METEORS AND SHOOTING STARS
"Falling stars," or "shooting stars," have been familiar sights in all
ages of the world, but the ancient philosophers thought them scarcely
worthy of notice. According to Aristotle they were mere nothings of the
upper atmosphere, of no more account than the general happenings of the
weather. But about the end of the eighteenth century and the beginning
of the nineteenth the insufficiency of this view began to be fully
recognized, and interplanetary space was conceived as tenanted by shoals
of moving bodies exceedingly small in mass and dimension as compared
with the planets.
Millions of these bodies are all the time in collision with the outlying
regions of our atmosphere; and by their impact upon it and their
friction in passing swiftly through it, they become heated to
incandescence, thus creating the luminous appearances commonly known as
shooting stars. For the most part they are consumed or dissipated in
vapor before reaching the solid surface of the earth; but occasionally a
luminous cloud or streak is left glowing in the wake of a large meteor,
which sometimes remains visible for half an hour after the passage of
the meteor itself. These mistlike clouds projected upon the dark sky
have been especially studied by Trowbridge of Columbia University.
Many more meteors are seen during the morning hours, say from four to
six, than at any other nightly period of equal length, because the
visible sky is at that time nearly centered around the general direction
toward which the earth is moving in its orbit round the sun; so that the
number of meteors that would fall upon the earth if at rest is increased
by those which the earth overtakes by its own motion. Also from January
to July while the earth is traveling from perihelion to aphelion, fewer
meteors are seen than in the last half of the year; but this is chiefly
because of the rich showers encountered in August and November.
Although the descent of meteoric bodies from the sky was pretty
generally discredited until early in the nineteenth century, such falls
had nevertheless been recorded from very early times. They were usually
regarded as prodigies or miracles, and such stones were commonly objects
of worship among ancient peoples. For example, the Phrygian Stone, known
as the "Diana of the Ephesians which fell down from Jupiter," was a
famous stone built into the Kaaba at Mecca, and even to-day it is
revered by Mohammedans as a holy relic. Perhaps the earliest known
meteoric fall is that historically recorded in the Parian Chronicle as
having occurred in the island of Crete, B. C. 1478. Also in the imperial
museum of Petrograd is the Pallas or Krasnoiarsk iron, perhaps
three-quarters of a ton in weight, found in 1772 by Pallas, the famous
traveler, at Krasnoiarsk, Siberia.
But a fall of meteoric stones that chanced upon the department of Orne,
France, in 1805, led to a critical investigation by Biot, the
distinguished physicist and academician. According to his report a
violent explosion in the neighborhood of L'Aigle had been heard for a
distance of seventy-five miles around, and lasting five or six minutes,
about 1 P. M. on Tuesday, April 26. From several adjoining towns a
rapidly moving fireball had been seen in a sky generally clear, and
there was absolutely no room for doubt that on the same day many stones
fell in the neighborhood of L'Aigle. Biot estimated their number between
two and three thousand, and they were scattered over an elliptical area
more than six miles long, and two and a half miles broad. Thenceforward
the descent of meteoric matter from outer space upon the earth has been
recognized as an unquestioned fact.
The origin of these bodies being cosmic, meteors may be expected to fall
upon the earth without reference to latitude, or season, or day and
night, or weather. On entering our upper atmosphere their temperature
must be that of space, many hundred degrees below zero; and their
velocities range from ten miles per second upward. But atmospheric
resistance to their flight is so great that their velocity is quickly
reduced: at ground impact it does not exceed a few hundred feet per
second. On January 1, 1869, several meteoric stones fell on ice only a
few inches thick in Sweden, rebounding without either breaking through
the ice or being themselves fractured.
Naturally the flight of a meteor through the atmosphere will be only a
few seconds in duration, and owing to the sudden reduction of velocity,
it will continue to be luminous throughout only the upper part of its
course. Visibility generally begins at an elevation of about seventy
miles, and ends at perhaps half that altitude.
What is the origin of meteors? Theories there are in great abundance:
that they come from the sun, that they come from the moon, that they
come from the earth in past ages as a result of volcanic action, and so
on. But there are many difficulties in the way of acceptance of these
and several other theories. That all meteors were originally parts of
cometary masses is however a theory that may be accepted without much
hesitation.
Comets have been known to disintegrate. Biela's comet even disappeared
entirely, so that during a shower of Biela meteors in November, 1885, an
actual fragment of the lost comet fell upon the earth, at Mazapil,
Mexico. And as the Bielid meteors encounter the earth with the
relatively low velocity of ten miles a second, we may expect to capture
other fragments in the future. Numerous observers saw the weird
disintegration of the nucleus of the great comet of 1882, well
recognized as a member of the family of the comet of 1843. As these
comets are fellow voyagers through space along the same orbit, probably
all five members of the family, with perhaps others, were originally a
single comet of unparalleled magnitude.
The Brooks comet of 1890 affords another instance of fragmentary
nucleus. The oft-repeated action of solar forces tending to disrupt the
mass of a comet more and more, and scatter its material throughout
space, the secular dismemberment of all comets becomes an obvious
conclusion. During the hundreds of millions of years that these forces
are known to have been operant, the original comets have been broken up
in great numbers, so that elliptical rings of opaque meteoric bodies now
travel round the sun in place of the comets.
These bodies in vast numbers are everywhere through space, each too
small to reflect an appreciable amount of sunlight, and becoming
visible only when they come into collision with our outer atmosphere.
