_COMETS AND COMET-SEEKING._
Ideas concerning Comets.—Appearance.—Large number
visible.—Nature of Apparition.—Tenuity of Comets.—Differences
of Orbit.—Discoveries of Comets.—Large Comets.—Periodical
Comets.—The Comets of Halley, Encke, Biela, Brorsen, Faye,
D’Arrest, Pons-Winnecke, and Tuttle.—Grouping.—Further Observations
required.—Nomenclature of Comets.—Curiosities of Comets.—Naked-eye
Comets.—Comet-seeking.—English weather.—Aperture and Power
required.—Annual rate of Discovery.—Telescopic Comets and
Nebulæ.—Ascertaining Positions.—Dr. Doberck’s hints.—Prizes.
Superstitious ideas with regard to comets as the harbingers of disaster
have long since been discarded for more rational opinions. They are no
longer looked upon as ill-omened presages of evil, or as
“From Saturnius sent,
To fright the nations with a dire portent.”
Many references are to be found among old writings to the supposed evil
influence of these bodies, and to the dread which their appearance
formerly incited in the popular mind. Shakespeare makes an allusion to
the common belief:—
“Hung be the heavens with black, yield day to night!
Comets, importing change of time and states,
Brandish your crystal tresses in the sky;”
and in relation to the habit of connecting historical events with their
apparition, he further says:—
“When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.”
But, happily, the notions prevalent in former times have been
superseded by the more enlightened views naturally resulting from
the acquirement and diffusion of knowledge; so that comets, though
still surrounded by a good deal of mystery, are now regarded with
considerable interest, and welcomed, not only as objects devoid of
malevolent character, but as furnishing many useful materials for
study. Mere superstition has been put aside as an impediment to real
progress, and a more intelligent age has recognized the necessity of
dealing only with _facts_ and explaining them according to the laws of
nature; for it is on facts, and their just interpretation, that all
true searchers after knowledge must rely. Comets are properly regarded
as bodies which, though far from being thoroughly understood in all the
details of their physical structure and behaviour, have yet a wonderful
history, and one which, could it be clearly elucidated, would unfold
some new and marvellous facts. Under these circumstances we need evince
no surprise that these visitors are invariably hailed with enthusiasm,
not only by scientific men, who make them the special subjects of close
observation, but by everyone who regards celestial “sights and signs”
with occasional attention.
_Appearance._—From whatever point of view a large comet is considered,
it deserves all the interest manifested in it and all the labour
expended in its investigation. Whilst its grand appearance in the
firmament arrests the notice of all classes alike, and is the subject
of much curious speculation amongst the uninformed, it merits, apart
from other considerations, the most assiduous observation on account
of the singular features it displays and the striking variations they
undergo. Indeed, the visible deportment of a comet during its rapid
career near perihelion is so extraordinary as to form a problem, the
solution of which continues to defy the most ingenious theories. The
remarkable changes in progress, the quickness and apparent irregularity
of their development, are the immediate result of a combination of
forces, the operations of which can neither be defined nor foreseen.
Jets of flame and wreaths of vapour start from the brilliant nucleus;
while, streaming away from the latter, in a direction opposite to the
Sun, is the fan-shaped tail, often traceable over a large span of the
heavens and commingling its extreme fainter limits with the star-dust
in the background.
_Large number visible._—The orbits of 400 comets have now been
computed, and more than 500 others have been observed; so that these
bodies are extremely plentiful. Kepler described them to be as numerous
as the fishes in the sea, and no doubt the allegory is justly applied.
Their vagaries of form, size, and place are equally noteworthy; and
those who enter upon the discussion of facts relating to these objects
will find an endless store of interesting materials, opening up a wide
field for conjecture.
_Nature of Apparition._—The apparition of a comet may be either gradual
or sudden. Usually the telescope gives us the earliest intimation
that one of these bodies is approaching us[41]. It is first seen as
a small round nebulosity, with probably a central condensation or
stellar nucleus of the 10th or 11th mag. The whole object brightens and
expands as its distance grows less, and it assumes an elongated form
preparatory to the formation of a tail. The latter varies greatly in
different instances: it may either be a narrow ray, as shown in the
southern comet of January 1887, or a fan-shaped extension like that of
the great comet of 1744. Barnard’s Comet of December 1886 exhibited
a duple tail. Occasionally a fine comet bursts upon us suddenly,
like that of 1843 or 1861. The former was sufficiently bright to be
discovered when only 4° from the Sun, and the latter presented itself
quite unexpectedly as a magnificent object even in the strong twilight
of a June sky.
