SHOOTING STARS AND FIERY BALLS
All the substances which we are accustomed to see and handle in our
daily lives belong to our world. There are vegetables which grow in the
earth, minerals which are dug out of it, and elementary things, such as
air and water, which have always made up a part of this planet since man
knew it. These are obvious, but there are other things not quite so
obvious which also help to form our world. Among these we may class all
the elements known to chemists, many of which have difficult names, such
as oxygen and hydrogen. These two are the elements which make up water,
and oxygen is an important element in air, which has nitrogen in it too.
There are numbers and numbers of other elements perfectly familiar to
chemists, of which many people never even hear the names. We live in the
midst of these things, and we take them for granted and pay little
attention to them; but when we begin to learn about other worlds we at
once want to know if these substances and elements which enter so
largely into our daily lives are to be found elsewhere in the universe
or are quite peculiar to our own world. This question might be answered
in several ways, but one of the most practical tests would be if we
could get hold of something which had not been always on the earth, but
had fallen upon it from space. Then, if this body were made up of
elements corresponding with those we find here, we might judge that
these elements are very generally diffused throughout the bodies in the
solar system.
It sounds in the highest degree improbable that anything should come
hurling through the air and alight on our little planet, which we know
is a mere speck in a great ocean of space; but we must not forget that
the power of gravity increases the chances greatly, for anything coming
within a certain range of the earth, anything small enough, that is, and
not travelling at too great a pace, is bound to fall on to it. And,
however improbable it seems, it is undoubtedly true that masses of
matter do crash down upon the earth from time to time, and these are
called meteorites. When we think of the great expanse of the oceans, of
the ice round the poles, and of the desert wastes, we know that for
every one of such bodies seen to fall many more must have fallen unseen
by any human being. Meteors large enough to reach the earth are not very
frequent, which is perhaps as well, and as yet there is no record of
anyone's having been killed by them. Most of them consist of masses of
stone, and a few are of iron, while various substances resembling those
that we know here have been found in them. Chemists in analyzing them
have also come across certain elements so far unknown upon earth, though
of course there is no saying that these may not exist at depths to which
man has not penetrated.
A really large meteor is a grand sight. If it is seen at night it
appears as a red star, growing rapidly bigger and leaving a trail of
luminous vapour behind as it passes across the sky. In the daytime this
vapour looks like a cloud. As the meteor hurls itself along there may be
a deep continuous roar, ending in one supreme explosion, or perhaps in
several explosions, and finally the meteor may come to the earth in one
mass, with a force so great that it buries itself some feet deep in the
soil, or it may burst into numbers of tiny fragments, which are
scattered over a large area. When a meteor is found soon after its fall
it is very hot, and all its surface has 'run,' having been fused by
heat. The heat is caused by the friction of our atmosphere. The meteor
gets entangled in the atmosphere, and, being drawn by the attraction of
the earth, dashes through it. Part of the energy of its motion is turned
to heat, which grows greater and greater as the denser air nearer to the
earth is encountered; so that in time all the surface of the meteor runs
like liquid, and this liquid, rising to a still higher temperature, is
blown off in vapour, leaving a new surface exposed. The vapour makes the
trail of fire or cloud seen to follow the meteor. If the process went on
for long the meteor would be all dissipated in vapour, and in any case
it must reach the earth considerably reduced in size.
Numbers and numbers of comparatively small ones disappear, and for every
one that manages to come to earth there must be hundreds seen only as
shooting stars, which vanish and 'leave not a wrack behind.' When a
meteor is seen to fall it is traced, and, whenever possible, it is found
and placed in a museum. Men have sometimes come across large masses of
stone and iron with their surfaces fused with heat. These are in every
way like the recognized meteorites, except that no eye has noted their
advent. As there can be no reasonable doubt that they are of the same
origin as the others, they too are collected and placed in museums, and
in any large museum you would be able to see both kinds--those which
have been seen to come to earth and those which have been found
accidentally.
The meteors which appear very brilliant in their course across the sky
are sometimes called fire-balls, which is only another name for the same
thing. Some of these are brighter than the full moon, so bright that
they cause objects on earth to cast a shadow. In 1803 a fiery ball was
noticed above a small town in Normandy; it burst and scattered stones
far and wide, but luckily no one was hurt. The largest meteorites that
have been found on the earth are a ton or more in weight; others are
mere stones; and others again just dust that floats about in the
atmosphere before gently settling. Of course, meteors of this last kind
could not be seen to fall like the larger ones, yet they do fall in such
numbers that calculations have been made showing that the earth must
catch about a hundred millions of meteors daily, having altogether a
total weight of about a hundred tons. This sounds enormous, but
compared with the weight of the earth it is very small indeed.
