? Certainly _something_ must be in that
quarter of space to occasion the perturbation. What was it?
It would appear that the elder Bouvart, the French astronomer referred
to above, was the first to suggest that the disturbances in the orbit
of Uranus, throwing that planet from his pathway outward, might be and
probably were to be explained by the presence in outer space of an
unknown ultra-Uranian planet. Bouvart prepared tables to show the
perturbations in question, and declared his opinion that they were
caused by an unknown planet beyond. No observer, however, undertook to
verify this suggestion or to disprove it. Nor did Bouvart go so far as
to indicate the particular part of the heavens which should be
explored in order to find the undiscovered world. His tables, however,
do show from the perturbations of the orbits of Jupiter, Saturn and
Uranus that the same are caused by the mutual influence of the planets
upon one another.
It seems to have remained for Dr. T.J. Hussey, of Hayes, England, to
suggest the actual discovery of the unknown planet by following the
clew of the disturbance produced by its presence in a certain field of
space. Dr. Hussey, in 1834, wrote to Sir George Biddell Airy,
astronomer royal at Greenwich, suggesting that the perturbation of the
orbit of Uranus might be used as the clew for the discovery of the
planet beyond. But Sir George was one of those safe, conservative
scholars who scorn to follow the suggestions of genius, preferring
rather to explore only what is known already. He said in answer that
he doubted if the irregularity in the Uranian orbit was in such a
state of demonstration as to give any hope of the discovery of the
disturbing cause. He doubted even that there was such irregularity in
the Uranian orbit. He was of opinion that the observers had been
mistaken in the alleged detection of perturbations. So the Greenwich
observatory was not used on the line of exploration suggested by
Hussey.
Three years afterward, and again in 1842, Sir George received letters
from the younger Bouvart, again suggesting the possibility and
probability of discovering the ultra-Uranian planet. These hints were
strengthened by a letter from Bessel, of K�nigsberg. But Sir George B.
Airy refused to be led in the direction of so great a possibility.
It was in 1844 that Professor James Challis, of the Cambridge
observatory, appealed to Sir George for the privilege of using or
examining the recorded observations made at Greenwich of the movements
of Uranus, saying that he wished these tables for a young friend of
his, Mr. John C. Adams, of Cambridge, who had but recently taken his
degree in mathematics. Adams was at that date only twenty-five years
of age. The royal astronomer granted the request, and for about a year
Adams was engaged in making his calculations. These were completed,
and in September of 1845, Challis informed Sir George Airy that
according to the calculations of Adams the perturbations of Uranus
were due to the influence of an unknown planet beyond.
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