_Tenuity of Comets._—Comets are noteworthy for the extreme thinness of
their material. The smallest stars may be discerned through the denser
portions of the head, without suffering any apparent diminution of
light. Yet such stars would be quite obscured by the interposition of
a minute speck of cloud or by a little fog or any vapour of trifling
density. Comets are visible in the form of transparent nebulosities;
and their mass must be inconceivably small relatively to the enormous
space over which they frequently extend. Sir J. Herschel has described
the “all but spiritual texture” of comets; and other authorities have
referred to them as feeble wreaths of vapour, which, though obeying
the laws of gravitation and suffering much perturbation, are yet
themselves incapable of exercising any disturbing influence upon the
other bodies near which they pass. It has been asserted that comets
would show phases were they rendered luminous by reflected sunlight,
and that, such features being absent, these bodies must possess a
phosphorescence of their own sufficient to cause the glow observed.
This idea, however, is hardly consistent with our present knowledge.
Comets are not compact and coherent masses of matter; they more
likely represent vast groups of planetary atoms, more or less loosely
dispersed and sometimes forming streams. The effect of sunshine
upon such assemblages will be that the whole mass becomes illumined
according to density, and that no phase will be apparent, inasmuch as
the light is enabled to penetrate through its entirety.
_Differences of Orbit._—When three trustworthy observations of a
comet’s place have been made, its orbit may be computed. This may be
either an ellipse, a parabola, or hyperbola. If an _ellipse_ the comet
is periodical, and the period depends upon the degree of eccentricity.
If a _parabola_ the comet will not be seen again, because this form of
orbit does not reunite; it consists of branches equally divergent and
uniting at perihelion, but extending outwards indefinitely in nearly
parallel lines and without convergence. If a _hyperbola_, the comet is
also not returnable; the branches of the orbit are widely divergent,
and show no tendency to parallelism. These several forms of orbit are
somewhat different as applied to various comets, but they are the same
in effect. Thus Tempel’s Comet of 1867 revolves in an ellipse having
an eccentricity of about 0·4630, while that of Halley’s Comet is
0·9674. No doubt some of the parabolic orbits applied to comets really
represent very eccentric ellipses; but the parabola is a convenient
form of orbit for computation, and unless ellipticity is very decided
it indicates the path with sufficient accuracy.
_Discoveries of Comets._—In the latter part of the last century
Messier, Mechain, and Miss Herschel shared nearly all the cometary
discoveries between them. Then Pons entered the field, and he may be
said to have monopolized this branch during the period from 1802 to
1827, for he was the first to announce thirty comets. Pons died in
1831, but the search was actively continued by others. In about 1843
a great rise became apparent in the rate of these discoveries; and we
find Di Vico, Mauvais, and Brorsen very successful at this period.
Later on, the work was sustained with the same prolific results by
Klinkerfues, Bruhns, and Donati, and subsequently by Winnecke, Tempel,
and Coggia. Swift and Borrelly also assisted materially to swell our
knowledge; while during the last few years Barnard and Brooks have
exhibited a surprising amount of zeal in this department. Since 1881 no
less than twenty-six comets are to be enumerated as the fruits of their
endeavours, and they are still engaged in nightly explorations of the
sky with similar ends in view. Their diligent pursuit of these fugitive
bodies will doubtless result in many further additions during ensuing
years.
It is a curious circumstance that Sir W. Herschel, during all his
star-gaugings and sweeps for nebulæ, never discovered a comet. He found
a nebula on Dec. 18, 1783, near δ Ceti, which he described as “small
and cometic.” In Sir J. Herschel’s ‘General Catalogue of Nebulæ,’
1864, p. 17, this object is presumed to have been a comet, as it could
not be identified; but at p. 45 the doubts are cleared up, and Sir W.
