FOUR LARGE WORLDS
I have told you about the four lesser worlds of which our earth is one,
and you know that beyond Mars, the last of them, there lies a vast
space, in which are found the asteroids, those strange small planets
circling near to each other, like a swarm of bees. After this there
comes Jupiter, who is so enormous, so superb in size compared with us,
that he might well serve as the sun of a little system of his own. You
remember that we represented him by a football, while the earth was only
a greengage plum. But Jupiter himself is far less in comparison with the
sun than we are in comparison with him. He differs from the planets we
have heard about up to the present in that he seems to glow with some
heat that he does not receive from the sun. The illumination which makes
him appear as a star to us is, of course, merely reflected sunlight, and
what we see is the external covering, his envelope of cloud.
There is every reason to believe that the great bulk of Jupiter is
still at a high temperature. We know that in the depths of the earth
there is still plenty of heat, which every now and then makes its
presence felt by bursting up through the vents we call volcanoes, the
weak spots in the earth's crust; but our surface long ago cooled, for
the outside of any body gets cool before the inside, as you may have
found if ever you were trying to eat hot porridge, and circled round the
edge of the plate with a spoon. A large body cools more slowly than a
small one, and it is possible that Jupiter, being so much larger than we
are, has taken longer to cool. One reason we have for thinking this is
that he is so very light compared with his size--in other words, his
density is so small that it is not possible he could be made of
materials such as the earth is made of.
As I said, when we study him through telescopes we see just the
exterior, the outer envelope of cloud, and as we should expect, this
changes continually, and appears as a series of belts, owing to the
rotation of the planet. Jupiter's rotation is very rapid; though he is
so much greater than the earth, he takes less than half the time the
earth does to turn round--that is to say, only ten hours. His days and
nights of five hours each seem short to us, accustomed to measure
things by our own estimates. But we must remember that everything is
relative; that is to say, there is really no such thing as fast or slow;
it is all by comparison. A spider runs fast compared with a snail, but
either is terribly slow compared with an express train; and the speed of
an express train itself is nothing to the velocity of light.
In the same way there is nothing absolutely great or small; it is all by
comparison. We say how marvellous it is that a little insect has all the
mechanism of life in its body when it is so tiny, but if we imagine that
insect magnified by a powerful microscope until it appears quite large,
the marvel ceases. Again, imagine a man walking on the surface of the
earth as seen from a great distance through a telescope: he would seem
less than an insect, and we might ask how could the mechanism of life be
compressed into anything so small? Thus, when we say enormous or tiny we
must always remember we are only speaking by the measurements of our own
standards.
There is nothing very striking about Jupiter's orbit. He takes between
eleven and twelve of our years to get round the sun, so you see, though
his day is shorter, his year is longer than ours. And this is not only
because his path is much larger, but because by the law of gravity the
more distant a planet is from the sun the more slowly it travels, so
that while the earth speeds over eighteen miles Jupiter has only done
eight. Of course, we must be careful to remember the difference between
rotation and revolution. Jupiter rotates much quicker than the
earth--that is to say, he turns round more quickly--but he actually gets
over the ground more slowly. The sun appears much smaller to him than it
does to us, and he receives considerably less light and heat. There are
various spots on his surface, and one remarkable feature is a dark mark,
which is called the 'great red spot.' If as we suppose what we see of
the planet is merely the cloudy upper atmosphere, we should not expect
to find anything permanent there, for the markings would change from day
to day, and this they do with this exception--that this spot, dark red
in colour, has been seen for many years, turning as the planet turned.
It was first noticed in 1878, and was supposed to be some great mountain
or excrescence peeping up through the clouds. It grew stronger and
darker for several years, and then seemed to fade, and was not so easily
seen, and though still remaining it is now pale. But, most startling
to say, it has shifted its position a little--that is, it takes a few
seconds longer to get round the planet than it did at first. A few
seconds, you will say, but that is nothing! It does not seem much, but
it shows how marvellously accurate astronomers are. Discoveries of vast
importance have been made from observing a few seconds' discrepancy in
the time the heavenly bodies take in their journeys, and the fact that
this spot takes a little longer in its rotation than it did at first
shows that it cannot be attached to the body of the planet. It is
impossible for it to be the summit of a mountain or anything of that
sort. What can it be? No one has yet answered that question.
