William F. Denning.

Telescopic Work for Starlight Evenings online

. (page 22 of 32)
Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 22 of 32)
Font size
QR-code for this ebook


with a moderate power, say of 40, and to keep a higher magnifier at
hand to examine any suspicious objects that may be picked up. With
power 32 I often encounter forms, the real character of which is
uncertain. In such cases I clamp the telescope and apply the power
60, which generally exhibits the objects as several minute stars
grouped together, or possibly nebulæ, in which case I proceed to
identify them. With lower magnifiers than 30 there must always be
considerable danger of sweeping over faint comets. Some of these are
only of the 10th, 11th, or 12th mag., and less than 1′ diameter, and
must certainly elude detection unless adequate power is brought to
bear upon them. Dr. Doberck mentioned in the L. A. S. Journal, vol.
vi. p. 236, an instrument for comet-seeking, 3½ inches in aperture,
power about 10, and field of 5°, which was bought in 1842 by the late
Mr. Cooper at Markree. But though with such a telescope a very large
portion of the firmament might be swept in one night, there would be
serious disadvantages; for small faint comets would pass through the
field unseen, and render the work abortive. The necessary conditions
of the case go far to support the view that moderate powers and
fields are best; for a search, to be thorough and satisfactory, must
be done critically, and with a power capable of revealing the smallest
specimens of comets.

_Annual Rate of Discovery._—Arranging cometary discoveries during the
century from 1782 to 1881 into periods of 20 years, and comparing
the annual average with that during the last eight years, we get the
following numbers:—

Comets Annual
Period. found. average.
1782-1801 25 1·25
1802-1821 26 1·30
1822-1841 36 1·80
1842-1861 83 4·15
1862-1881 79 3·95
1882-1889 40 5·00

These discoveries seem to have been greatly accelerated about the year
1845. The yearly average between 1842 and 1881 was about 4; but between
1882 and 1889 it increased to 5, owing mainly to the diligence of
Barnard and Brooks.

The months in which the largest number of cometary discoveries have
been effected are July and August, the figures since 1782 being—

Comets
Month. found. Percentage.
January 22 7·6
February 20 6·9
March 18 6·2
April 25 8·7
May 17 5·9
June 21 7·3
July 34 11·8
August 38 13·2
September 22 7·6
October 20 6·9
November 26 9·0
December 26 9·0

Of 289 comets discovered during the last 108 years, 123 belonged to
the first six months, while no less than 166 belonged to the last half
of the year.

Though comets are not confined to any special region of the heavens,
there is no doubt that the vicinity of the Sun is the spot to which
the comet-seeker should direct his chief attention. It is here where
the majority of the discoveries have been made; and theoretically this
should be so, seeing that the Sun is the controlling influence of the
cometary flights, and that his position must be regarded as a sort of
focus of their convergence and divergence. Hence the most likely spots
are over the western horizon after sunset and the eastern horizon
before sunrise. The twilight and zodiacal light, together with the
mist at low altitudes, are impediments which are inseparable from this
work; but they need not interfere to any serious extent if the observer
is careful to make the best of his opportunities. But though special
attention is recommended to the neighbourhood of the Sun, other regions
should not be altogether neglected, for comets are occasionally found
in nearly the opposite part of the heavens to the Sun’s place, as, for
example, Zona’s Comet of November 1890. In order to save time, and to
prevent troublesome references during the progress of sweeping, the
brighter nebulæ should be marked upon a star-chart, so that, as they
enter the field, they may be instantly identified.