Herschel’s nebula, the position of which was only roughly given, is
shown to be the same as another very near; it is No. 1055 of the new
‘General Catalogue’ published by the Royal Astronomical Society in
January 1888. Quite possibly Sir W. Herschel’s lists of nebulæ contain
several comets, as some of his objects are missing; but errors of
observation in ascribing positions may explain this. Herschel himself,
in speaking of a comet visible in the winter of 1807-8, says:—“If I had
met the comet in one of my sweeps, as it appeared between Dec. 6 and
Feb. 21, I should have put it down as a nebula. Perhaps my lists of
nebulæ, then, contain some comets.”
_Large Comets._—The most widely observed and attractive class of
comets includes those of large proportions, as they are not only
visible to the naked eye, but exhibit features having the lustre
necessary to permit of their examination with high magnifying powers.
A brief summary of some of the finest comets of modern times is
subjoined; but, to save space, a few only of the more salient facts
concerning them are given:—
1577, Nov. and Dec.—Observed by Tycho Brahe. At the end of November it
had a double tail; the longest of the two branches was about 20°. This
comet was visible in the daytime.
1618 II., Nov.—“The length of its tail equalled in extent one sixth
part of the zodiac.” On Nov. 18 it was estimated as 40°. Longomontanus,
however, described it as 104° long, and Cysatus estimated it as 75°.
Kepler referred to it as the largest comet that had appeared for a
hundred and fifty years.
1680, Dec.—A fine comet, which on Dec. 12 had a narrow tail about 80°
long. The nucleus was equal to a 1st mag. star. Hooke remarked jets of
flame issuing from the nucleus. At perihelion the comet approached very
near the Sun’s surface, similarly to the fine comets of 1843, 1880, and
1882.
1744, Jan.-Feb.—Probably the largest comet of the 18th century. At
one time it displayed six tails, each of which was 4° in breadth. The
head was so bright that it was perceived with the naked eye in full
sunshine. At the middle of February the tail was 24° long, and it was
divided into two branches.
1769, Sept.—Discovered on Aug. 8 by Messier. On Aug. 30 the comet had a
trifid tail; there was a central ray of 24° and two outlying ones of 4°
each. On Sept. 19 the tail had increased to 75°, and a few nights later
Pingre estimated it as 90° and 97°.
1811 I., Sept.-Oct.—A very fine comet. The tail was branched; it did
not, however, exceed 25° in length and about 6° in breadth. Sir W.
Herschel found the nucleus to be 428 miles in diameter. This remarkable
comet remained visible during a period extending over seventeen months.
Its period is approximately 3000 years.
1843, Mar.—Visible in the daytime. On Mar. 4 its tail was 69° in
length; it was very narrow, being only 1-1/4° in breadth throughout.
At perihelion this object passed very near to the Sun, like the great
comet of 1680. it revolves in an elliptical orbit; period about 376
years. This comet swept past perihelion with a velocity of 366 miles
per second! The real length of its tail was 200 millions of miles!
1858 VI., Sept.-Oct.—Donati’s Comet: one of the most brilliant comets
of the 19th century. Early in October it displayed a tail about 40°
long, and on the 5th it passed over the star Arcturus. Its period of
revolution appears to be about 2000 years.
1861 II., June-July.—Became suddenly visible at the end of June. In
the opinion of Sir John Herschel this comet surpassed in grandeur
the comets of 1811 and 1858. On June 30 the nucleus was equal to the
brightness of Venus, and the tail was 80° long; but early in July it
increased to 90°. One observer estimated its length as 100° on July 2.
This comet remained visible during twelve months. It appears to have an
elliptical orbit, with a period of 409 years.
1874, July.—Coggia’s Comet: a fine object in the northern sky. On July
14 the tail was 35° long, and it remained visible several days after
the nucleus had disappeared below the horizon. The nucleus was about
equal to a star of the 1st mag. Orbit probably elliptical, with a
period of about 5711 years.
1880 I., Jan.-Feb.—A southern comet, with a long narrow tail, variously
estimated from 30° to 40° in length. It passed very near to the Sun,
and presents an orbital resemblance to the fine comets of 1680 and 1843.
1881 III., June-July.—This large comet appeared in the northern heavens
on June 22, and became generally visible to observers in England. On
the 27th it had a tail 15° long. Its period of visibility extended over
nine months.
1881 IV., Aug.—This comet is scarcely entitled to rank as one of
exceptional character; but it was a conspicuous object for several
weeks in August, and had a tail 6° long on the 19th.