Now that we have arrived at the fact that strange bodies do come
hurtling down upon us out of space, and that we can actually handle and
examine them, the next question is, Where do they come from? At one time
it was thought that they were fragments which had been flung off by the
earth herself when she was subject to violent explosions, and that they
had been thrown far enough to resist the impulse to drop down upon her
again, and had been circling round the sun ever since, until the earth
came in contact with them again and they had fallen back upon her. It is
not difficult to imagine a force which would be powerful enough to
achieve the feat of speeding something off at such a velocity that it
passed beyond the earth's power to pull it back, but nothing that we
have on earth would be nearly strong enough to achieve such a feat.
Imaginative writers have pictured a projectile hurled from a cannon's
mouth with such tremendous force that it not only passed beyond the
range of the earth's power to pull it back, but so that it fell within
the influence of the moon and was precipitated on to her surface! Such
things must remain achievements in imagination only; it is not possible
for them to be carried out. Other ideas as to the origin of meteors
were that they had been expelled from the moon or from the sun. It would
need a much less force to send a projectile away from the moon than from
the earth on account of its smaller size and less density, but the
distance from the earth to the moon is not very great, and any
projectile hurled forth from the moon would cross it in a comparatively
short time. Therefore if the meteorites come from the moon, the moon
must be expelling them still, and we might expect to see some evidence
of it; but we know that the moon is a dead world, so this explanation is
not possible. The sun, for its part, is torn by such gigantic
disturbances that, notwithstanding its vast size, there is no doubt
sufficient force there to send meteors even so far as the earth, but the
chances of their encountering the earth would be small. Both these
theories are now discarded. It is believed that the meteors are merely
lesser fragments of the same kind of materials as the planets, circling
independently round the sun; and a proof of this is that far more
meteorites fall on that part of the earth which is facing forward in its
journey than on that behind, and this is what we should expect if the
meteors were scattered independently through space and it was by reason
of our movements that we came in contact with them. There is no need to
explain this further. Everyone knows that in cycling or driving along a
road where there is a good deal of traffic both ways the people we meet
are more in number than those who overtake us, and the same result would
follow with the meteors; that is to say, in travelling through space
where they were fairly evenly distributed we should meet more than we
should be overtaken by.
You remember that it was suggested the sun's fuel might be obtained from
meteors, and this was proved to be not possible, even though there are
no doubt unknown millions of these strange bodies circling throughout
the solar system.
There are so many names for these flashing bodies that we may get a
little confused: when they are seen in the sky they are meteors, or
fire-balls; when they reach the earth they are called meteorites, and
also aerolites. Then there is another class of the same bodies called
shooting stars, and these are in reality only meteors on a smaller
scale; but there ought to be no confusion in our thoughts, for all these
objects are small bodies travelling round the sun, and caught by the
earth's influence.
When you watch the sky for some time on a clear night, you will seldom
fail to see at least one star flash out suddenly in a path of thrilling
light and disappear, and you cannot be certain whether that star had
been shining in the sky a minute before, or if it had appeared suddenly
only in order to go out. The last idea is right. We must get rid at once
of the notion that it would be possible for any fixed star to behave in
this manner. To begin with, the fixed stars are many of them actually
travelling at a great velocity at present, yet so immeasurably distant
are they that their movement makes no perceptible difference to us. For
one of them to appear to dash across the heavens as a meteor does would
mean a velocity entirely unknown to us, even comparing it with the speed
of light. No, these shooting stars are not stars at all, though they
were so named, long before the real motions of the fixed stars were even
dimly guessed at. As we have seen, they belong to the same class as
meteors.
I remember being told by a clergyman, years ago, that one night in
November he had gone up to bed very late, and as he pulled up his blind
to look at the sky, to his amazement he saw a perfect hail of shooting
stars, some appearing every minute, and all darting in vivid trails of
light, longer or shorter, though all seemed to come from one point. So
marvellous was the sight that he dashed across the village street,
unlocked the church door, and himself pulled the bell with all his
might. The people in that quiet country village had long been in bed,
but they huddled on their clothes and ran out of their pretty thatched
cottages, thinking there must be a great fire, and when they saw the
wonder in the sky they were amazed and cried out that the world must be
coming to an end. The clergyman knew better than that, and was able to
reassure them, and tell them he had only taken the most effectual means
of waking them so that they might not miss the display, for he was sure
as long as they lived they would never see such another sight. A star
shower of this kind is certainly well worth getting up to see, but
though uncommon it is not unique. There are many records of such showers
having occurred in times gone by, and when men put together and examined
the records they found that the showers came at regular intervals. For
instance, every year about the same time in November there is a star
shower, not comparable, it is true, with the brilliant one the clergyman
saw, but still noticeable, for more shooting stars are seen then than at
other times, and once in every thirty-three years there is a specially
fine one. It happened in fact to be one of these that the village people
were wakened up to see.
