THE FARTHEST PLANETS
On the 13th of March, 1781, between 10 and 11 P. M., as Sir William
Herschel was sweeping the constellation Gemini with one of his great
reflecting telescopes, one star among all that passed through the field
of view attracted his attention. Removing the eyepiece and applying
another with a higher magnifying power, he found that, unlike all the
other stars, this one had a small disk and was not a mere point of
light, as all the fixed stars seem to be.
A few nights' observation showed that the stranger was moving among the
stars, so he thought it must be a comet; but a week's observation
following showed that he had discovered a new member of the planetary
system, far out beyond Saturn, which from time immemorial had been
assumed to be the outermost planet of all. This, then, was the first
real discovery of a planet, as the finding of the satellites of Jupiter
had been the first of all astronomical discoveries. Herschel's discovery
occasioned great excitement, and he named the new planet Georgium Sidus
or the Georgian, after his King. The King created him a knight and gave
him a pension, besides providing the means for building a huge
telescope, 40 feet long, with which he subsequently made many other
astronomical discoveries. The planet that Herschel discovered is now
called Uranus.
Uranus is an object not wholly impossible to see with the naked eye, if
the sky background is clear and black, and one knows exactly where to
look for it. Its brightness is about that of a sixth magnitude star or a
little fainter. Its average distance from the sun is about 1,800 million
miles and it takes eighty-four years to complete its journey round the
sun, traveling only a little more than four miles a second. When we
examine Uranus closely with a large telescope, we find a small disk
slightly greenish in tint, very slightly flattened, and at times faint
bands or belts are apparently seen. Uranus is about 30,000 miles in
diameter, and is probably surrounded by a dense atmosphere. Its rotation
time is 10 h. 50 m.
Uranus is attended by four moons or satellites, named Ariel, Umbriel,
Titania, and Oberon, the last being the most remote from the planet.
This system of satellites has a remarkable peculiarity: the plane of the
orbits in which they travel round Uranus is inclined about 80 degrees to
the plane of the ecliptic, so that the satellites travel backward, or in
a retrograde direction; or we might regard their motion as forward, or
direct, if we considered the planes of their orbits inclined at 100
degrees.
For many years after the discovery of Uranus it was thought that all the
great bodies of the solar system had surely been found. Least of all was
any planet suspected beyond Uranus until the mathematical tables of the
motion of Uranus, although built up and revised with the greatest care
and thoroughness, began to show that some outside influence was
disturbing it in accordance with Newton's law of gravitation. The
attraction of a still more distant planet would account for the
disturbance, and since no such planet was visible anywhere a
mathematical search for it was begun.
NEPTUNE
Wholly independently of each other, two young astronomers, Adams of
England and Le Verrier of France, undertook to solve the unique problem
of finding out the position in the sky where a planet might be found
that would exactly account for the irregular motion of Uranus. Both
reached practically identical results. Adams was first in point of time,
and his announcement led to the earliest observation, without
recognition of the new planet (July 30, 1846), although it was Le
Verrier's work that led directly to the new planet's being first seen
and recognized as such (September 23, 1846). Figuring backward, it was
found that the planet had been accidentally observed in Paris in 1795,
but its planetary character had been overlooked.
Neptune is the name finally assigned to this historical planet. It is
thirty times farther from the sun than the earth, or 2,800 million
miles; its velocity in orbit is a little over three miles per second,
and it consumes 164 years in going once completely round the sun. So
faint is it that a telescope of large size is necessary to show it
plainly. The brightness equals that of a star of the eighth magnitude,
and with a telescope of sufficient magnifying power, the tiny disk can
be seen and measured. The planet is about 30,000 miles in diameter, and
is not known to possess more than one moon or satellite. If there are
others, they are probably too faint to be seen by any telescope at
present in existence.