THE SOLAR SYSTEM
We have seen, in the course of the last chapter, that the solar system
is composed as follows:--there is a central body, the sun, around which
revolve along stated paths a number of important bodies known as
planets. Certain of these planets, in their courses, carry along in
company still smaller bodies called satellites, which revolve around
them. With regard, however, to the remaining members of the system, viz.
the comets and the meteors, it is not advisable at this stage to add
more to what has been said in the preceding chapter. For the time being,
therefore, we will devote our attention merely to the sun, the planets,
and the satellites.
Of what shape then are these bodies? Of one shape, and that one alone
which appears to characterise all solid objects in the celestial spaces:
they are spherical, which means _round like a ball_.
Each of these spherical bodies rotates; that is to say, turns round and
round, as a top does when it is spinning. This rotation is said to take
place "upon an axis," a statement which may be explained as
follows:--Imagine a ball with a knitting-needle run right through its
centre. Then imagine this needle held pointing in one fixed direction
while the ball is turned round and round. Well, it is the same thing
with the earth. As it journeys about the sun, it keeps turning round and
round continually as if pivoted upon a mighty knitting needle
transfixing it from North Pole to South Pole. In reality, however, there
is no such material axis to regulate the constant direction of the
rotation, just as there are no actual supports to uphold the earth
itself in space. The causes which keep the celestial spheres poised, and
which control their motions, are far more wonderful than any mechanical
device.
At this juncture it will be well to emphasise the sharp distinction
between the terms _rotation_ and _revolution_. The term "rotation" is
invariably used by astronomers to signify the motion which a celestial
body has upon an axis; the term "revolution," on the other hand, is used
for the movement of one celestial body around another. Speaking of the
earth, for instance, we say, that it _rotates_ on its axis, and that it
_revolves_ around the sun.
So far, nothing has been said about the sizes of the members of our
system. Is there any stock size, any pattern according to which they may
be judged? None whatever! They vary enormously. Very much the largest of
all is the Sun, which is several hundred times larger than all the
planets and satellites of the system rolled together. Next comes Jupiter
and afterwards the other planets in the following order of
size:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
Very much smaller than any of these are the asteroids, of which Ceres,
the largest, is less than 500 miles in diameter. It is, by the way, a
strange fact that the zone of asteroids should mark the separation of
the small planets from the giant ones. The following table, giving
roughly the various diameters of the sun and the principal planets in
miles, will clearly illustrate the great discrepancy in size which
prevails in the system.
Sun 866,540 miles
Mercury 2,765 "
Venus 7,826 "
Earth 7,918 "
Mars 4,332 "
ZONE OF ASTEROIDS
Jupiter 87,380 "
Saturn 73,125 "
Uranus[3] 34,900 "
Neptune[3] 32,900 "
It does not seem possible to arrive at any generalisation from the above
data, except it be to state that there is a continuous increase in size
from Mercury to the earth, and a similar decrease in size from Jupiter
outwards. Were Mars greater than the earth, the planets could then with
truth be said to increase in size up to Jupiter, and then to decrease.
But the zone of asteroids, and the relative smallness of Mars, negative
any attempt to regard the dimensions of the planets as an orderly
sequence.
Next with respect to relative distance from the sun, Venus circulates
nearly twice as far from it as Mercury, the earth nearly three times as
far, and Mars nearly four times. After this, just as we found a sudden
increase in size, so do we meet with a sudden increase in distance.
Jupiter, for instance, is about thirteen times as far as Mercury, Saturn
about twenty-five times, Uranus about forty-nine times, and Neptune
about seventy-seven. (See Fig. 2, p. 21.)
It will thus be seen how enormously the solar system was enlarged in
extent by the discovery of the outermost planets. The finding of Uranus
plainly doubled its breadth; the finding of Neptune made it more than
half as broad again. Nothing indeed can better show the import of these
great discoveries than to take a pair of compasses and roughly set out
the above relative paths in a series of concentric circles upon a large
sheet of paper, and then to consider that the path of Saturn was the
supposed boundary of our solar system prior to the year 1781.
We have seen that the usual shape of celestial bodies themselves is
spherical. Of what form then are their paths, or _orbits_, as these are
called? One might be inclined at a venture to answer "circular," but
this is not the case. The orbits of the planets cannot be regarded as
true circles. They are ovals, or, to speak in technical language,
"ellipses." Their ovalness or "ellipticity" is, however, in each case
not by any means of the same degree. Some orbits--for instance, that of
the earth--differ only slightly from circles; while others--those of
Mars or Mercury, for example--are markedly elliptic. The orbit of the
tiny planet Eros is, however, far and away the most elliptic of all, as
we shall see when we come to deal with that little planet in detail.
It has been stated that the sun and planets are always rotating. The
various rates at which they do so will, however, be best appreciated by
a comparison with the rate at which the earth itself rotates.
But first of all, let us see what ground we have, if any, for asserting
that the earth rotates at all?
If we carefully watch the heavens we notice that the background of the
sky, with all the brilliant objects which sparkle in it, appears to turn
once round us in about twenty-four hours; and that the pivot upon which
this movement takes place is situated somewhere near what is known to us
as the _Pole Star_. This was one of the earliest facts noted with regard
to the sky; and to the men of old it therefore seems as if the heavens
and all therein were always revolving around the earth. It was natural
enough for them to take this view, for they had not the slightest idea
of the immense distance of the celestial bodies, and in the absence of
any knowledge of the kind they were inclined to imagine them
comparatively near. It was indeed only after the lapse of many
centuries, when men had at last realised the enormous gulf which
separated them from even the nearest object in the sky, that a more
reasonable opinion began to prevail. It was then seen that this
revolution of the heavens about the earth could be more easily and more
satisfactorily explained by supposing a mere rotation of the solid earth
about a fixed axis, pointed in the direction of the polar star. The
probability of such a rotation on the part of the earth itself was
further strengthened by the observations made with the telescope. When
the surfaces of the sun and planets were carefully studied these bodies
were seen to be rotating. This being the case, there could not surely
be much hesitation in granting that the earth rotated also; particularly
when it so simply explained the daily movement of the sky, and saved men
from the almost inconceivable notion that the whole stupendous vaulted
heaven was turning about them once in every twenty-four hours.