Not all at once, but gradually, the mystery of these shower displays was
solved. It was realized that the meteors need not necessarily come from
one fixed place in the sky because they seemed to us to do so, for that
was only an effect of perspective. If you were looking down a long,
perfectly straight avenue of tree-trunks, the avenue would seem to close
in, to get narrower and narrower at the far end until it became a point;
but it would not really do so, for you would know that the trees at the
far end were just the same distance from each other as those between
which you were standing. Now, two meteors starting from the same
direction at a distance from each other, and keeping parallel, would
seem to us to start from a point and to open out wider and wider as they
approached, but they would not really do so; it would only be, as in the
case of the avenue, an effect of perspective. If a great many meteors
did the same thing, they would appear to us all to start from one point,
whereas really they would be on parallel lines, only as they rushed to
meet us or we rushed to meet them this effect would be produced.
Therefore the first discovery was that these meteors were thousands and
thousands of little bodies travelling in lines parallel to each other,
like a swarm of little planets. To judge that their path was not a
straight line but a circle or ellipse was the next step, and this was
found to be the case. From taking exact measurements of their paths in
the sky an astronomer computed they were really travelling round the sun
in a lengthened orbit, an ellipse more like a comet's orbit than that of
a planet. But next came the puzzling question, Why did the earth
apparently hit them every year to some extent, and once in thirty-three
years seem to run right into the middle of them? This also was answered.
One has only to imagine a swarm of such meteors at first hastening
busily along their orbit, a great cluster all together, then, by the
near neighbourhood of some planet, or by some other disturbing causes,
being drawn out, leaving stragglers lagging behind, until at last there
might be some all round the path, but only thinly scattered, while the
busy, important cluster that formed the nucleus was still much thicker
than any other part. Now, if the orbit that the meteors followed cut the
orbit or path of the earth at one point, then every time the earth came
to what we may call the level crossing she must run into some of the
stragglers, and if the chief part of the swarm took thirty-three years
to get round, then once in about thirty-three years the earth must
strike right into it. This would account for the wonderful display. So
long drawn-out is the thickest part of the swarm that it takes a year to
pass the points at the level crossing. If the earth strikes it near the
front one year, she may come right round in time to strike into the rear
part of the swarm next year, so that we may get fine displays two years
running about every thirty-three years. The last time we passed through
the swarm was in 1899, and then the show was very disappointing. Here in
England thick clouds prevented our seeing much, and there will not be
another chance for us to see it at its best until 1932.
These November meteors are called Leonids, because they _seem_ to come
from a group of stars named Leo, and though the most noticeable they are
not the only ones. A shower of the same kind occurs in August too, but
the August meteors, called Perseids, because they seem to come from
Perseus, revolve in an orbit which takes a hundred and forty-two years
to traverse! So that only every one hundred and forty-second year could
we hope to see a good display. When all these facts had been gathered
up, it seemed without doubt that certain groups of meteors travelled in
company along an elliptical orbit. But there remained still something
more--a bold and ingenious theory to be advanced. It was found that a
comet, a small one, only to be seen with the telescope, revolved in
exactly the same orbit as the November meteors, and another one, larger,
in exactly the same orbit as the August ones; hence it could hardly be
doubted that comets and meteors had some connection with each other,
though what that connection is exactly no one knows. Anyway, we can have
no shadow of doubt when we find the comet following a marked path, and
the meteors pursuing the same path in his wake, that the two have some
mysterious affinity. There are other smaller showers besides these of
November and August, and a remarkable fact is known about one of them.
This particular stream was found to be connected with a comet named
Biela's Comet, that had been many times observed, and which returned
about every seven years to the sun. After it had been seen several
times, this astonishing comet split in two and appeared as two comets,
both of which returned at the end of the next seven years. But on the
next two occasions when they were expected they never came at all, and
the third time there came instead a fine display of shooting stars, so
it really seemed as if these meteors must be the fragments of the lost
comet.
It is very curious and interesting to notice that in these star showers
there is no certain record of any large meteorite reaching the earth;
they seem to be made up of such small bodies that they are all
dissipated in vapour as they traverse our air.