1882 III., Oct.—Well visible in the morning sky, with a tail 22° long.
The nucleus underwent remarkable changes, and on Oct. 23 it showed four
or five bright points or nuclei, looking like “a string of beads.” The
comet threw off several small condensations, which were observed as
separate comets near the parent mass. At perihelion this comet passed
very close to the Sun, like the comets of 1680, 1843, and 1880; and
these bodies were suspected to have an intimate relation, if not an
absolute identity. But subsequent inquiries disproved this startling
supposition; for the comet of 1882 was shown to have a period of about
718 years.
1887 I., Jan.—A fine southern comet, presenting many points of
resemblance to that of 1880 I. On Jan. 22, as observed at Adelaide,
the comet had a long narrow tail of about 30°, but no well-defined
nucleus. On the same date, at the Cape of Good Hope, the tail appeared
as a narrow ribbon of light, quite straight, and of nearly uniform
brightness throughout its length. It was visible in the same region of
the sky as the comet 1880 I., and came into view with equal suddenness.
_Periodical Comets._—On page 235 is a list of the periodical comets as
at present known. Some of these, marked with an asterisk, have only
been observed at one return, and therefore await complete confirmation.
Many other comets have shown indications of pursuing elliptical orbits.
Amongst those of short period may be mentioned 1743 I., 1766 II., 1783
I., 1819 IV., 1844 I., and 1873 VII. The following are examples of
longer periods:—
Comet. Period.
1862 III. 121 years.
1857 IV. 234 ”
1861 I. 415 ”
1860 III. 1089 ”
1889 IV. 5100 ”
1877 II. 8393 ”
1847 III. 13918 ”
1877 III. 28,000 ”
1850 I. 29,000 ”
1780 I. 75,314 ”
1844 II. 102,050 ”
1744 122,683 ”
1849 I. 382,801 ”
1882 I. 400,000 ”
These figures are to be regarded as approximations only.
+——————————————+——————-+——————————————-+——————————-+————————-+
| |Period,| Perihelion | Long. of | Long. of|
| Name. | in | Passage. |Perihelion.|Ascending|
| |years. | | | Node. |
+——————————————+——————-+——————————————-+——————————-+————————-+
| | | | ° ′ | ° ′ |
| Encke | 3·29 |1888, June 28 | 158 36 | 334 39 |
| Tempel (1873)| 5·20 |1878, Sept. 7 | 306 8 | 121 1 |
|*Barnard | 5·40 |1884, Aug. 16 | 301 2 | 5 9 |
| Brorsen | 5·46 |1879, Mar. 30 | 116 15 | 101 19 |
| Pons-Winnecke| 5·73 |1886, Sept. 16 | 276 4 | 101 56 |
| Tempel(1867) | 5·98 |1879, May 7 | 238 11 | 78 46 |
| Tempel-Swift | 5·99 |1880, Nov. 8 | 43 0 | 296 42 |
|*Brooks (1886)| 6·30 |1886, June 7 | 229 46 | 53 3 |
|*Spitaler | 6·40 |1890, Oct. 26 | 58 24 | 45 8 |
| Biela | 6·62 |1852, Sept. 23 | 109 8 | 245 52 |
| D’Arrest | 6·64 |1890, September| 319 9 | 146 9 |
|*Finlay | 6·67 |1886, Nov. 22 | 7 34 | 52 30 |
|*Wolf | 6·78 |1884, Sept. 27 | 352 31 | 206 18 |
|*Swift | 6·91 |1889, Nov. 29 | 69 29 | 331 27 |
|*Brooks (1889)| 7·07 |1889, Sept. 30 | 1 26 | 17 59 |
| Faye | 7·57 |1888, Aug. 19 | 50 56 | 209 42 |
|*Denning | 8·69 |1881, Sept. 13 | 312 31 | 65 57 |
|*Peters | 12·80 |1846, June 1 | 240 7 | 260 28 |
| Tuttle | 13·66 |1885, Sept. 11 | 116 28 | 269 42 |
|*Tempel (1866)| 33·18 |1866, Jan. 11 | 60 28 | 231 26 |
|*Stephan | 33·62 |1867, Jan. 20 | 75 52 | 78 36 |
|*Westphal | 67·77 |1852, Oct. 13 | 43 12 | 346 13 |
| Pons | 71·48 |1884, Jan. 25 | 93 21 | 254 6 |
| Olbers | 72·45 |1887, Oct. 8 | 149 45 | 84 30 |
