THE MODERN VIEW
It is still well under four hundred years since the modern, or
Copernican, theory of the universe supplanted the Ptolemaic, which had
held sway during so many centuries. In this new theory, propounded
towards the middle of the sixteenth century by Nicholas Copernicus
(1473-1543), a Prussian astronomer, the earth was dethroned from its
central position and considered merely as one of a number of planetary
bodies which revolve around the sun. As it is not a part of our purpose
to follow in detail the history of the science, it seems advisable to
begin by stating in a broad fashion the conception of the universe as
accepted and believed in to-day.
The Sun, the most important of the celestial bodies so far as we are
concerned, occupies the central position; not, however, in the whole
universe, but only in that limited portion which is known as the Solar
System. Around it, in the following order outwards, circle the planets
Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
(see Fig. 2, p. 21). At an immense distance beyond the solar system, and
scattered irregularly through the depth of space, lie the stars. The two
first-mentioned members of the solar system, Mercury and Venus, are
known as the Inferior Planets; and in their courses about the sun, they
always keep well inside the path along which our earth moves. The
remaining members (exclusive of the earth) are called Superior Planets,
and their paths lie all outside that of the earth.
[Illustration: FIG. 2.--The Copernican theory of the Solar System.]
The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
known from all antiquity. Nothing then can bring home to us more
strongly the immense advance which has taken place in astronomy during
modern times than the fact that it is only 127 years since observation
of the skies first added a planet to that time-honoured number. It was
indeed on the 13th of March 1781, while engaged in observing the
constellation of the Twins, that the justly famous Sir William Herschel
caught sight of an object which he did not recognise as having met with
before. He at first took it for a comet, but observations of its
movements during a few days showed it to be a planet. This body, which
the power of the telescope alone had thus shown to belong to the solar
family, has since become known to science under the name of Uranus. By
its discovery the hitherto accepted limits of the solar system were at
once pushed out to twice their former extent, and the hope naturally
arose that other planets would quickly reveal themselves in the
immensities beyond.
For a number of years prior to Herschel's great discovery, it had been
noticed that the distances at which the then known planets circulated
appeared to be arranged in a somewhat orderly progression outwards from
the sun. This seeming plan, known to astronomers by the name of Bode's
Law, was closely confirmed by the distance of the new planet Uranus.
There still lay, however, a broad gap between the planets Mars and
Jupiter. Had another planet indeed circuited there, the solar system
would have presented an appearance of almost perfect order. But the void
between Mars and Jupiter was unfilled; the space in which one would
reasonably expect to find another planet circling was unaccountably
empty.
On the first day of the nineteenth century the mystery was however
explained, a body being discovered[1] which revolved in the space that
had hitherto been considered planetless. But it was a tiny globe hardly
worthy of the name of planet. In the following year a second body was
discovered revolving in the same space; but it was even smaller in size
than the first. During the ensuing five years two more of these little
planets were discovered. Then came a pause, no more such bodies being
added to the system until half-way through the century, when suddenly
the discovery of these so-called "minor planets" began anew. Since then
additions to this portion of our system have rained thick and fast. The
small bodies have received the name of Asteroids or Planetoids; and up
to the present time some six hundred of them are known to exist, all
revolving in the previously unfilled space between Mars and Jupiter.
In the year 1846 the outer boundary of the solar system was again
extended by the discovery that a great planet circulated beyond Uranus.
The new body, which received the name of Neptune, was brought to light
as the result of calculations made at the same time, though quite
independently, by the Cambridge mathematician Adams, and the French
astronomer Le Verrier. The discovery of Neptune differed, however, from
that of Uranus in the following respect. Uranus was found merely in the
course of ordinary telescopic survey of the heavens. The position of
Neptune, on the other hand, was predicted as the result of rigorous
mathematical investigations undertaken with the object of fixing the
position of an unseen and still more distant body, the attraction of
which, in passing by, was disturbing the position of Uranus in its
revolution around the sun. Adams actually completed his investigation
first; but a delay at Cambridge in examining that portion of the sky,
where he announced that the body ought just then to be, allowed France
to snatch the honour of discovery, and the new planet was found by the
observer Galle at Berlin, very near the place in the heavens which Le
Verrier had mathematically predicted for it.
Nearly fifty years later, that is to say, in our own time, another
important planetary discovery was made. One of the recent additions to
the numerous and constantly increasing family of the asteroids, a tiny
body brought to light in 1898, turned out after all not to be
circulating in the customary space between Mars and Jupiter, but
actually in that between our earth and Mars. This body is very small,
not more than about twenty miles across. It has received the name of
Eros (the Greek equivalent for Cupid), in allusion to its insignificant
size as compared with the other leading members of the system.
