SHINING VISITORS
Our solar system is set by itself in the midst of a great space, and so
far as we have learnt about it in this book everything in it seems
orderly: the planets go round the sun and the satellites go round the
planets, in orbits more or less regular; there seems no place for
anything else. But when we have considered the planets and the
satellites, we have not exhausted all the bodies which own allegiance to
the sun. There is another class, made up of strange and weird members,
which flash in and out of the system, coming and going in all directions
and at all times--sometimes appearing without warning, sometimes
returning with a certain regularity, sometimes retiring to infinite
depths of space, where no human eye will ever see them more. These
strange visitors are called comets, and are of all shapes and sizes and
never twice alike. Even as we watch them they grow and change, and then
diminish in splendour. Some are so vast that men see them as flaming
signs in the sky, and regard them with awe and wonder; some cannot be
seen at all without the help of the telescope. From the very earliest
ages those that were large enough to be seen without glasses have been
regarded with astonishment. Men used to think that they were signs from
heaven foretelling great events in the world. Timid people predicted
that the end of the world would come by collision with one of them.
Others, again, fancifully likened them to fishes in that sea of space in
which we swim--fishes gigantic and terrifying, endowed with sense and
will.
It is perhaps unnecessary to say that comets are no more alive than is
our own earth, and as for causing the end of the world by collision,
there is every reason to believe the earth has been more than once right
through a comet's tail, and yet no one except scientific men even
discovered it. These mysterious visitors from the outer regions of space
were called comets from a Greek word signifying hair, for they often
leave a long luminous trail behind, which resembles the filaments of a
woman's hair. It is not often that one appears large and bright enough
to be seen by the naked eye, and when it does it is not likely to be
soon forgotten. In the year 1910 such a comet is expected, a comet
which at its former appearance compelled universal attention by its
brilliancy and strangeness. At the time of the Norman Conquest of
England a comet believed to be the very same one was stretching its
glorious tail half across the sky, and the Normans seeing it, took it as
a good omen, fancying that it foretold their success. The history of the
Norman Conquest was worked in tapestry--that is to say, in what we
should call crewels on a strip of linen--and in this record the comet
duly appears. Look at him in the picture as the Normans fancied him. He
has a red head with blue flames starting from it, and several tails. The
little group of men on the left are pointing and chattering about him.
We can judge what an impression this comet must have made to be recorded
in such an important piece of work.
[Illustration: THE COMET IN THE BAYEUX TAPESTRY.]
But we are getting on too fast. We have yet to learn how anyone can know
that the comet which appeared at the time of the Norman Conquest is the
same as that which has come back again at different times, and above
all, how anyone can tell that it will come again in the year 1910. All
this involves a long story.
Before the invention of telescopes of course only those comets could be
seen which were of great size and fine appearance. In those days men
did not realize that our world was but one of a number and of no great
importance except to ourselves, and they always took these blazing
appearances in the heavens as a particular warning to the human race.
But when astronomers, by the aid of the telescope, found that for one
comet seen by the eye there were hundreds which no mortal eye unaided
could see, this idea seemed, to say the least of it, unlikely. Yet even
then comets were looked upon as capricious visitors from outer space;
odd creatures drawn into our system by the attraction of the sun, who
disappeared, never to return. It was Newton, the same genius who
disclosed to us the laws of gravity, who first declared that comets
moved in orbits, only that these orbits were far more erratic than any
of those followed by the planets.
So far we have supposed that the planets were all on what we should call
a level--that is to say, we have regarded them as if they were floating
in a sea of water around the sun; but this is only approximately
correct, for the orbits of the planets are not all at one level. If you
had a number of slender hoops or rings to represent the planetary
orbits, you would have to tilt one a little this way and another a
little that way, only never so far but that a line through the centre
of the hoop from one side to another could pass through the sun. The way
in which the planetary orbits are tilted is slight in comparison with
that of the orbits of comets, for these are at all sorts of angles--some
turned almost sideways, and others slanting, and all of them are
ellipses long drawn out and much more irregular than the planetary
orbits; but erratic as they are, in every case a line drawn through the
sun and extended both ways would touch each side of the orbits.
