[Illustration: FIG. 79.--A Great Shower of Shooting Stars.]
Occasionally we have the superb spectacle of a shower of shooting
stars. None of you, my young friends, can as yet have had the good
fortune to witness one of the specially grand displays, but you may
live in hope; there are still showers to come. Astronomers have
ventured on the prophecy that in or about the year 1899 you will have
the opportunity of seeing a magnificent exhibition of this kind. There
is only one ground for anxiety, and that is as to whether the clouds
will keep out of the way for the occasion. I think I cannot explain
my subject better than by taking you into my confidence and showing
you the reasons on which we base this prediction. The last great
shooting-star shower took place in the year 1866, or, perhaps, I should
rather say that this was the last display from the same shooting-star
system as that about which we are now going to speak. On the night of
the 13th of November, 1866, astronomers were everywhere delighted by
a superb spectacle. Enjoyment of the wondrous sight was not only for
astronomers. Every one who loves to see the great sights of nature
will have good reason for remembering that night. I certainly shall
never forget it. It was about ten o’clock when a brilliant meteor or
two first flashed across the sky, then presently they came in twos and
threes, and later on in dozens, in scores, in hundreds. These meteors
were brilliant objects, any one of which would have extorted admiration
on an ordinary night. What, then, was the splendor of the display when
they came on in multitudes? For two or three hours the great shower
lasted, and then gradually subsided.
We were not taken unawares on this occasion, for the shower was
expected, and had been, in fact, awaited with eager anticipation. It
should first be noticed that each year some shooting stars may always
be looked for on or about the 13th of November. Every thirty-three
years, or thereabouts, the ordinary spectacle breaks out into a
magnificent display. It has also been found that for nearly 1000 years
there have been occasional grand showers of meteors at the time of year
mentioned, and all these incidents agree with the supposition that
they are merely repetitions of the regular thirty-three-year shower.
The first was in the year A.D. 902, which an old chronicle speaks
of as the “year of the stars,” from the extraordinary display which
then took place. I do not think the good people 1000 years ago fully
appreciated the astronomical interest of such spectacles; in fact,
they were often frightened out of their wits, and thought the end of
the world had come. Doubtless many ancient showers have taken place
of which we have no record whatever. In more modern days we have had
somewhat fuller information; for example, on the night between the 12th
and 13th of November, 1833, a shower was magnificently seen in America.
Mr. Kirkwood tells us that a gentleman of South Carolina described the
effect on the negroes of his plantation as follows: “I was suddenly
awakened by the most distressing cries that ever fell on my ears.
Shrieks of horror and cries for mercy I could hear from most of the
negroes of the three plantations, amounting in all to about 600 or 800.
While earnestly listening for the cause, I heard a faint voice near the
door calling my name. I arose, and taking my sword, stood at the door.
At this moment I heard the same voice still beseeching me to arise,
and crying out that the world was on fire. I then opened the door, and
it is difficult to say which excited me the most--the awfulness of the
scene or the distressed cries of the negroes. Upwards of a hundred lay
prostrate on the ground, some speechless, and some with the bitterest
cries, but with their hands raised praying for mercy. The scene was
truly awful, for never did rain fall much thicker than the meteors fell
towards the earth.”
By the study of many records of great showers it was learned that the
interval at which these grand displays succeeded one another was about
thirty-three years; and when it was remembered that the last great
shower was in 1833 it was confidently expected that another similar
display would take place in 1866. This was fully confirmed. Yet another
thirty-three years brings us to 1899, when we have good reason for
looking forward to a grand shower of these bodies. It may be expected
to occur on November 14 or November 15. It may, however, possibly be
that a shower will occur on the same days of the succeeding year.
[Illustration: FIG. 80.--The Earth crossing the Track of Meteors.]
We know a good deal now as regards the movements of these little
objects. I want you to think of a vast swarm, something like a flock
of birds, which I dare say you have often seen flying high in the air;
the difference, however, is that the flock of meteors is enormously
greater than any flock of birds ever was; and the meteors, too, are
scattered so widely apart, that each one may be miles away from its
next neighbors. Usually the meteoric shoal is many millions of miles
long, and perhaps a hundred thousand miles in width. The great flock
of meteors travels through space in a certain definite track. We have
learned how the sun guides a planet, and forces the planet to move
around him in an ellipse. But our sun will also condescend to guide
an object no bigger than a shooting star. A bullet, a pea, or even
a grain of sand will be held to an elliptic course around the sun as
carefully as the great Jupiter himself. The entire shoal of meteors may
therefore pursue their common journey around the sun as if inspired by
a common purpose, each individual member of the host being, however,
guided by the sun, and performing its path in real independence of its
neighbors. The orbit followed by this shoal of meteors is enormously
large and wide. Here is a sketch of the path (Fig. 80), and I have
laid down the position of the orbit of the earth, but not on the same
scale. The ellipse is elongated, so that while the shoal approaches
comparatively close to the sun at one end of its journey, at the other
end it goes out to an enormous distance, far beyond the orbit of the
earth--beyond, indeed, the orbit of Jupiter or Saturn; in fact, it
reaches to the path of Uranus. To accomplish so vast a journey as this
thirty-three years and a quarter are required, and now you will be
easily able to see why we get periodical visits from the shoal.