|*Di Vico | 73·25 |1846, Mar. 6 | 90 35 | 77 36 |
|*Brorsen | 74·97 |1847, Sept. 10 | 79 13 | 309 49 |
| Halley | 76·37 |1835, Nov. 15 | 304 32 | 55 10 |
+——————————————+——————-+——————————————-+——————————-+————————-+
+——————————————+——————-+————-+——————-+
| |Incli- | Mo- | Next |
| Name. |nation.|tion.|Return.|
| | | | |
+——————————————+——————-+————-+——————-+
| | ° ′ | | |
| Encke | 12 53 | D | 1891 |
| Tempel (1873)| 12 46 | D | 1894 |
|*Barnard | 5 28 | D | 1895 |
| Brorsen | 29 23 | D | 1895 |
| Pons-Winnecke| 14 27 | D | 1892 |
| Tempel(1867) | 9 47 | D | 1891 |
| Tempel-Swift | 5 31 | D | 1892 |
|*Brooks (1886)| 12 56 | D | 1892 |
|*Spitaler | 12 52 | D | 1897 |
| Biela | 12 33 | D | ? |
| D’Arrest | 15 43 | D | 1897 |
|*Finlay | 3 2 | D | 1893 |
|*Wolf | 25 16 | D | 1891 |
|*Swift | 10 3 | D | 1896 |
|*Brooks (1889)| 6 4 | D | 1896 |
| Faye | 11 20 | D | 1896 |
|*Denning | 6 51 | D | 1899 |
|*Peters | 30 24 | R | ? |
| Tuttle | 54 19 | D | 1899 |
|*Tempel (1866)| 17 18 | R | 1899 |
|*Stephan | 18 13 | D | 1900 |
|*Westphal | 40 59 | D | 1920 |
| Pons | 74 3 | D | 1955 |
| Olbers | 44 35 | D | 1960 |
|*Di Vico | 84 57 | D | 1919 |
|*Brorsen | 19 8 | D | 1922 |
| Halley | 17 45 | R | 1912 |
+——————————————+——————-+————-+——————-+
_Halley’s Comet._—A fine comet with a tail about 15° long appeared in
the summer of 1682, and Halley computed the orbit according to the
method explained by Newton. He then consulted observations of previous
comets, and discovered a great similarity in the paths of large comets
seen in 1531 and 1607 to that of the one he himself had observed in
1682. He thereupon suspected the three bodies to be one and the same,
and advised posterity to maintain a strict watch for the comet’s return
in about 1758 or 1759. On pursuing his investigations still further, he
alighted upon records of comets in 1305, 1380, and 1456, which greatly
strengthened his opinion that the comet of 1682 moved in an elliptical
path with a period of about 75½ years. He termed this body “the
Mercury[42] of comets, revolving round the Sun in the smallest orbit,”
and said that, should it reappear according to his prediction in about
the year 1758, “impartial posterity must needs allow this to be the
discovery of an Englishman.”
As the time drew near for the return of the comet, interest became
intensified, and computations were made by Clairaut with a view to
determine the precise epoch when it would arrive at perihelion. He
found that the comet would be retarded by the action of Jupiter and
Saturn, but that perihelion would be reached at the middle of April
1759, subject to an uncertainty of 30 days. The comet was rediscovered
on Dec. 25, 1758, by Palitzch, an amateur astronomer at Politz, near
Dresden, who employed a telescope of 8 feet focal length, and appears
to have anticipated Messier and others who were on the alert for it.
It arrived at perihelion on March 12, 1759, and within a month of the
date announced by Clairaut. Early in May it had a tail nearly 50° long,
and presented a fine aspect in the heavens. Thus the sagacity of Halley
had revealed a periodical comet—the first known. It duly returned again
in 1835, and received all the attention which a body so replete with
historical associations deserved.
[Illustration: Fig. 47.
Comet 1862 III. (Aug. 19, 1862).]
[Illustration: Fig. 48.
Sawerthal’s Comet, 1888 I. (March 25, Brooks).]