The practical identity of several such meteor streams and cometary
orbits has already been established, and there is every reason for
assigning a similar origin to all meteoric bodies. Meteors, then, were
originally parts of comets, which have trailed themselves out to such
extent that particles of the primal masses are liable to be picked up
anywhere along the original cometary paths. The historic records of all
countries contain trustworthy accounts of meteoric showers. Making due
allowances for the flowery imagery of the oriental, it is evident that
all have at one time or another seen much the same thing. In A. D. 472,
for instance, the Constantinople sky was reported alive with flying
stars. In October, 1202, "stars appeared like waves upon the sky; and
they flew about like grasshoppers." During the reign of King William II
occurred a very remarkable shower in which "stars seemed to fall like
rain from heaven."
But the showers of November, 1799 and 1833, are easily the most striking
of all. The sky was filled with innumerable fiery trails and there was
not a space in the heavens a few times the size of the moon that was not
ablaze with celestial fireworks. Frequently huge meteors blended their
dazzling brilliancy with the long and seemingly phosphorescent trails of
the shooting stars.
The interval of thirty-four years between 1799 and 1833 appeared to
indicate the possibility of a return of the shower in November of 1866
or 1867, and all the people of that day were aroused on this subject and
made every preparation to witness the spectacle. Extemporized
observatories were established, watchmen were everywhere on the
lookout, and bells were to be rung the minute the shower began. The
newspapers of the day did little to allay the fears of the multitude,
but the critical days of November, 1866, passed with disappointment in
America. In Europe, however, a fine shower was seen, though it was not
equal to that of 1833. The astronomers at Greenwich counted many
thousand meteors. In November of 1867, however, American astronomers
were gratified by a grand display, which, although failing to match the
general expectation, nevertheless was a most striking spectacle, and the
careful preparation for observing it afforded data of observation which
were of the greatest scientific value. The actual orbits of these bodies
in space became known with great exactitude, and it was found that their
general path was identical with that of the first comet of 1866, which
travels outward somewhat beyond the planet Uranus. When the visible
paths of these meteors are traced backward, all appear as if they
originated from the constellation Leo. So they are known as Leonids, and
a return of the shower was confidently predicted for November,
1900-1901, which for unknown reasons failed to appear.
[Illustration: TWO VIEWS OF HALLEY'S COMET. Taken with the same
camera from the same position, one on May 12, and the other on
May 15, 1910. (_Photo, Mt. Wilson Solar Observatory._)]
[Illustration: SWIFT'S COMET OF 1892. This comet showed
extraordinary and rapid transformations, one day having a dozen
streamers in its tail, another only two. (_Photo by Prof. E. E.
Barnard._)]
[Illustration: A LARGE METEOR TRAIL IN THE FIELD WITH FINE NEBULAE.
(_Photo, Yerkes Observatory._)]
During the last half century meteors have been pretty systematically
observed, especially by the astronomers of Italy and Denning of England,
so that several hundred distinct showers are now known, their radiant
points fall in every part of the heavens, and there is scarcely a clear
moonless night when careful watching for meteors will be unrewarded.
Besides November, the months of August (Perseids), April (Lyrids), and
December (Geminids) are favorable. Following in tabular form is a fairly
comprehensive list of the meteoric showers of the year, with the
positions of the radiant points and the epochs of the showers according
to Denning:
RADIANT POINT
============================================================
Name of Shower | R. A. | Decl. | Date of Shower
-----------------------+---------+--------+-----------------
Quadrantids | 230 deg. | +53 deg. | Jan. 2-4
Zeta Cepheids | 331 deg. | +56 deg. | Jan. 25
Alpha Leonids | 155 deg. | +14 deg. | Feb. 19-March 1
Tau Leonids | 166 deg. | +4 deg. | March 1-4
Beta Ursids | 161 deg. | +58 deg. | March 13-24
Lyrids | 271 deg. | +33 deg. | April 20-22
Gamma Aquarids | 338 deg. | -2 deg. | May 1-6
Zeta Herculids | 246 deg. | +29 deg. | May 18-26
Eta Pegasids | 330 deg. | +28 deg. | May 30-June 4
Theta Booetids | 213 deg. | +53 deg. | June 27-28
Alpha Capricornids | 304 deg. | -12 deg. | July 15-28
Delta Aquarids | 339 deg. | -11 deg. | July 25-30
Perseids | 45 deg. | +57 deg. | Aug. 10-12
Omicron Draconids | 291 deg. | +60 deg. | Aug. 15-25
Zeta Draconids | 262 deg. | +63 deg. | Aug. 21-Sept. 2
Piscids | 348 deg. | +2 deg. | Sept. 4-14
Alpha Andromedids | 4 deg. | +28 deg. | Sept. 27
Epsilon Arietids | 40 deg. | +20 deg. | Oct. 11-24
Orionids | 92 deg. | +15 deg. | Oct. 17-24
Epsilon Perseids | 61 deg. | +35 deg. | Nov. 5
Leonids | 150 deg. | +23 deg. | Nov. 13-15
Epsilon Taurids | 64 deg. | +22 deg. | Nov. 14-25
Andromedids | 25 deg. | +43 deg. | Nov. 17-23
Beta Geminids | 119 deg. | +31 deg. | Dec. 1-12
Geminids | 108 deg. | +33 deg. | Dec. 1-14
Alpha Ursae Majorids | 161 deg. | +58 deg. | Dec. 18-21
Kappa Draconids | 194 deg. | +68 deg. | Dec. 18-28
------------------------------------------------------------
The year 1916 was exceptional in providing an abundant and previously
unknown shower on June 28, and its stream has nearly the same orbit as
that of the Pons-Winnecke periodic comet. Useful observations of meteors
are not difficult to make, and they are of service to professional
astronomers investigating the orbits of these bodies, among whom are
Mitchell and Olivier of the University of Virginia.