If the sun be regularly observed through a telescope, it will gradually
be gathered from the slow displacement of sunspots across its face,
their disappearance at one edge and their reappearance again at the
other edge, that it is rotating on an axis in a period of about
twenty-six days. The movement, too, of various well-known markings on
the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
these bodies are rotating in periods, which are about twenty-four hours
for the first, and about ten hours for each of the other two. With
regard, however, to Uranus and Neptune there is much more uncertainty,
as these planets are at such great distances that even our best
telescopes give but a confused view of the markings which they display;
still a period of rotation of from ten to twelve hours appears to be
accepted for them. On the other hand the constant blaze of sunlight in
the neighbourhood of Mercury and Venus equally hampers astronomers in
this quest. The older telescopic observers considered that the rotation
periods of these two planets were about the same as that of the earth;
but of recent years the opinion has been gaining ground that they turn
round on their axes in exactly the same time as they revolve about the
sun. This question is, however, a very doubtful one, and will be again
referred to later on; but, putting it on one side, it will be seen from
what we have said above, that the rotation periods of the other planets
of our system are usually about twenty-four hours, or under. The fact
that the rotation period of the sun should run into _days_ need not seem
extraordinary when one considers its enormous size.
The periods taken by the various planets to revolve around the sun is
the next point which has to be considered. Here, too, it is well to
start with the earth's period of revolution as the standard, and to see
how the periods taken by the other planets compare with it.
The earth takes about 365-1/4 days to revolve around the sun. This
period of time is known to us as a "year." The following table shows in
days and years the periods taken by each of the other planets to make a
complete revolution round the sun:--
Mercury about 88 days.
Venus " 226 "
Mars " 1 year and 321 days.
Jupiter " 11 years and 313 days.
Saturn " 29 years and 167 days.
Uranus " 84 years and 7 days.
Neptune " 164 years and 284 days.
From these periods we gather an important fact, namely, that the nearer
a planet is to the sun the faster it revolves.
Compared with one of our years what a long time does an Uranian, or
Neptunian, "year" seem? For instance, if a "year" had commenced in
Neptune about the middle of the reign of George II., that "year" would
be only just coming to a close; for the planet is but now arriving back
to the position, with regard to the sun, which it then occupied. Uranus,
too, has only completed a little more than 1-1/2 of its "years" since
Herschel discovered it.
Having accepted the fact that the planets are revolving around the sun,
the next point to be inquired into is:--What are the positions of their
orbits, or paths, relatively to each other?
Suppose, for instance, the various planetary orbits to be represented by
a set of hoops of different sizes, placed one within the other, and the
sun by a small ball in the middle of the whole; in what positions will
these hoops have to be arranged so as to imitate exactly the true
condition of things?
First of all let us suppose the entire arrangement, ball and hoops, to
be on one level, so to speak. This may be easily compassed by imagining
the hoops as floating, one surrounding the other, with the ball in the
middle of all, upon the surface of still water. Such a set of objects
would be described in astronomical parlance as being _in the same
plane_. Suppose, on the other hand, that some of these floating hoops
are tilted with regard to the others, so that one half of a hoop rises
out of the water and the other half consequently sinks beneath the
surface. This indeed is the actual case with regard to the planetary
orbits. They do not by any means lie all exactly in the same plane. Each
one of them is tilted, or _inclined_, a little with respect to the plane
of the earth's orbit, which astronomers, for convenience, regard as the
_level_ of the solar system. This tilting, or "inclination," is, in the
larger planets, greatest for the orbit of Mercury, least for that of
Uranus. Mercury's orbit is inclined to that of the earth at an angle of
about 7°, that of Venus at a little over 3°, that of Saturn 2-1/2°;
while in those of Mars, Neptune, and Jupiter the inclination is less
than 2°. But greater than any of these is the inclination of the orbit
of the tiny planet Eros, viz. nearly 11°.
The systems of satellites revolving around their respective planets
being, as we have already pointed out, mere miniature editions of the
solar system, the considerations so far detailed, which regulate the
behaviour of the planets in their relations to the sun, will of
necessity apply to the satellites very closely. In one respect, however,
a system of satellites differs materially from a system of planets. The
central body around which planets are in motion is self-luminous,
whereas the planetary body around which a satellite revolves is not.
True, planets shine, and shine very brightly too; as, for instance,
Venus and Jupiter. But they do not give forth any light of their own, as
the sun does; they merely reflect the sunlight which they receive from
him. Putting this one fact aside, the analogy between the planetary
system and a satellite system is remarkable. The satellites are
spherical in form, and differ markedly in size; they rotate, so far as
we know, upon their axes in varying times; they revolve around their
governing planets in orbits, not circular, but elliptic; and these
orbits, furthermore, do not of necessity lie in the same plane. Last of
all the satellites revolve around their primaries at rates which are
directly comparable with those at which the planets revolve around the
sun, the rule in fact holding good that the nearer a satellite is to its
primary the faster it revolves.
[3] As there seems to be much difference of opinion concerning the
diameters of Uranus and Neptune, it should here be mentioned that the
above figures are taken from Professor F.R. Moulton's _Introduction to
Astronomy_ (1906). They are there stated to be given on the authority of
"Barnard's many measures at the Lick Observatory."