Uranus, a planet unknown to the ancients, was discovered by its
movement among the stars by William Herschel in 1781. It was not until
1846 that another major planet was added to the solar system, and this
discovery was one of the sensations of the century.
The story of the independent discovery of Neptune by Adams and Le
Verrier, who were both driven to the conclusion that certain apparent
regularities in the motion of Uranus were due to the attraction of
another body travelling on an orbit outside it, has been often told.
The subsequent discovery of the external body not far from the place at
which their mathematical analysis had led them to believe it would be
seen, will forever be regarded as a fine triumph of the human intellect.
But the results of the inquiries which now concern us are generally
of not so sensational a character, although they lie at the root of
our knowledge of celestial motions. They more often take the shape of
tables and discussions relating to the movements of the bodies which
make up our solar system.
Gauss may be said to have led the way during the nineteenth century by
his _Theoria molus corporum coelestium solem ambientium_. This was a
worthy sequel to the _Méchanique Céleste_, in which work, towards the
end of the eighteenth century, Laplace had enshrined all that was known
on the planetary results of gravitation.
In later years Le Verrier and Newcomb have been among the chief workers
on whom the mantle of such distinguished predecessors has fallen. From
them the planet and satellite tables now in use have been derived.
But the motion of our own satellite, the moon, has had fascinations for
other analysts besides those we have named.
The problem, indeed, of the moon’s motion is one of the most difficult,
and has taxed the ingenuity of astronomers from an early date. Even at
the present day it is impossible to predict the exact position of the
moon at any one moment owing to inequalities and perturbations, the
exact varying values of which are not known.
The two most important theories of the motion of the moon completed
towards the middle of the century were due to Hansen and Delaunay.
The former’s appeared in 1838, the lunar tables being published later
(1857), while the latter’s was published in 1860.
Hansen’s theory had for its chief object the formation of tables;
to avoid the inconvenience of using in his calculations series which
slowly converge, he inserted numerical values throughout. In Hansen’s
solution the problem is one actually presented by nature, allowance
being made for every known cause of disturbance. There is one
disadvantage, namely, that should observations demand a change in any
of the constants used, there is no means of making any correction in
the results.
Delaunay’s theory surmounted this difficulty, but at the expense
of still greater inconvenience for making an ephemeris. The slow
convergence of certain series involved an immense amount of labor to
give sufficiently approximate results.
More recently, as the century was closing, Dr. Brown took up the
subject and made a fresh attempt to calculate the motion of our
satellite. It may be stated that he adopts all Delaunay’s modifications
of the problem and works them out algebraically; but there are many
technical differences which it would be out of place to mention here.
Enough has been stated to show that there is not likely to be any
breach of continuity in the treatment of this most important problem.
Another attack on the moon, and, incidentally, its motion, has recently
been made by another analyst, Professor George Darwin; grappling with
all the consequences of tidal friction, he has been able to present to
us the past and future history of our satellite. Beginning as a part of
the material congeries from which subsequently some fifty million years
ago both earth and moon, as separate bodies, were formed, it has ever
since been extending its orbit, and so retreating farther away from its
centre of motion, while the period of the earth’s rotation has been
increasing at the same time, from a possible period of some three hours
when the moon was born, to one of one thousand four hundred hours when
the day and month will be equal, something like one hundred and fifty
million years being required for the process.