_Encke’s Comet._—Until the year 1819 Halley’s Comet was the only one
certainly known to be periodical. Then the able deductions of Encke
presented us with a veritable “Mercury[43] of comets.” He showed that
a small comet, discovered by the unwearying Pons of Marseilles on Nov.
26, 1818, was really identical with three previous comets—viz. 1786 I.
(Mechain), 1795 (C. Herschel), and 1805 (Thulis),—and that its period
was a very short one of about 3-1/3 years. Its return to perihelion
was predicted to occur on May 24, 1822, and this was observed in the
southern hemisphere. It returned again on Sept. 16, 1825, and on this
occasion the circumstances were more favourable. Since 1825 this
object has effected nineteen returns to perihelion, and one of the
most singular facts noticed in connection with it is that its period
is gradually shortening. In 1795 it was 1212 days, while in 1858 it
was 1210. In order to explain this contraction of orbit, it has been
necessary to assume the existence of a thin medium in space capable
of affording a slight resistance to the tenuous materials of a comet,
though not dense enough to appreciably affect the motions of planets.
If this closing-up of the orbit and shortening of period continue
to operate through a vast interval of time, Encke’s Comet must be
ultimately precipitated upon the Sun![44]
_Biela’s Comet._—This comet was discovered on Feb. 27, 1826, by Wilhelm
von Biela, an Austrian officer, at Josephstadt in Bohemia, and ten
days later by Gambart at Marseilles. It was found to be revolving in
an orbit of short period, and its elements presented an agreement with
those of the comet of 1772 (Montaigne) and 1806 I. (Pons). Identity
was inferred, and the next return was fixed for Nov. 27, 1832, when
the object reappeared with great punctuality. At the end of 1845 this
comet displayed some startling phenomena; for it divided into two
portions, apparently quite disconnected, and which travelled side by
side, separated by an interval of more than 150,000 miles! The double
comet was observed again in 1852, when the interval separating them
had, however, increased eightfold, for the dark space between measured
1,250,000 miles. This instance of a divided comet is by no means
unique. The great comet of 1882 underwent a process of disruption, by
throwing off small masses of nebulosity, which, however, survived
the separation only a few days. Brooks’s Comet (1889 V.) was found by
Barnard, on Aug. 1, 1889, to be divided into four parts! Two of these
had a brief existence; but one of the minor fragments retained a very
distinct appearance near the parent mass during the ensuing months
of September and October. The phenomena of Biela’s Comet found an
excellent counterpart here.
[Illustration: Fig. 49.
Brooks’s double Comet, Sept. 17, 1889.
(10-inch reflector, power 60. W. F. Denning.) ]
Since 1852 Biela’s Comet has been lost. The most assiduous observations
have failed to recover it, and the conclusion seems irresistible that
further disintegrations have occurred and that its material has been
dispersed beyond recognition. The great meteoric storms of Nov. 27,
1872 and 1885, were derived from this comet, and there is little reason
to hope that _as a comet_ it will ever be seen again.
_Brorsen’s Comet._—A small comet was discovered in Pisces by Theodor
Brorsen, at Kiel, on the evening of Feb. 26, 1846, Its observed path
soon gave traces of an elliptical orbit; and the period was found to be
about 5·58 years. The comet was re-observed at its return to perihelion
in 1857, 1868, 1873, and 1879; but in 1884 it was looked for in vain.
This comet was expected in February 1890, and several observers swept
for it diligently, but to no purpose. Are we, therefore, to regard
this as another lost member of our system? Has the comet of Brorsen,
like that of Biela, suffered dispersion in such degree as to be no
longer within the reach of our powerful telescopes? Should negative
results again attend observers in 1895, when the comet ought to return,
there will be no reason to doubt its actual disappearance. It may be
mentioned that owing to planetary perturbations, the period of this
body has rapidly become shorter since 1846. It was then 2034 days, but
in 1879 was reduced to 1995 days.
_Faye’s Comet._—First seen at the Paris Observatory on the night of
Nov. 22, 1843, when it was near the star Bellatrix in Orion. The
observations clearly proved the comet to be moving in an elliptical
path, and Dr. Goldschmidt of Göttingen determined its period as 7½
years. It was re-observed in 1851, and also during each of its five
subsequent returns, the last of which occurred in August 1888. The
orbit of this body approaches nearer to the circular form than that of
any other known comet, except Tempel’s of 1867. Its perihelion distance
is considerable, for it never comes within the orbit of Mars. Prof.