_Telescopic Comets_ vary in size to a considerable degree. In diameter
they generally range from about 1′ to 7′, and are usually round, with
a bright centre like the globular clusters Messier 2, 3, 13, 15, 49,
and 92, as seen with a low power; but occasionally they are faint
diffused masses, like the planetary nebula near β Ursæ Majoris, M. 97,
or the large nebula S. of ξ Cassiopeiæ, in the New General Catalogue,
No. 185, R.A. 0^h 33^m, Dec. 47° 44′ N. In brightness they range
from being visible to the naked eye to objects of the last degree of
faintness. They average some 2′ or 3′ diameter, but are sometimes less
than 1′; so that the power of the sweeper should be capable of readily
showing an object of this size as it passes through the field. The
observer should turn his instrument upon the small planetary nebula
N.G.C. 1501, R.A. 3^h 57^m, Dec. 60° 37′ N. in Camelopardus. It is
about 1′ diameter. He should also pick up N.G.C. 6654, R.A. 18^h 27^m,
Dec. 73° 6′ N., which is a star of about 12½ mag. involved in a pretty
conspicuous nebulosity. Swift describes the latter as looking just
like a comet. N.G.C. 6217, R.A. 16^h 38^m, Dec. 78° 25′ N., is also a
small nebulosity which might easily be overlooked with a low power.
Let the observer examine the three objects named, and he will gather
a good idea of a small telescopic comet, especially from N.G.C. 6654,
which may be readily found, as it is in the same field as χ Draconis,
and visible at any time of the year and night. N.G.C. 6643, R.A. 18^h
23^m, Dec. 74° 32′ N., is near the latter, but it is a brighter object.
The observer will find two tolerably plain nebulæ in the same field at
about R.A. 6^h 52^m, Dec. 85° 56′; so that they are only 4° from the
pole. They are N.G.C. 2276 and 2300. These objects ought not to elude
detection in any instrument properly adapted for comet-seeking.

_Ascertaining Positions._—No observer should be without the means
of determining exact positions. A ring-micrometer and comprehensive
star-catalogues are most important accessories of the amateur. When a
suspicious object is found its precise position should be instantly
measured; but if no micrometer is at hand, the observer should
carefully note the place relatively to adjoining stars, and then,
after a short interval, re-observe it for traces of motion. In these
comparisons the low-power eyepiece should be exchanged for one of
greater amplification, because this will render a slight motion more
readily sensible. If the suspicious object proves to be a comet, the
extent and direction of its daily motion should be computed from the
change in the observed places, and the information telegraphed to the
Royal Observatory, Greenwich. A statement should also be given as to
the diameter and brightness of the object; we may then be satisfied
that it will be readily picked up at some of the many stations where
prompt attention is given to this class of observation. Amateurs who do
not attempt to obtain exact positions are sometimes condemned for their
negligence in this respect, and most unjustly so. By far the hardest
part of the work falls to them, and professional astronomers ought to
be indebted to amateurs for leaving to their care an important feature
of these observations. If the latter are to undertake the labour of
measuring as well as discovering comets, then there will be nothing
left in this line for the elaborate instruments of observatories to do.
Yet, while thus objecting to amateurs, with their generally incomplete
and inefficient appliances, being expected to perform the work both
of discovery and exact observation, it cannot be denied that there is
a great necessity for them to have the means of measurement, and to
utilize them during the first few observations, which are usually made
before the comet has been seen elsewhere, and will therefore possess
great value if precise.

_Dr. Doberck’s Hints._—Dr. Doberck has given some useful hints in
connection with this subject:—“In order to be as sure as possible of
ultimate success it is not enough to sweep with the instrument and
watch any suspicious object for proper motion. It is better to procure
a large map such as Argelander’s, and, comparing the image seen in
the comet-seeker with the map, to insert all the nebulous objects
according as they are discovered. At the end of the watch they are
then compared with the catalogues of nebulæ and clusters of stars. A
general catalogue facilitates this, but is never quite sufficient, as
there seems to be no limit to the number of objects in the sky, and
more are constantly being catalogued. In the course of time an observer
learns to remember the objects he has seen before in the seeker, and
at last he need not consult the map at all. The subsequent observation
of a newly-found comet is best made with the ring-micrometer if the
telescope is not equatoreally mounted. In the latter case it should be
made by aid of a steel-bar micrometer. As soon as three observations
are available the first approximation to a parabolic orbit can
generally be determined: the calculation of which is quite elementary,
and would be enjoyed by many amateur astronomers who are fond of
figures and would easily get used to Olbers’s method. Only the three
positions must not be so near each other as to lie on a great circle.”

_Prizes for Discoveries._—The Vienna Academy of Sciences formerly gave
a gold medal to the discoverer of every new comet. These presentations
were discontinued in about 1880; and Mr. H. H. Warner then offered a
prize of $200 for every unexpected comet found in the United States
or in Canada. This prize was continued in subsequent years, and the
conditions were amended so as to include observers in Europe. Many
of these prizes were gained by Barnard and Brooks; but they have not
been re-offered during the past year or two. Mr. Warner, however,
contemplates renewing them. The Astronomical Society of the Pacific now
awards a bronze medal to all such discoverers.


