OTHER METEORIC RINGS.
II. The Meteors of August 6th-11th.
Muschenbroek, in his _Introduction to Natural Philosophy_, published
in 1762, called attention to the fact that shooting-stars are more
abundant in August than in any other part of the year. The annual
periodicity of the maximum on the 9th or 10th of the month was
first shown, however, by Quetelet, shortly after the discovery of
the yearly return of the November phenomenon. Since that time an
extraordinary number of meteors has been regularly observed, both
in Europe and America, from the 7th to the 11th of the month; the
greatest number being generally seen on the 10th. In 1839, Edward Heis,
of Aix-la-Chapelle, saw 160 meteors in one hour on the night of the
10th. In 1842, he saw 34 in ten minutes at the time of the maximum. In
1861, on the night of the 10th, four observers, watching together at
New Haven, saw in three hours--from ten to one o'clock--289 meteors.
On the same night, at Natick, Massachusetts, two observers saw 397
in about seven hours. At London, Mercer County, Pennsylvania, on the
night of August 9th, 1866, Samuel S. Gilson, Esq., watching alone, saw
72 meteors in forty minutes, and, with an assistant, 117 in one hour
and fifteen minutes. Generally, the number observed per hour, at the
time of the August maximum, is about nine times as great as on ordinary
nights. Like the November meteors, they have a common "radiant;" that
is, their tracks, when produced backward, meet, or nearly meet, in a
particular point in the constellation Perseus.
Of the 315 meteoric displays given in Quetelet's "Catalogue des
principales apparitions d'étoiles filantes," 63 seem to have been
derived from the August ring. The first 11 of these, with one
exception, were observed in China during the last days of July, as
follows:
1 A.D. 811, July 25th.
2 820, " 25th-30th.
3 824, " 26th-28th.
4 830, " 26th.
5 833, " 27th.
6 835, " 26th.
7 841, " 25th-30th.
8 924, " 27th-30th.
9 925, " 27th-30th.
10 926, " 27th-30th.
11 933, " 25th-30th.
The next dates are 1243, August 2d, and 1451, August 7th. A comparison
of these dates indicates a forward motion of the node of the ring along
the ecliptic. This was pointed out several years since by Boguslawski.
A similar motion of the node has also been found in the case of the
November ring. That these points should be stationary is, indeed,
altogether improbable. The nodes of all the planetary orbits, it is
well known, have a secular variation.
On the evening of August 10th, 1861, at about 11h. 30m., a meteor
was seen by Mr. E. C. Herrick and Prof. A. C. Twining, at New
Haven, Connecticut, which "was much more splendid than Venus, and
left a train of sparks which remained luminous for twenty seconds
after the meteor disappeared." The same meteor was also accurately
observed at Burlington, New Jersey, by Mr. Benjamin V. Marsh. It was
"conformable,"--that is, its track produced backward passed through the
common radiant--and it was undoubtedly a member of the August group.
The observations were discussed by Professor H. A. Newton, of Yale
College, who deduced from them the following approximate elements of
the ring:[8]
Semi-axis major 0·84
Eccentricity 0·28
Perihelion distance 0·60
Inclination 84°
Period 281 days.
Motion, retrograde.
The earth moving at the rate of 68,000 miles per hour, is at least five
days in passing entirely through the ring. This gives a thickness of
more than 8,000,000 miles.
The result of Professor Newton's researches on the orbit of this ring,
though undertaken with inadequate data, and hence, in some respects,
probably far from correct, is nevertheless highly interesting as being
the first attempt to determine the orbit of shooting-stars. More recent
investigations have shown a remarkable resemblance between the elements
of these meteors and those of the third comet of 1862. The former, by
Schiaparelli, and the latter, by Oppolzer, are as follows:
Meteors of August 10th. Comet III., 1862.
Longitude of perihelion 343° 38´ 344° 41
Ascending node 138 16 137 27
Inclination 63 3 66 25
Perihelion distance 0·9643 0·9626
Period 105 years(?). 123 years(?).
Motion Retrograde. Retrograde.
This similarity is too great to be accidental. _The August meteors and
the third comet of 1862 probably belong to the same ring._
III. The Meteors of April 18th-26th.
The following dates of the April meteoric showers are extracted from
Quetelet's table previously referred to:
1 A.D. 401, April 9th.
2 538, " 7th.
3 839, " 17th.
4 927, " 17th.
5 934, " 18th.
6 1009, " 16th.
7 1094, " 10th.
8 1096, " 10th.
9 1122, " 11th.
10 1123, " 11th.
11 1803, " 20th.
12 1838, " 20th.
13 1841, " 19th.
14 1850, " 11th-17th.
The display of 401 was witnessed in China, and is described as "very
remarkable." That of 1803 was best observed in Virginia, and was at its
maximum between one and three o'clock. The alarm of fire had called
many of the inhabitants of Richmond from their houses, so that the
phenomenon was generally witnessed. The meteors "seemed to fall from
every point in the heavens, in such numbers as to resemble a shower of
sky-rockets." Some were of extraordinary magnitude. "One in particular,
appeared to fall from the zenith, of the apparent size of a ball 18
inches in diameter, that lighted the whole hemisphere for several
seconds."
The probability that the meteoric falls about the 20th of April are
derived from a ring which intersects the earth's orbit, was first
suggested by Arago, in 1836. The preceding list indicates a forward
motion of the node. The radiant, according to Mr. Greg, is about
_Corona_. The number of meteors observed in 1838, 1841, and 1850, was
not very extraordinary. Recent observations indicate April 9th-12th as
another epoch. The radiant is in Virgo.
IV. The Meteors of December 6th-13th.
On the 13th of December, 1795, a large meteoric stone fell in England.
