URANUS.
To the Ancients Saturn was the outermost planet of the System, nothing
beyond it being known. Nor indeed was it to be assumed that any more
could possibly exist, because Mercury, Venus, the Earth, Mars, Jupiter,
and Saturn, with the Sun, made 7 celestial bodies of prime importance;
and 7 was the number of perfection; and there was thus provided one
celestial body to give a name to each of the days of the week.
But Science is not sentimental; and when men of Science come upon what
looks like a discovery they do their best to bring their discovery to a
successful issue, however much people’s prejudices may seem to stand in
the way at the moment.
On a certain evening in March, 1781, Sir William Herschel, then
gradually coming into notice as a practical astronomer, was engaged in
looking at different fields of stars in the constellation Gemini when he
lighted on one which at once attracted his special attention. Altering
his eyepiece, and substituting a higher magnifying power he found the
apparent size of the mysterious object enlarged, which conclusively
proved that it was not a star; for it is a well-known optical property
of all stars that whatever be the size of telescope employed on them,
and however high the magnifying power no definite disc of light can be
obtained when in focus. Herschel’s new find, therefore, was plainly not
a star, and no idea having in those days come into men’s minds of there
being any new planets awaiting discovery, he announced as a matter of
course that he had found a new comet, so soon as he ascertained that the
new body was in motion. The announcement was not made to the Royal
Society till April 26, more than six weeks after the date of the actual
discovery, an indication, by the way, of the dilatory circulation of
news a hundred years ago. The supposed comet was observed by Maskelyne,
the Astronomer Royal, four days after Herschel had first seen it, and
Maskelyne seems to have at once got the idea into his head that he was
looking at a planet and not at a comet. As soon as possible after the
discovery of a new comet the practice of astronomers is to endeavour to
determine what is the shape of the orbit which it is pursuing. All
attempts to carry out this in the case of Herschel’s supposed new comet
proved abortive, because it was found impossible to harmonise, except
for a short period of time, the movements of the new body with the form
of curve usually affected by most comets, namely, the parabola. It is
true, as we shall see later on in speaking of comets, that a certain
number of those bodies do revolve in the closed curve known as the
ellipse, but it is usual to calculate the parabolic form first of all,
because it is the easier to calculate; and to persevere with it until it
plainly appears that the parabola will not fit in with the observed
movements of the new object. This practice was carried out in the case
of Herschel’s new body, and it was eventually found that not only was
its orbit not parabolic; that not only was its orbit not an elongated
ellipse of the kind affected by comets; but that it was nearly a circle,
and as the body itself showed a defined disc the conclusion was
inevitable: it was in real truth a new planet. It has not taken long to
write this statement, and it will take still less time for the reader to
read what has been written, but the result just mentioned occupied the
attention of astronomers many months in working out, step by step, in
such a way as to make sure that no mistake had been made.
When it was once clearly determined that Herschel had added a new planet
to the list of known planets it became an interesting matter of inquiry
to find out whether it had ever been seen before; and to settle the name
it should bear. A little research soon showed that the new planet had
been seen and recorded as a fixed star by various observers on 20
previous occasions, beginning as far back as Dec. 13, 1690, when
Flamstead registered at Greenwich as a star. These various observations,
spread over a period of 91 years, and all recorded by observers of skill
and eminence materially helped astronomers in their efforts to calculate
accurately the shape and nature of the new planet’s orbit. One observer,
a Frenchman named Le Monnier, saw the planet no less than 12 times
between 1750 and 1771, and if he had had (which it is known he had not)
an orderly and methodical mind, the glory of this discovery would have
been lost to England and obtained by France. Arago has left it on record
that he was once shown one of these chance observations of Uranus, which
had been recorded by Le Monnier on an old paper bag in which hair powder
had been sold by a perfumer.
A long discussion took place on the question of a name for the new
planet. Bode’s suggestion of “Uranus” is now in universal use, but it is
within the recollection of many persons living that this planet bore
sometimes the name of the “Georgium Sidus” and sometimes the name of
“Herschel.” The former designation was proposed by Herschel himself in
compliment to his sovereign and patron George III. of England; whilst a
French astronomer suggested the latter name. However, neither of these
appellations was acceptable to the astronomers of the Continent, who
declared in favour of a mythological name, though it was a long time
before they agreed to accept Bode’s “Uranus.” The symbol commonly used
to represent the planet is formed of Herschel’s initial with a little
circle added below, though the Germans employ something else, “made in
Germany,” to quote a too familiar phrase.
The visible disc of Uranus is so small that none but telescopes of the
very largest size can make anything of it. A few sentences therefore
will dispose of this part of the subject. The disc is usually bluish in
tinge, and most observers who look at it consider it uniformly bright,
but there is satisfactory testimony to the effect that under the most
favourable circumstances of instrument and atmosphere two or more belts,
not unlike the belts of Jupiter, may be traced. From the position in
which these belts have been seen it is inferred that the satellites of
Uranus (presently to be mentioned) are unusually much inclined to the
planet’s equator, and revolve in a retrograde direction, contrary to
what is the ordinary rule of the planets and satellites. It is assumed
as the basis of these ideas, (and by analogy it is reasonable to do
this) that the belts are practically parallel to the planet’s equator,
and at right angles to the planet’s axis of rotation. To speak of the
planet’s axis of rotation is, in one sense, another assumption, because
available observations can scarcely be said to enable us to demonstrate
that the planet does rotate on its axis, yet we can have no moral doubt
about it. Taylor has suggested grounds for the opinion that “there can
be very little doubt that Uranus is to a very large extent
self-luminous, and that we do not see it wholly by reflected light.” To
this Gore adds the idea that there is “strong evidence in favour of the
existence of intrinsic heat in the planet.”
Uranus is attended by several satellites. It was once thought that there
were eight, of which six were due to Sir W. Herschel, the other two
being of modern discovery. Astronomers are, however, now agreed that no
more than four satellites can justly be recognised as known to exist,
and they are so minute in size that only the very largest telescopes
will show them; and therefore our knowledge of them is extremely
limited. Sir W. Herschel’s idea that he had seen six satellites appears
to have resulted from his having on some occasions mistaken some very
small stars for satellites. Two only of his six are thought to have been
real satellites. The other two recognised satellites were found both in
1847, one by Lassell, and the other by O. Struve.
Uranus revolves round the Sun in rather more than 84 years, at a mean
distance of 1781 millions of miles. Its apparent diameter, seen from the
Earth, does not vary much from 3½″ which corresponds to about 31,000
miles. It has been calculated that the light received from the Sun by
Uranus would be about the amount furnished by 300 full Moons seen by us
on the Earth, though another authority increases this to 1670 full
Moons. From Uranus Saturn can be seen, and perhaps Jupiter, both as
inferior planets, just as we see Venus and Mercury; but all the other
inner planets, including Mars and the Earth, would be hopelessly lost to
view, because perpetually too close to the Sun. Possibly, however, they
might, on rare occasions, be seen in transit across the Sun’s disc.
Neptune, of course, would be visible and be the only superior planet.
The Sun itself would appear to an observer on Uranus as a very bright
star, with a disc of 1¾′ of arc in diameter.