[Illustration: JUPITER AND ONE OF HIS MOONS]
When we get to the chapter on the sun, we shall find curiosities
respecting the spots there as well.
Jupiter has seven moons, and four of these are comparatively large. They
have the honour of having been the first heavenly bodies ever actually
discovered, for the six large planets nearest the sun have been known so
long that there is no record of their first discovery, and of course our
own moon has always been known. Galileo, who invented the telescope,
turned it on to the sky in 1610, when our King Charles I. was on the
throne, and he saw these curious bodies which at first he could not
believe to be moons. The four which he saw vary in size from two
thousand one hundred miles in diameter to nearly three thousand six
hundred. You remember our own moon is two thousand miles across, so even
the smallest is larger than she. They go round at about the same level
as the planet's Equator, and therefore they cross right in front of him,
and go behind him once in every revolution. Since then the other three
have been discovered in the band of Jupiter's satellites--one a small
moon closer to him than any of the first set, and two others further
out. It was by observation of the first four, however, that very
interesting results were obtained. Mathematicians calculated the time
that these satellites ought to disappear behind Jupiter and reappear
again, but they found that this did not happen exactly at the time
predicted; sometimes the moons disappeared sooner than they should have
done, and sometimes later. Then this was discovered to have some
relation to the distance of our earth from Jupiter. When he was at the
far side of his immense orbit he was much more distant from us than when
he was on the nearer side--in fact, the difference may amount to more
than three hundred millions of miles. And it occurred to some clever man
that the irregularities in time we noticed in the eclipses of the
satellites corresponded with the distance of Jupiter from us. The
further he drew away from us, the later were the eclipses, and as he
came nearer they grew earlier. By a brilliant inspiration, this was
attributed to the time light took to travel from them to us, and this
was the first time anyone had been able to measure the velocity or speed
of light. For all practical purposes, on the earth's surface we hold
light to be instantaneous, and well we may, for light could travel more
than eight times round the world in one second. It makes one's brain
reel to think of such a thing. Then think how far Jupiter must be away
from us at the furthest, when you hear that sometimes these eclipses
were delayed seventeen minutes--minutes, not seconds--because it took
that time for light to cross the gulf to us!
[Illustration: JUPITER AND HIS PRINCIPAL MOONS.]
Sound is very slow compared with light, and that is why, if you watch a
man hammering at a distance, the stroke he gives the nail does not
coincide with the bang that reaches you, for light gets to you
practically at once, and the sound comes after it. No sound can travel
without air, as we have heard, therefore no sound reaches us across
space. If the moon were to blow up into a million pieces we should see
the amazing spectacle, but should hear nothing of it. Light travels
everywhere throughout the universe, and by the use of this universal
carrier we have learnt all that we know about the stars and planets.
When the time that light takes to travel had been ascertained by means
of Jupiter's satellites, a still more important problem could be
solved--that was our own distance from the sun, which before had only
been known approximately, and this was calculated to be ninety-two
millions seven hundred thousand miles, though sometimes we are a little
nearer and sometimes a little further away.
Jupiter is marvellous, but beyond him lies the most wonderful body in
the whole solar system. We have found curiosities on our way out: we
have studied the problem of the asteroids, of the little moon that goes
round Mars in less time than Mars himself rotates; we have considered
the 'great red spot' on Jupiter, which apparently moves independently
of the planet; but nothing have we found as yet to compare with the
rings of Saturn. May you see this amazing sight through a telescope one
day!
Look at the picture of this wonderful system, and think what it would be
like if the earth were surrounded with similar rings! The first question
which occurs to all of us is what must the sky look like from Saturn?