FOOTNOTES:

[41] Donati’s Comet of 1858 and Coggia’s Comet of 1874 may be mentioned
as good examples of the gradual approach and development of these
visitors witnessed by means of the telescope.

[42] It ought, perhaps, in the present state of our knowledge, to be
termed “the Neptune of comets;” for it has the longest period of any
comet whose path has been definitely ascertained by multiple returns to
perihelion.

[43] Encke’s Comet has the shortest period of all the known comets.

[44] Newton conjectured that comets formed “the aliment by which suns
are sustained,” his opinion being that the former bodies finally
coalesced with the suns round which they revolved. He remarked:—“I
cannot say when the Comet of 1680 will fall into the Sun,—possibly
after five or six revolutions; but whenever that time shall arrive, the
heat of the Sun will be raised by it to such a point that our globe
will be burnt and all the animals upon it will perish.”




CHAPTER XV.

_METEORS AND METEORIC OBSERVATIONS._

Ancient ideas concerning Meteors.—Meteoric
Apparitions.—Radiation of Meteors.—Identity of Meteors
and Comets.—Aerolites.—Fireballs.—Differences of
Motion.—Nomenclature of Meteor-Systems.—Meteor-Storms.—Telescopic
Meteors.—Meteor-Showers.—Varieties of Meteors.—Heights.—Meteoric
Observations.

“As oft along the still and pure serene
At nightfall, glides a sudden trail of fire,
Attracting with involuntary heed
The eye to follow it, erewhile it rest;
And seems some star that shifted place in heaven.”
DANTE.


No one can contemplate the firmament for long on a clear moonless
night without noticing one or more of those luminous objects called
shooting-stars. They are particularly numerous in the autumnal months,
and will sometimes attract special attention either by their frequency
of apparition or by their excessive brilliancy in individual cases.
For many ages little was known of these bodies, though some of the
ancient philosophers appear to have formed correct ideas as to their
astronomical nature. Humboldt says that Diogenes of Apollonia, who
probably belonged to the period intermediate between Anaxagoras and
Democritus, expressed the opinion that, “together with the visible
stars, there are invisible ones which are therefore without names.
These sometimes fall upon the Earth and are extinguished, as took place
with the star of stone which fell at Ægos Potamoi.” Plutarch, in the
‘Life of Lysander,’ remarks:—“Falling stars are not emanations or
rejected portions thrown off from the ethereal fire, which when they
come into our atmosphere are extinguished after being kindled: they
are, rather, celestial bodies which, having once had an impetus of
revolution, fall, or are cast down to the Earth, and are precipitated,
not only on inhabited countries, but also, and in greater numbers,
beyond these into the great sea, so that they remain concealed.”

In later times, however, opinions became less rational. Falling stars
were considered to be of a purely terrestrial nature, and originated by
exhalations in the upper regions of the air. Shakespeare expressed the
popular belief when he wrote:—

“I shall fall
Like a bright exhalation in the evening,
And no man see me more.”

Another theory, attributed to Laplace, Arago, and others, was that
meteors were ejections from lunar volcanoes. But these explanations
were not altogether satisfactory in their application. The truth is,
that men had commenced to theorize before they had begun to observe and
accumulate facts. They had learnt little or nothing as to the numbers,
directions, and appearances of meteors, and therefore possessed no
materials on which to found any plausible hypothesis to account for
them.

_Meteoric Apparitions._—The occasional apparition of brilliant
detonating fireballs, the occurrence of remarkable star-showers, the
precipitation upon the Earth’s surface of stony masses, were facts
which could be verified from many independent sources, and they set
men thinking how to account for the strange and startling freaks of
nature as exhibited in such phenomena. But though records existed of
exceptionally large meteors and of meteor-showers, the descriptions
were imperfect and failed in the most important details. The observers
were usually unprepared for witnessing such events, and gave
exaggerated and inaccurate accounts of what they had seen. The vivid
brightness of a fireball (overpowering the lustre of the stars, and
even vieing with the Moon in splendour), the flaming train left in its
wake (curling itself up into grotesque shapes, as it drifted and died
away), the form of the nucleus with its jets and sparks, and the final
explosion, with the reverberations it caused, were all alluded to by
the enthusiastic observer; but it was only in rare cases that the more
valuable features were placed on record. The _direction_ and _duration_
of the meteor’s flight amongst the stars were facts of greater
significance than the mere visible aspect of the object; but they were
seldom regarded. Hence the early observations proved of little weight
in inducing just conceptions as to the phenomena of meteors.