A great astronomer called Halley, who was born in the time of the
Commonwealth, was lucky enough to see a very brilliant comet, and the
sight interested him so much that he made all the calculations necessary
to find out just in what direction it was travelling in the heavens. He
found out that it followed an ellipse which brought it very near to the
sun at one part of its journey, and carried it far beyond the orbit of
the earth, right out to that of Neptune, at the other. Then he began to
search the records for other comets which had been observed before his
time. He found that two particularly bright ones had been carefully
noted--one about seventy-five years before that which he had seen, and
the other seventy-five years before that again. Both these comets had
been watched so scientifically that the paths in which they had
travelled could be computed. A brilliant inspiration came to Halley. He
believed that instead of these three, his own and the other two, being
different comets, they were the same one, which returned to the sun
about every seventy-five years. This could be proved, for if this idea
were correct, of course the comet would return again in another
seventy-five years, unless something unforeseen occurred. But Halley was
in the prime of life: he could not hope to live to see his forecast
verified. The only thing he could do was to note down exact particulars,
by means of which others who lived after him might recognize his comet.
And so when the time came for its return, though Halley was in his
grave, numbers of astronomers were watching eagerly to see the
fulfilment of his prediction. The comet did indeed appear, and since
then it has been seen once again, and now we expect it to come back in
the year 1910, when you and I may see it for ourselves. When the
identity of the comet was fully established men began to search further
back still, to compare the records of other previous brilliant comets,
and found that this one had been noticed many times before, and once as
I said, at the time of the Norman Conquest. Halley's comet is peculiar
in many ways. For instance, it is unusual that so large and interesting
a comet should return within a comparatively limited time. It is the
smaller comets, those that can only be seen telescopically, that usually
run in small orbits. The smallest orbits take about three and a half
years to traverse, and some of the largest orbits known require a period
of one hundred and ten thousand years. Between these two limits lies
every possible variety of period. One comet, seen about the time
Napoleon was born, was calculated to take two thousand years to complete
its journey, and another, a very brilliant one seen in 1882, must
journey for eight hundred years before it again comes near to the sun.
But we never know what might happen, for at any moment a comet which has
traversed a long solitary pathway in outer darkness may flash suddenly
into our ken, and be for the first time noted and recorded, before
flying off at an angle which must take it for ever further and further
from the sun.
Everything connected with comets is mysterious and most fascinating.
From out of the icy regions of space a body appears; what it is we know
not, but it is seen at first as a hairy or softly-glowing star, and it
was thus that Herschel mistook Uranus for a comet when he first
discovered it. As it draws nearer the comet sends out some fan-like
projections toward the sun, enclosing its nucleus in filmy wrappings
like a cocoon of light, and it travels faster and faster. From its head
shoots out a tail--it may be more than one--growing in splendour and
width, and always pointing away from the sun. So enormous are some of
these tails that when the comet's head is close to the sun the tail
extends far beyond the orbit of the earth. Faster still and faster flies
the comet, for as we have seen it is a consequence of the law of
gravitation that the nearer planets are to the sun the faster they move
in their orbits, and the same rule applies to comets too. As the comet
dashes up to the sun his pace becomes something indescribable; it has
been reckoned for some comets at three hundred miles a second! But
behold, as the head flies round the sun the tail is always projected
outwards. The nucleus or head may be so near to the sun that the heat it
receives would be sufficient to reduce molten iron to vapour; but this
does not seem to affect it: only the tail expands. Sometimes it becomes
two or more tails, and as it sweeps round behind the head it has to
cover a much greater space in the same time, and therefore it must
travel even faster than the head. The pace is such that no calculations
can account for it, if the tail is composed of matter in any sense as we
know it. Then when the sun is passed the comet sinks away again, and as
it goes the tail dies down and finally disappears. The comet itself
dwindles to a hairy star once more and goes--whither? Into space so
remote that we cannot even dream of it--far away into cold more
appalling than anything we could measure, the cold of absolute space.