What must it be to look up overhead and see several great hoops or
arches extending from one horizon to another, reflecting light in
different degrees of intensity? It would be as if we saw several immense
rainbows, far larger than any earthly rainbow, and of pure light, not
split into colours, extending permanently across the sky, and now and
then broken by the black shadow of the planet itself as it came between
them and the sun. However, we must begin at the beginning, and find out
about Saturn himself before we puzzle ourselves over his rings. Saturn
is not a very great deal less than Jupiter, though, so small are the
other planets in comparison, that if Saturn and all the rest were rolled
together, they would not make one mass so bulky as Jupiter! Saturn is
so light--in other words, his density is so small--that he is actually
lighter than water. He is the lightest, in comparison with his size, of
any of the planets. Therefore he cannot be made largely of solid land,
as our earth is, but must be to a great extent, composed of air and
gaseous vapour, like his mighty neighbour. He approaches at times as
near to Jupiter as Jupiter does to us, and on these occasions he must
present a splendid spectacle to Jupiter. He takes no less than
twenty-nine and a half of our years to complete his stately march around
the sun, and his axis is a little more bent than ours; but, of course,
at his great distance from the sun, this cannot have the same effect on
the seasons that it does with us. Saturn turns fast on his axis, but not
so fast as Jupiter, and in turning his face, or what we call his
surface, presents much the same appearance to us that we might expect,
for it changes very frequently and looks like cloud belts.
The marvellous feature about Saturn is, of course, the rings. There are
three of these, lying one within the other, and separated by a fine line
from each other. The middle one is much the broadest, probably about ten
thousand miles in width, and the inner one, which is the darkest, was
not discovered until some time after the others. As the planet swings in
his orbit the rings naturally appear very different to us at different
times. Sometimes we can only see them edgewise, and then even in the
largest telescope they are only like a streak of light, and this shows
that they cannot be more than fifty or sixty miles in thickness. The one
which is nearest to Saturn's surface does not approach him within ten
thousand miles. Saturn has no less than ten satellites, in addition to
the rings, so that his midnight sky must present a magnificent
spectacle. The rings, which do not shine by their own light but by
reflected sunlight, are solid enough to throw a shadow on the body of
the planet, and themselves receive his shadow. Sometimes for days
together a large part of Saturn must suffer eclipse beneath the
encircling rings, but at other times, at night, when the rings are clear
of the planet's body, so that the light is not cut off from them, they
must appear as radiant arches of glory spanning the sky.
The subject of these rings is so complicated by the variety of their
changes that it is difficult for us even to think about it. It is one of
the most marvellous of all the features of our planetary system. What
are these rings? what are they made of? It has been positively proved
that they cannot be made of continuous matter, either liquid or solid,
for the force of gravity acting on them from the planet would tear them
to pieces. What, then, can they be? It is now pretty generally believed
that they are composed of multitudes of tiny bodies, each separate, and
circling separately round the great planet, as the asteroids circle
round the sun. As each one is detached from its neighbour and obeys its
own impulses, there is none of the strain and wrench there would be were
they all connected. According to the laws which govern planetary bodies,
those which are nearest to the planet will travel more quickly than
those which are further away. Of course, as we look at them from so
great a distance, and as they are moving, they appear to us to be
continuous. It is conjectured that the comparative darkness of the
inside ring is caused by the fact that there are fewer of the bodies
there to reflect the sunlight. Then, in addition to the rings, enough
themselves to distinguish him from all other planets, there are the ten
moons of richly-endowed Saturn to be considered. It is difficult to
gather much about these moons, on account of our great distance from
them. The largest is probably twice the diameter of our own moon. One of
them seems to be much brighter--that is to say, of higher reflecting
power--on one side than the other, and by distinguishing the sides
and watching carefully, astronomers have come to the conclusion that it
presents always the same face to Saturn in the same way as our own moon
does to us; in fact, there is reason to think that all the moons of
large planets do this.
[Illustration: THE PLANET SATURN WITH TWO OF HIS MOONS.]
All the moons lie outside the rings, and some at a very great distance
from Saturn, so that they can only appear small as seen from him. Yet at
the worst they must be brighter than ordinary stars, and add greatly to
the variations in the sky scenery of this beautiful planet. In
connection with Saturn's moons there is another of those astonishing
facts that are continually cropping up to remind us that, however much
we know, there is such a vast deal of which we are still ignorant. So
far in dealing with all the planets and moons in the solar system we
have made no remark on the way they rotate or revolve, because they all
go in the same direction, and that direction is called
counter-clockwise, which means that if you stand facing a clock and turn
your hand slowly round the opposite direction to that in which the hands
go, you will be turning it in the same way that the earth rotates on its
axis and revolves in its orbit. It is, perhaps, just as well to give
here a word of caution. Rotating of course means a planet's turning on
its own axis, revolving means its course in its orbit round the sun.