There is, perhaps, no celestial event which can compare, as regards
its striking aspect and interesting features, with that of a meteoric
display of the most brilliant kind. A large comet, a total solar
eclipse, a bright display of aurora, have each their attractive and
imposing forms; but the effect produced is hardly equal to that during
the Earth’s _rencontre_ with a dense meteor-swarm. The firmament
becomes alive with shooting-stars of every magnitude; their incessant
flights are directed to every point of the compass for several hours;
and the scene is so animated, and one of such peculiarly impressive and
novel character, that it can never be forgotten by those who have been
among its fortunate spectators.

_Radiation of Meteors._—Heis, in Germany, was the pioneer in this
branch of practical astronomy. About half a century ago he began
systematic observations, and gathered many useful data. Schmidt, at
Bonn and Athens, followed his example; and in England Prof. Alexander
Herschel and Mr. R. P. Greg devoted themselves to the subject with
highly successful results. Their collective labours revealed a large
number of well-defined systems of meteors, and enabled them to publish
tables of the radiant-points. The investigations were more precise than
formerly, and conducted on methods ensuring more accurate and plentiful
materials. The radiation of meteors from fixed points in the sky had
been observed before in regard to the great display which occurred
in November 1833; but the meteors that fell on ordinary nights were
regarded as sporadic, until Heis and his immediate successors showed
they were reducible to an orderly arrangement and that every one of
them had its radiant-point and its origin in a definite meteor-stream.
The apparently divergent flights from a common centre are simply due
to the effects of perspective on bodies really moving in parallel
directions and collected into groups more or less scattered.

[Illustration: Fig. 52.

Radiation of Meteors

(Shower of early Perseids from 32°+53°, July 28-Aug. 1, 1878.)]

_Identity of Meteors and Comets._—The mystery concerning these
fugitive objects and their vagaries of appearance was not always to
remain concealed. Denison Olmsted had, in his work on ‘The Mechanism
of the Heavens,’ published in 1850, stated that the constitution of
the body to which the meteors of 1833 belonged bore “a strong analogy
to comets.” Reichenbach, in 1858, wrote a paper in which it was sought
to prove that a comet is a swarm of meteorites. Prof. Kirkwood, in
1861, also concluded that “meteors and meteoric rings are the debris
of ancient but now disintegrated comets, whose matter has become
distributed around their orbits.” But it remained for Schiaparelli, of
Milan, in 1866, to demonstrate the identity of meteoric and cometary
systems. Others had reasoned up to it, and observers had amassed many
useful observations bearing on the subject; but absolute proof was
wanting until Schiaparelli supplied it. He computed elements for a
well-known shower of meteors occurring on August 10th, and found the
orbit presented a very close resemblance to that of Comet III. 1862;
and he detected a similar analogy between the November meteors and
Comet I. 1866. The orbit of the April meteors was afterwards shown
by Galle and Weiss to agree with the path of Comet I. 1861; and a
meteor-shower occurring at the end of November was found to coincide
with Biela’s Comet. Facts like these could not be disproved. Comets
were thenceforth known to be the parents—the derivative source—of
meteors. Thus two important classes of objects became as one, the
differences observed being merely those of aspect due to the variable
conditions under which they were presented. The great meteor-shower of
November was found to be the dispersed materials of Tempel’s Comet of
1866 seen in detail and from a near standpoint. Every meteoric display
was known to be the visible effects of the collision of the Earth with
a comet or with the great stream of planetary fragments describing a
cometary orbit.

[Illustration: Fig. 53.

1. Double meteor, Dec. 29, 1886. 2. Curved meteor, Dec. 25, 1886.
3. Fireball, Sept. 7, 1888.
]

[Illustration: Fig. 54.

Meteorite found in Chili in 1866.]