Möller, of Lund, has investigated the path with all the critical acumen
of a profound mathematician, and, chiefly owing to his labours, it is
now regarded as one of the best known members of our system.
_D’Arrest’s Comet._—Discovered at the Leipsic Observatory on June
27, 1851. M. Villarceau discussed the orbit, and announced it as an
elliptic one with a periodic time of about 6½ years. The comet was
redetected at its return in 1857-8, 1870, 1877, and 1890. It is a very
faint object.
_Pons-Winnecke’s Comet._—Discovered at Bonn on March 8, 1858, and on
the elements being computed they were found nearly coincident with
those of Pons’s Comet, 1819 III. Encke had assigned a period of 5·62
years for the latter, but it managed to escape observation during the
six returns that occurred in the 39 years between 1819 and 1858. Its
identity was fully established in 1869, when it was again observed.
_Tuttle’s Comet._—A faint, diffused comet was discovered in the
northern part of Hercules by H. P. Tuttle, of Cambridge, U.S.A., on
Jan. 4, 1858. Its elements on being calculated were found by Pape to
be similar to those of a comet discovered by Mechain on Jan. 9, 1790,
and an elliptic orbit with a period of 13·66 years was derived from the
new observations. On the assumption that the two bodies were one and
the same there must have occurred four unobserved returns to perihelion
between 1790 and 1858. The year 1871 was awaited in settlement of the
question. When it came the comet returned, and the predictions received
exact verification. Thus the comets of Mechain and Tuttle were placed
in the inseverable bonds of identity.
Of the other periodical comets it will be unnecessary to give details.
Some of them are still without full corroboration, only one return
to perihelion having been observed. The reappearance of Pons’s Comet
(1812) in 1883-4, and of Olbers’s Comet (1815) in 1887 furnished two
excellent examples of well-determined comets belonging to the same
class as that of Halley. Tempel’s discoveries in 1867, 1869, and 1873
afforded some interesting additions to the family of short-period
comets, and the list of these is continually extending owing to the
assiduity of observers, though the lost comets of Biela and Brorsen
will have to be removed from it. Peters’s Comet of 1846 is also
doubtful, as it escaped rediscovery in about 1859, 1872, and 1884; but
this object may yet be captured at one of its succeeding apparitions.
These bodies often evade redetection when their periods and paths are
not accurately known. This has been fully exemplified in the case of
the comets of Pons-Winnecke and Tuttle, which were unseen at several
consecutive returns. It has been supposed, and not without reason, that
the periodical comets are in process of wearing away. They apparently
grow fainter at each return. Halley’s Comet in 1835 was only moderately
bright, whereas in ancient times its appearance was magnificent.
[Illustration: Fig. 50.
Pons’s Comet (1812). Telescopic view. 1884, January 6, 5^h 50^m.
(10-inch reflector, power 60. W. F. Denning.)]
[Illustration: Fig. 51.
Pons’s Comet (1812). Telescopic view. 1884, January 21, 13^h 18^m.
(5-inch refractor; comet-eyepiece, field 1-1/4°. E. E. Barnard.)]
_Grouping of Periodical Comets._—It is a curious circumstance that
these bodies are assorted into groups having their aphelia near the
orbits of major planets. The short-period comets comprised within
the orbits of Encke’s (3·29 years) and Denning’s (8·69 years)
have aphelia in the region of the path of Jupiter, hence they
are occasionally referred to as Jovian comets. The next group is
represented by the comets of Peters and Tuttle, with aphelia near
Saturn. The third group includes the comets of Tempel and Stephan,
with aphelia just outside the orbit of Uranus. The fourth group is
shown by the comets of Halley, Pons, and Olbers, with three others
less certainly ascertained, with aphelia exterior to Neptune. There
are unmistakable indications of other groups far outside the known
boundaries of the solar system, but these are not so well defined.
This clustering of cometary orbits has been ascribed to the attractive
influences of the large superior planets, which are so capable of
disturbing the paths of comets passing near that the orbits become
transformed, and the aphelia henceforth lie near the points of extreme
perturbation. This has been called the “capture theory;” and there is
also an “ejection theory,” which supposes the periodical comets to have
had their birth in planetary ejections.