More and more slowly it travels, always away and away, until the sun, a
short time back a huge furnace covering all the sky, is now but a faint
star. Thus on its lonely journey unseen and unknown the comet goes.
This comet which we have taken as an illustration is a typical one, but
all are not the same. Some have no tails at all, and never develop any;
some change utterly even as they are watched. The same comet is so
different at different times that the only possible way of identifying
it is by knowing its path, and even this is not a certain method, for
some comets appear to travel at intervals along the same path!
Now we come to the question that must have been in the mind of everyone
from the beginning of this chapter, What are comets? This question no
one can answer definitely, for there are many things so puzzling about
these strange appearances that it is difficult even to suggest an
explanation. Yet a good deal is known. In the first place, we are
certain that comets have very little density--that is to say, they are
indescribably thin, thinner than the thinnest kind of gas; and air,
which we always think so thin, would be almost like a blanket compared
with the material of comets. This we judge because they exercise no sort
of influence on any of the planetary bodies they draw near to, which
they certainly would do if they were made of any kind of solid matter.
They come sometimes very close to some of the planets. A comet was so
near to Jupiter that it was actually in among his moons. The comet was
violently agitated; he was pulled in fact right out of his old path, and
has been going on a new one ever since; but he did not exercise the
smallest effect on Jupiter, or even on the moons. And, as I said earlier
in this chapter, we on the earth have been actually in the folds of a
comet's tail. This astonishing fact happened in June, 1861. One evening
after the sun had set a golden-yellow disc, surrounded with filmy
wrappings, appeared in the sky. The sun's light, diffused throughout
our atmosphere, had prevented its being seen sooner. This was apparently
the comet's head. It is described as 'though a number of light, hazy
clouds were floating around a miniature full moon.' From this a cone of
light extended far up into the sky, and when the head disappeared below
the horizon this tail was seen to reach to the zenith. But that was not
all. Strange shafts of light seemed to hang right overhead, and could
only be accounted for by supposing that they were caused by another tail
hanging straight above us, so that we looked up at it foreshortened by
perspective. The comet's head lay between the earth and the sun, and its
tail, which extended over many millions of miles, stretched out behind
in such a way that the earth must have gone right through it. The fact
that the comet exercised no perceptible influence on the earth at all,
and that there were not even any unaccountable magnetic storms or
displays of electricity, may reassure us so that if ever we do again
come in contact with one of these extremely fine, thin bodies, we need
not be afraid.
There is another way in which we can judge of the wonderful tenuity or
thinness of comets--that is, that the smallest stars can be seen
through their tails, even though those tails must be many thousands of
miles in thickness. Now, if the tails were anything approaching the
density of our own atmosphere, the stars when seen through them would
appear to be moved out of their places. This sounds odd, and requires a
word of explanation. The fact is that anything seen through any
transparent medium like water or air is what is called refracted--that
is to say, the rays coming from it look bent. Everyone is quite familiar
with this in everyday life, though perhaps they may not have noticed it.
You cannot thrust a stick into the water without seeing that it looks
crooked. Air being less dense than water has not quite so strong a
refracting power, but still it has some. We cannot prove it in just the
same way, because we are all inside the atmosphere ourselves, and there
is no possibility of thrusting a stick into it from the outside! The
only way we know it is by looking at something which is 'outside'
already, and we find plenty of objects in the sky. As a matter of fact,
the stars are all a little pulled out of their places by being seen
through the air, and though of course we do not notice this, astronomers
know it and have to make allowance for it. The effect is most
noticeable in the case of the sun when he is going down, for the
atmosphere bends his rays up, and though we see him a great glowing red
ball on the horizon, and watch him, as we think, drop gradually out of
sight, we are really looking at him for the last moment or two when he
has already gone, for the rays are bent up by the air and his image
lingers when the real sun has disappeared.