[Illustration: Fig. 55.

Meteorite which fell at Orgueil in 1864.]

_Aerolites._—Meteors enter our atmosphere with such great velocity
that the friction induced by their impact is sufficient to destroy
them by combustion. They rarely approach the Earth’s surface within 15
miles. Occasionally, however, a slow-moving meteor of large size, and
formed of a very compact substance, will penetrate entirely through the
air-strata and fall upon the Earth’s surface. Many instances of the
kind have been recorded, and a few of these are quoted below:—

1478 B.C. The Parian chronicle records that an aerolite or
thunder-stone fell in the island of Crete. This appears to be the
earliest stone-fall described in history.

654 B.C. A shower of stones descended near Rome.

465 B.C. A stone, surrounded with fire, fell in Thrace. This stone is
referred to by several ancient writers. It was termed the “Mother of
the Gods” and is said to have fallen at the feet of the poet Pindar.

52 B.C. A shower of iron descended at Lucania, in the time of Crassus.

1492 A.D. A stone weighing 262 lb. fell at Ensisheim, in Alsace.

1642. A stone of 4 lb. fell near Woodbridge, in Suffolk.

1795, Dec. 13. A stone of 56 lb. fell at Wold Cottage, Thwing,
Yorkshire.

1860, July 14. A shower of aerolites fell at Dhurmsala, in India. A
tremendous detonation attended their descent, and the natives became
greatly alarmed. They supposed the stones to have been thrown by some
of their deities from the summit of the Himalayas, and many of them
were preserved as objects of religious veneration.

1864, May 14. A very large meteor was observed in France. At Montauban
and the neighbourhood deafening explosions occurred, and showers of
stones fell near the villages of Orgueil and Nohic.

1876, April 20. A piece of iron weighing 7-3/4 lb. fell at Rowton,
Shropshire.

1881, March 14. A stone weighing 3 lb. 8-1/4 oz. fell at
Middlesborough, Yorkshire, on a part of the North-Eastern Railway
Company’s branch line. The descent of the aerolite was witnessed by an
inspector and three platelayers, who were working about fifty yards
distant. At first they became aware of a whizzing or rushing noise in
the air, immediately followed by the sudden blow of a body striking
the ground near. The hole, 11 inches deep, which the stone made was
found directly after, and the stone was extracted.

Many other examples might be given, but the above will be sufficient
for our purpose. Records of this nature were discredited in former
times; but more modern researches have long since placed their reality
beyond all question. The fall of stones from the sky is no longer
regarded as a mere legendary tale, but as one of the well-assured
operations of nature.

Meteoric stones and irons have been classified according to the
ingredients of their composition. Those in which iron is found in
considerable amount are termed siderites, those containing an admixture
of iron and stone, siderolites, and those consisting almost entirely of
stone are known as aerolites. The siderite which fell in Shropshire on
April 20, 1876, forms only the seventh recorded instance where a mass
of meteoric iron has been actually seen to fall.

_Fireballs._—The table on p. 268 gives the dates, heights, &c. of
fifteen fireballs observed during the last quarter of a century.

Fireballs are sometimes detonating, though more often silent. The
fireball of Nov. 23, 1877, gave a sound like salvoes of artillery,
and doors and windows were shaken violently. At Chester the noise of
its explosion was compared to loud but distant thunder. Lieut.-Col.
Tupman says that “thunder, to be loud, must be within five miles;
hence it appears that the violence of the explosion must have been at
least a hundred times greater than a peal of thunder, the intensity of
sound-waves diminishing as the square of the distance.” “The explosion
of a 13-inch bomb-shell, consisting of some 200 lb. of iron, would not
have produced a sound of one hundredth part of the intensity of the
meteor-explosion.” This fireball must therefore have been an object of
considerable mass before its dissolution; and it is fortunate that such
bodies are usually destroyed by the effects of combustion before they
reach the Earth’s surface.

These phenomena exhibit many varieties of appearance.

+——————————————+——————+————————————————————+————————+——————+
| | | Height. | Real | |
| Date of | +——————————+————————-+ Length |Velo- |
| Apparition. |G.M.T.| At Ap- |At Disap-|of Path.|city. |
| | |pearance. |pearance.| | |



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 22 of 32)