Mercury, Venus, Earth, Mars, Jupiter, and all their moons, as well as
Saturn himself, rotate on their axes in this one
direction--counter-clockwise--and revolve in the same direction as they
rotate. Even the queer little moon of Mars, which runs round him quicker
than he rotates, obeys this same rule. Nine of Saturn's moons follow
this example, but one independent little one, which has been named
Phoebe, and is far out from the planet, actually revolves in the
opposite way. We cannot see how it rotates, but if, as we said just now,
it turns the same face always to Saturn, then of course it rotates the
wrong way too. A theory has been suggested to account for this curious
fact, but it could not be made intelligible to anyone who has not
studied rather high mathematics, so there we must just leave it, and put
it in the cabinet of curiosities we have already collected on our way
out to Saturn.
For ages past men have known and watched the planets lying within the
orbit of Saturn, and they had made up their minds that this was the
limit of our system. But in 1781 a great astronomer named Herschel was
watching the heavens through a telescope when he noticed one strange
object that he was certain was no star. The vast distance of the stars
prevents their having any definite outline, or what is called a disc.
The rays dart out from them in all directions and there is no 'edge' to
them, but in the case of the planets it is possible to see a disc with a
telescope, and this object which attracted Herschel's attention had
certainly a disc. He did not imagine he had discovered a new planet,
because at that time the asteroids had not been found, and no one
thought that there could be any more planets. Yet Herschel knew that
this was not a star, so he called it a comet! He was actually the first
who discovered it, for he knew it was not a fixed star, but it was after
his announcement of this fact that some one else, observing it
carefully, found it to be a real planet with an orbit lying outside that
of Saturn, then the furthest boundary of the solar system. Herschel
suggested calling it Georgius Sidus, in honour of George III., then
King; but luckily this ponderous name was not adopted, and as the other
planets had been called after the Olympian deities, and Uranus was the
father of Saturn, it was called Uranus. It was subsequently found that
this new planet had already been observed by other astronomers and
catalogued as a star no less than seventeen times, but until Herschel's
clear sight had detected the difference between it and the fixed stars
no one had paid any attention to it. Uranus is very far away from the
sun, and can only sometimes be seen as a small star by people who know
exactly where to look for him. In fact, his distance from the sun is
nineteen times that of the earth.
Yet to show at all he must be of great size, and that size has actually
been found out by the most delicate experiments. If we go back to our
former comparison, we shall remember that if the earth were like a
greengage plum, then Uranus would be in comparison about the size of one
of those coloured balloons children play with; therefore he is much
larger than the earth.
In this far distant orbit the huge planet takes eighty-four of our years
to complete one of his own. A man on the earth will have grown from
babyhood to boyhood, from boyhood to the prime of life, and lived longer
than most men, while Uranus has only once circled in his path.
But in dealing with Uranus we come to another of those startling
problems of which astronomy is full. So far we have dealt with planets
which are more or less upright, which rotate with a rotation like that
of a top. Now take a top and lay it on one side on the table, with one
of its poles pointing toward the great lamp we used for the sun and the
other pointing away. That is the way Uranus gets round his path, on his
side! He rotates the wrong way round compared with the planets we have
already spoken of, but he revolves the same way round the sun that all
the others do. It seems wonderful that even so much can be found out
about a body so far from us, but we know more: we have discovered that
Uranus is made of lighter material than the earth; his density is less.
How can that be known? Well, you remember every body attracts every
other body in proportion to the atoms it contains. If, therefore, there
were any bodies near to Uranus, it could be calculated by his influence
on them what was his own mass, which, as you remember, is the word we
use to express what would be weight were it at the earth's surface; and
far away as Uranus is, the bodies from which such calculations may be
made have been discovered, for he has no less than four satellites, or
moons. Considering now the peculiar position of the planet, we might
expect to find these moons revolving in a very different way from
others, and this is indeed the case. They turn round the planet at
about its Equator--that is to say, if you hold the top representing
Uranus as was suggested just now, these moons would go above and below
the planet in passing round it. Only we must remember there is really no
such thing as above and below absolutely. We who are on one side of the
world point up to the sky and down to the earth, while the people on the
other side of the earth, say at New Zealand, also point up to the sky
and down to the earth, but their pointings are directly the opposite of
ours. So when we speak of moons going above and below that is only
because, for the moment, we are representing Uranus as a top we hold in
our hands, and so we speak of above and below as they are to us.