[Illustration: A STICK THRUST INTO THE WATER APPEARS CROOKED.]
Therefore in looking through the luminous stuff that forms a comet's
tail astronomers might well expect to see the stars displaced, but not a
sign of this appears. It is difficult to imagine, therefore, what the
tail can be made of. The idea is that the sun exercises a sort of
repulsive effect on certain elements found in the comet's head--that is
to say, it pushes them away, and that as the head approaches the sun,
these elements are driven out of it away from the sun in vapour. This
action may have something to do with electricity, which is yet little
understood; anyway, the effect is that, instead of attracting the matter
toward itself, in which case we should see the comet's tails stretching
toward the sun, the sun drives it away! In the chapter on the sun we had
to imagine something of the same kind to account for the corona, and the
corona and the comet's tails may be really akin to each other, and
could perhaps be explained in the same way. Now we come to a stranger
fact still. Some comets go right through the sun's corona, and yet do
not seem to be influenced by it in the smallest degree. This may not
seem very wonderful at first perhaps, but if you remember that a dash
through anything so dense as our atmosphere, at a pace much less than
that at which a comet goes, is enough to heat iron to a white heat, and
then make it fly off in vapour, we get a glimpse of the extreme fineness
of the materials which make the corona.
Here is Herschel's account of a comet that went very near the sun:
'The comet's distance from the sun's centre was about the 160th part of
our distance from it. All the heat we enjoy on this earth comes from the
sun. Imagine the heat we should have to endure if the sun were to
approach us, or we the sun, to one 160th part of its present distance.
It would not be merely as if 160 suns were shining on us all at once,
but, 160 times 160, according to a rule which is well known to all who
are conversant with such matters. Now, that is 25,600. Only imagine a
glare 25,600 times fiercer than that of the equatorial sunshine at noon
day with the sun vertical. In such a heat there is no substance we know
of which would not run like water, boil, and be converted into smoke or
vapour. No wonder the comet gave evidence of violent excitement, coming
from the cold region outside the planetary system torpid and ice-bound.
Already when arrived even in our temperate region it began to show signs
of internal activity; the head had begun to develop, and the tail to
elongate, till the comet was for a time lost sight of--not for days
afterwards was it seen; and its tail, whose direction was reversed, and
which could not possibly be the same tail it had before, had already
lengthened to an extent of about ninety millions of miles, so that it
must have been shot out with immense force in a direction away from the
sun.'
We remember that comets have sometimes more than one tail, and a theory
has been advanced to account for this too. It is supposed that perhaps
different elements are thrust away by the sun at different angles, and
one tail may be due to one element and another to another. But if the
comet goes on tail-making to a large extent every time it returns to the
sun, what happens eventually? Do the tails fall back again into the head
when out of reach of the sun's action? Such an idea is inconceivable;
but if not, then every time a comet approaches the sun he loses
something, and that something is made up of the elements which were
formerly in the head and have been violently ejected. If this be so we
may well expect to see comets which have returned many times to the sun
without tails at all, for all the tail-making stuff that was in the head
will have been used up, and as this is exactly what we do see, the
theory is probably true.
Where do the comets come from? That also is a very large question. It
used to be supposed they were merely wanderers in space who happened to
have been attracted by our sun and drawn into his system, but there are
facts which go very strongly against this, and astronomers now generally
believe that comets really belong to the solar system, that their proper
orbits are ellipses, and that in the case of those which fly off at such
an angle that they can never return they must at some time have been
pulled out of their original orbit by the influence of one of the
planets.
[Illustration: _Royal Observatory, Cape of Good Hope._
A GREAT COMET.]
To get a good idea of a really fine comet, until we have the opportunity
of seeing one for ourselves, we cannot do better than look at this
picture of a comet photographed in 1901 at the Cape of Good Hope. It is
only comparatively recently that photography has been applied to comets.
When Halley's comet appeared last time such a thing was not thought
of, but when he comes again numbers of cameras, fitted up with all the
latest scientific appliances, will be waiting to get good impressions of
him.