It is, however, a mere piece of good fortune that we ever encounter the
November meteors. Probably there are numerous other shoals of meteors
quite as important which we never see, just in the same way as there
are many shoals of fish in the sea that never come into our net. The
earth moves round the sun in a path which is very nearly a circle, and
the shoal moves round in this long oval. We cannot easily represent
the true state of things by mere diagrams which show all these objects
on the same plane. This does not give an accurate representation of
the orbits. I think you will better understand what I mean by means of
some wire rings. Make a round one to represent the path of the earth,
and a long oval one to represent the path of the meteors. There is to
be a small opening in the circular ring so that we can slip one of the
orbits inside the other. If we are to see the meteors, it is of course
necessary that they should strike the earth’s atmosphere, for they are
not visible to us when they lie at a distance like the moon or like the
planets. It is necessary that there be a collision between the earth
and the shoal of meteors. But there never could be a collision between
two trains unless the lines on which these trains run meet each other;
therefore, it is necessary that this long ellipse shall actually cross
the earth’s track; it will not do to have it pass inside like the two
links of a chain; our earth would then miss the meteors altogether,
and we should never see them. There are very likely many of such shoals
of meteors revolving in this way, and thus escaping our notice entirely.
You will also understand why there is no use in looking for these
showers except on a particular day of November. On that day, and on
that day alone, the earth appears at that particular point of its
route where the latter crosses the track of the shoal. On the 1st of
November, for instance, the earth has not yet reached the point where
it could meet with these bodies. By the end of November it has passed
too far. But even supposing that the earth is crossing the track of
the meteors on the 13th of November, it is still possible that only
a few, or none at all, shall be seen. The shoal may not happen to be
at that spot at the right time. For a display of meteors to occur, it
is therefore necessary that the shoal shall happen to be passing this
particular stage of the journey on the 13th of November. In 1866 the
earth dipped through the shoal and caught a great many of these meteors
in its net. For a few hours the earth was engaged in the capture, until
it emerged on the other side of the shoal, and the display was at an
end.
Sometimes it happens that in two years following each other, grand
showers of meteors are seen. The reason of this is that the shoal is
very long and thin, and consequently if the earth passes through the
beginning of the shoal one year, it may have returned to the same point
next year before the whole length of the shoal has completely passed.
In this case we shall have two great showers in consecutive years.
Thus a very fine display was seen in America on the proper day in 1867,
while many stragglers were also observed during the three subsequent
recurrences of the same date.
Whenever the 13th of November comes round we generally meet with at
least five shooting stars belonging to this same system, and we must
explain how this occurs. Suppose there is a small racecourse so that
the competitors will have to run a great many times round before the
race is over. Let there be a very large number of entries, and let
the majority of the athletes be fairly good runners, while a few are
exceptionally good with varying degrees of excellence, and a few are
very bad, some being worse than others. The whole group starts together
in a cluster at the signal, and perhaps for the first round or two they
may keep tolerably well together. It will be noticed the cluster begins
to elongate as one circuit after another is made; the better runners
draw out to the front, and the slower runners lag further and further
behind; at last it may happen that those at the head will have gained
a whole round on those at the tail, while the other runners of varying
degrees of speed will be scattered all round the course. The majority
of the runners, if of nearly equal speed, may continue in a pretty
dense group.
Precisely similar has been the great celestial race which these meteors
are running. They started on their grand career centuries ago, and ever
since then they have been flying round and round their mighty course.
The greater proportion of the meteors still stay close together, and
their pace is nearly uniform. The exceptionally smart ones have shot
ahead, the exceptionally slow ones have lagged behind, and thus it
happens that, after fifty or more revolutions have been completed,
the shape of the original swarm has become considerably modified.
Its length has been drawn out, while the stragglers and the fastest
runners have been scattered all around the path. Across this course
our earth carries us every November; there we usually encounter some
of the members of this swarm which have strayed from the great host;
they flash into the air, and thus it is that some of these bodies are
generally seen every November.
[Illustration: FIG. 81.--The Radiant.]
During a shooting-star shower it is interesting to notice that all the
meteors seem to diverge from a single point. In the adjoining figure
(Fig. 81), which shows the directions of a number of meteors’ tracks,
you will notice that every one seems to radiate from a certain point of
the sky. In the case of the shower of the 13th–15th of November this
point lies in the constellation Leo. I must refer you to the Appendix
for a description of the way to find Leo or the Lion. The radiant
point, as we term it, of this system of meteors is there situated. It
is true that the meteors themselves do not generally seem to come all
the way from this place. It is the direction of their luminous trails
produced backward that carries the eye to the radiant (Fig. 81). If a
meteor were actually seen there, it would be certainly coming straight
towards us; it would not then appear as a streak of light at all; it
would merely seem like a star which suddenly blazed into splendor and
then again sank down into invisibility. Every meteor which appeared
near this point would be directed very nearly at the observer, and its
path would therefore seem very much foreshortened. I can illustrate
this with a long straight rod. If I point it directly at you, you can
only see the end. If I point it nearly at you, it will seem very much
shortened. During the great shower in 1866 many of the meteors could be
observed so close to the radiant in Leo that they seemed merely like
very short marks in the sky; some of them, indeed, seemed to be merely
starlike points swelling up into brilliance and then going out. Hence
it is that we call this system of shooting stars the “Leonids.” They
bear this name because their radiant lies in the constellation Leo,
and unless the direction of a shooting star emanates from this point it
does not belong to the Leonids. Even if it did so, the meteor would not
be a Leonid unless the date was right, namely, on the 13th of November,
or within a day thereof. We thus have two characteristics which belong
to a system of shooting stars; there is the date on which they occur
and the point from which they radiate.