This completes the list of the planets which, so far, have revealed
themselves to us. Whether others exist time alone will show. Two or
three have been suspected to revolve beyond the path of Neptune; and it
has even been asserted, on more than one occasion, that a planet
circulates nearer to the sun than Mercury. This supposed body, to which
the name of "Vulcan" was provisionally given, is said to have been
"discovered" in 1859 by a French doctor named Lescarbault, of Orgères
near Orleans; but up to the present there has been no sufficient
evidence of its existence. The reason why such uncertainty should exist
upon this point is easy enough to understand, when we consider the
overpowering glare which fills our atmosphere all around the sun's place
in the sky. Mercury, the nearest known planet to the sun, is for this
reason always very difficult to see; and even when, in its course, it
gets sufficiently far from the sun to be left for a short time above the
horizon after sunset, it is by no means an easy object to observe on
account of the mists which usually hang about low down near the earth.
One opportunity, however, offers itself from time to time to solve the
riddle of an "intra-Mercurial" planet, that is to say, of a planet which
circulates within the path followed by Mercury. The opportunity in
question is furnished by a total eclipse of the sun; for when, during an
eclipse of that kind, the body of the moon for a few minutes entirely
hides the sun's face, and the dazzling glare is thus completely cut off,
astronomers are enabled to give an unimpeded, though all too hurried,
search to the region close around. A goodly number of total eclipses of
the sun have, however, come and gone since the days of Lescarbault, and
no planet, so far, has revealed itself in the intra-Mercurial space. It
seems, therefore, quite safe to affirm that no globe of sufficient size
to be seen by means of our modern telescopes circulates nearer to the
sun than the planet Mercury.
Next in importance to the planets, as permanent members of the solar
system, come the relatively small and secondary bodies known by the name
of Satellites. The name _satellite_ is derived from a Latin word
signifying _an attendant_; for the bodies so-called move along always in
close proximity to their respective "primaries," as the planets which
they accompany are technically termed.
The satellites cannot be considered as allotted with any particular
regularity among the various members of the system; several of the
planets, for instance, having a goodly number of these bodies
accompanying them, while others have but one or two, and some again have
none at all. Taking the planets in their order of distance outward from
the Sun, we find that neither Mercury nor Venus are provided with
satellites; the Earth has only one, viz. our neighbour the Moon; while
Mars has but two tiny ones, so small indeed that one might imagine them
to be merely asteroids, which had wandered out of their proper region
and attached themselves to that planet. For the rest, so far as we at
present know, Jupiter possesses seven,[2] Saturn ten, Uranus four, and
Neptune one. It is indeed possible, nay more, it is extremely probable,
that the two last-named planets have a greater number of these secondary
bodies revolving around them; but, unfortunately, the Uranian and
Neptunian systems are at such immense distances from us, that even the
magnificent telescopes of to-day can extract very little information
concerning them.
From the distribution of the satellites, the reader will notice that the
planets relatively near to the sun are provided with few or none, while
the more distant planets are richly endowed. The conclusion, therefore,
seems to be that nearness to the sun is in some way unfavourable either
to the production, or to the continued existence, of satellites.
A planet and its satellites form a repetition of the solar system on a
tiny scale. Just as the planets revolve around the sun, so do these
secondary bodies revolve around their primaries. When Galileo, in 1610,
turned his newly invented telescope upon Jupiter, he quickly recognised
in the four circling moons which met his gaze, a miniature edition of
the solar system.
Besides the planets and their satellites, there are two other classes of
bodies which claim membership of the solar system. These are Comets and
Meteors. Comets differ from the bodies which we have just been
describing in that they appear filmy and transparent, whereas the others
are solid and opaque. Again, the paths of the planets around the sun and
of the satellites around their primaries are not actually circles; they
are ovals, but their ovalness is not of a marked degree. The paths of
comets on the other hand are usually _very_ oval; so that in their
courses many of them pass out as far as the known limits of the solar
system, and even far beyond. It should be mentioned that nowadays the
tendency is to consider comets as permanent members of the system,
though this was formerly not by any means an article of faith with
astronomers.
Meteors are very small bodies, as a rule perhaps no larger than pebbles,
which move about unseen in space, and of which we do not become aware
until they arrive very close to the earth. They are then made visible to
us for a moment or two in consequence of being heated to a white heat by
the friction of rushing through the atmosphere, and are thus usually
turned into ashes and vapour long before they reach the surface of our
globe. Though occasionally a meteoric body survives the fiery ordeal,
and reaches the earth more or less in a solid state to bury itself deep
in the soil, the majority of these celestial visitants constitute no
source of danger whatever for us. Any one who will take the trouble to
gaze at the sky for a short time on a clear night, is fairly certain to
be rewarded with the view of a meteor. The impression received is as if
one of the stars had suddenly left its accustomed place, and dashed
across the heavens, leaving in its course a trail of light. It is for
this reason that meteors are popularly known under the name of "shooting
stars."
[1] By the Italian astronomer, Piazzi, at Palermo.
[2] Probably eight. (See note, page 232.)