It was Herschel who discovered these satellites, as well as the planet,
and for these great achievements he occupies one of the grandest places
in the role of names of which England is proud. But he did much more
than this: his improvements in the construction of telescopes, and his
devotion to astronomy in many other ways, would have caused him to be
remembered without anything else.
Of Uranus's satellites one, the nearest, goes round in about two and a
half days, and the one that is furthest away takes about thirteen and a
half days, so both have a shorter period than our moon.
The discovery of Uranus filled the whole civilized world with wonder.
The astronomers who had seen him, but missed finding out that he was a
planet, must have felt bitterly mortified, and when he was discovered he
was observed with the utmost accuracy and care. The calculations made to
determine his path in the sky were the easier because he had been noted
as a star in several catalogues previously, so that his position for
some time past was known. Everybody who worked at astronomy began to
observe him. From these facts mathematicians set to work, and, by
abstruse calculations, worked out exactly the orbit in which he ought to
move; then his movements were again watched, and behold he followed the
path predicted for him; but there was a small difference here and there:
he did not follow it exactly. Now, in the heavens there is a reason for
everything, though we may not always be clever enough to find it out,
and it was easily guessed that it was not by accident that Uranus did
not precisely follow the path calculated for him. The planets all act
and react on one another, as we know, according to their mass and their
distance, and in the calculations the pull of Jupiter on Saturn and of
Saturn on Uranus were known and allowed for. But Uranus was pulled by
some unseen influence also.
A young Englishman named Adams, by some abstruse and difficult
mathematical work far beyond the power of ordinary brains, found out not
only the fact that there must be another planet nearly as large as
Uranus in an orbit outside his, but actually predicted where such a
planet might be seen if anyone would look for it. He gave his results to
a professor of astronomy at Cambridge. Now, it seems an easy thing to
say to anyone, 'Look out for a planet in such and such a part of the
sky,' but in reality, when the telescope is turned to that part of the
sky, stars are seen in such numbers that, without very careful
comparison with a star chart, it is impossible to say which are fixed
stars and which, if any, is an intruder. There happened to be no star
chart of this kind for the particular part of the sky wanted, and thus a
long time elapsed and the planet was not identified. Meantime a young
Frenchman named Leverrier had also taken up the same investigation, and,
without knowing anything of Adams' work, had come to the same
conclusion. He sent his results to the Berlin Observatory, where a star
chart such as was wanted was actually just being made. By the use of
this the Berlin astronomers at once identified this new member of our
system, and announced to the astonished world that another large planet,
making eight altogether, had been discovered. Then the English
astronomers remembered that they too held in their hands the means for
making this wonderful discovery, but, by having allowed so much time to
elapse, they had let the honour go to France. However, the names of
Adams and Leverrier will always be coupled together as the discoverers
of the new planet, which was called Neptune. The marvel is that by pure
reasoning the mind of man could have achieved such results.
If the observation of Uranus is difficult, how much more that of
Neptune, which is still further plunged in space! Yet by patience a few
facts have been gleaned about him. He is not very different in size from
Uranus. He also is of very slight density. His year includes one hundred
and sixty-five of ours, so that since his discovery in 1846 he has only
had time to get round less than a third of his path. His axis is even
more tilted over than that of Uranus, so that if we compare Uranus to a
top held horizontally, Neptune will be like a top with one end pointing
downwards. He rotates in this extraordinary position, in the same manner
as Uranus--namely, the other way over from all the other planets, but he
revolves, as they all do, counter-clockwise.
Seen from Neptune the sun can only appear about as large as Venus
appears to us at her best, and the light and heat received are but one
nine-hundreth part of what he sends us. Yet so brilliant is sunshine
that even then the light that falls on Neptune must be very
considerable, much more than that which we receive from Venus, for the
sun itself glows, and from Venus the light is only reflected. The sun,
small as it must appear, will shine with the radiance of a glowing
electric light. To get some idea of the brilliance of sunlight, sit near
a screen of leaves on some sunny day when the sun is high overhead, and
note the intense radiance of even the tiny rays which shine through the
small holes in the leaves. The scintillating light is more glorious than
any diamond, shooting out coloured rays in all directions. A small sun
the apparent size of Venus would, therefore, give enough light for
practical purposes to such a world as Neptune, even though to us a world
so illuminated would seem to be condemned to a perpetual twilight.