On the night, between the 6th and 7th of December, 1798, Professor
Brandes, then a student in Göttingen, saw 2000 shooting-stars. On
the 11th of the month, 1836, a fall of meteoric stones, described by
Humboldt as "enormous," occurred near the village of Macao, in Brazil.
During the last few years unusual numbers of shooting-stars have been
noticed by different observers from the 10th to the 13th; the maximum
occurring about the 11th. From A.D. 848, December 2d, to 1847, December
8th-10th, we find 14 star showers in Quetelet's catalogue, derived,
probably, from this meteoric stream. As in other cases, the dates
seem to show a progressive motion of the node. The position of the
radiant, as determined by Benjamin V. Marsh, Esq., of Philadelphia,
from observations in 1861 and 1862, and also by R. P. Greg, Esq., of
Manchester, England, is at a point midway between Castor and Pollux.
V. The Meteors of January 2d-3d.
About the middle of the present century, Mr. Julius Schmidt, of Bonn,
a distinguished and accurate observer, designated the 2d of January as
a meteoric epoch; characterizing it, however, as "probably somewhat
doubtful." Recent observations, especially those of R. P. Greg, Esq.,
have fully confirmed it. The meteors for several hours are said to be
as numerous as at the August maximum. The radiant is near the star
_Beta_ of the constellation Böotes.
Quetelet's list contains at least five exhibitions which belong to this
epoch. Two or three others may also be referred to it with more or less
probability.
* * * * *
Several other meteoric epochs have been indicated; some of which,
however, must yet be regarded as doubtful. In thirty years, from 1809
to 1839, 12 falls of bolides and meteoric stones occurred from the 27th
to the 29th of November. Such coincidences can hardly be accidental.
Unusual numbers of shooting-stars have also been seen about the 27th
of July; from the 15th to the 19th of October, and about the middle
of February. The radiant, for the last-mentioned epoch, is in _Leo
Minor_. The numbers observed in October are said to be at present
increasing. At least seven of the exhibitions in Quetelet's catalogue
are referable to this epoch. It is worthy of remark, moreover, that
three of the dates specified by Mr. Greg as _aerolite_ epochs are
coincident with those of shooting-stars; viz., February 15th-19th, July
26th, and December 13th. The whole number of exhibitions enumerated
in Quetelet's catalogue is 315. In eighty-two instances the day of
the month on which the phenomenon occurred is not specified. Nearly
two-thirds of the remainder, as we have seen, belong to established
epochs, and the periodicity of others will perhaps yet be discovered.
But reasons are not wanting for believing that our system is traversed
by numerous meteoric streams besides those which actually intersect the
earth's orbit. The asteroid region between Mars and Jupiter is probably
occupied by such an annulus. The number of these asteroids increases
as their magnitudes diminish; and this doubtless continues to be the
case far below the limit of telescopic discovery. The zodiacal light
is probably a dense meteoric ring, or rather, perhaps, a number of
rings. We speak of it as _dense_ in comparison with others, which are
invisible except by the ignition of their particles in passing through
the atmosphere. From a discussion of the motions of the perihelia of
Mercury and Mars, Leverrier has inferred the existence of two rings of
minute asteroids; one within the orbit of Mercury, whose mass is nearly
equal to that of Mercury himself; the other at the mean distance of the
earth, whose mass cannot _exceed_ the tenth part of the mass of the
earth.
Within the last few years a distinguished European savant, Buys-Ballot,
of Utrecht, has discovered a short period of variation in the amount
of solar heat received by the earth: the time from one maximum to
another exceeding the period of the sun's apparent rotation by about
twelve hours. The variation cannot therefore be due to any inequality
in the heating power of the different portions of the sun's surface.
The discoverer has suggested that it may be produced by a meteoric
ring, whose period slightly exceeds that of the sun's rotation. Such
a zone might influence our temperature by partially intercepting the
solar heat.
GENERAL REMARKS.
1. The average number of shooting-stars seen in a clear, moonless
night by a single observer, is about 8 per hour. _One_ observer,
however, sees only about one-fourth of those visible from his point of
observation. About 30 per hour might therefore be seen by watching the
entire hemisphere. In other words, 720 shooting-stars per day could be
seen by the naked eye at any one point of the earth's surface, did the
sun, moon, and clouds permit.
2. The mean altitude of shooting-stars above the earth's surface is
about 60 miles.
3. The number visible over the whole earth is about 10,460 times the
number to be seen at any one point. Hence the average number of those
daily entering the atmosphere and having sufficient magnitude to be
seen by the naked eye, is about 7,532,600.
4. The observations of Pape and Winnecke indicate that the number of
meteors visible through the telescope, employed by the latter, is about
53 times the number visible to the naked eye, or about 400,000,000
per day.[9] This is two per day, or 73,000 per century, for every
square mile of the earth's surface. By increasing the optical power,
this number would probably be indefinitely increased. At special
times, moreover, such as the epochs of the great meteoric showers,
the addition of foreign matter to our atmosphere is much greater than
ordinary. It becomes, therefore, an interesting question whether
sensible changes may not thus be produced in the atmosphere of our
planet.
5. In August, 1863, 20 shooting-stars were doubly observed in England;
that is, they were seen at two different stations. The average weight
of these meteors, estimated--in accordance with the mechanical theory
of heat--from the quantity of light emitted, was a little more than two
ounces.
6. A meteoric mass exterior to the atmosphere, and consequently
non-luminous, was observed on the evening of October 4th, 1864, by
Edward Heis, a distinguished European astronomer. It entered the field
of view as he was observing the milky way, and he was enabled to follow
it over 11 or 12 degrees of its path. It eclipsed, while in view, a
number of the fixed stars.