M. Hoek of Utrecht has found cases in which the orbits of two or more
comets exhibit a common point of intersection in distant space, and
infers their derivation from the same origin.
_Further Observations required._—One of the chief and essential
features in cometary work is the accurate determination of positions.
But this entails the possession of expensive instruments, and a
knowledge which amateurs have not always acquired. This department of
labour can well be left to the trained hands at large observatories,
where, fortunately, it meets with every attention. Ordinary observers
will merely require to know the approximate place, and this is to be
found by estimating the difference in R.A. and Dec. between a comet
and a known star. The position of the latter may be found in a good
catalogue and corrected for precession; then, allowing for the observed
differences, the comet’s place may be assigned to within very small
limits of error. A low power, embracing a field of 1° or more, is best
adapted for these observations, as it is more likely to include a
catalogued star, and will exhibit the comet, especially if a large one,
to the best effect.
The announcement of a new comet is always read with avidity by
amateurs, and their first desire is to see it for themselves. This
they may readily do by marking its place on a star-map or globe, and
noting its relative place amongst the stars near. The telescope should
then be directed towards the point indicated, and if the comet is not
presented in the field, the instrument should be moved a little so
that the surrounding region may be examined. If failure still attends
the effort, the observer should point the telescope a few degrees E.
or W. of the suspected point, and then carefully sweep over the place
of the comet. It will then be picked up, unless it is too faint for
his aperture. The first announcement of a comet generally gives the
position at discovery, and the daily rate and direction of motion. The
latter must of course be allowed for when the search is instituted.
The physical aspects of comets are as diversified as they are variable.
No two comets are exactly alike, nor does the same comet exhibit a
permanency of detail. Of course, when these objects are enormously
distant, and barely visible, many of them appear to present similar
characteristics; but under the closer and more expanded views
obtainable near perihelion the resemblance vanishes, and every comet is
seen to possess features peculiar to itself. To trace these features,
and to record them by delineation and description, forms one of the
most interesting branches in which amateurs may engage. Much has been
learnt of previous comets by successively noting their transitions of
form and brightness, and the same scrupulous attention should be given
to future comets.
The tails of comets are not always turned away from the Sun. Indeed,
the contrary effect is sometimes produced. Occasionally there is a
duple tail, the largest branch of which follows the normal direction,
while the other is turned towards the Sun. Forms of this character
require close watching from night to night. Is the sunward tail
developed suddenly? and has it a fairly durable existence? Instances of
singular curvature should also be noted. The tails are seldom perfectly
straight, especially those attached to naked-eye comets, and decided
changes affect their visible outlines at very short intervals. In large
comets the space over which the tail extends should be sketched upon a
star-map on successive evenings; its changes of position and curve will
then be manifested by comparisons, and its increasing or decreasing
length will also be apparent. Dark rifts, like shadows, often run
lengthwise through the tail, and occasion a fan-like appearance
analogous to that which distinguished the great comet of 1744 and gave
it a sextuple tail.
The light of comets sometimes fluctuates in a very extraordinary
manner, and too rapidly and irregularly to be consistent with theory.
In this respect, Pons’s Comet, at its last return in 1883-4 presented
an eccentric behaviour. Bigourdan found that during the nineteen days
from Sept. 5 to 24, 1883, the increase in the comet’s brilliancy
exceeded by thirty or forty times that resulting from reflected light
alone! This increase appears to have been due to a sudden outburst on
Sept. 22, which occurred some time within the four hours preceding
midnight. Dr. Müller, of Potsdam, witnessed a further outburst on Jan.
1, 1884, within 1-3/4 hour; and the extent of this was accurately
determined by means of a photometer. He found an augmentation of
seven tenths of a magnitude in the brightness of the comet, and an
equally sudden fall to its previous lustre. While these fluctuations
were in progress, he noticed variations in the shape of the nucleus
not less remarkable than its variations in light. Those who observe
future comets will do well, therefore, to be on the alert for similar
phenomena. The apparent brightness of the nuclei and alterations of
shape or size should be recorded on every night when observations are
feasible.
As a comet approaches the Sun its material apparently contracts, while
with increasing distance from that luminary it expands. Usually the
nucleus is extremely small and bright, and it often looks like a star
shining through nebulosity. High powers must therefore be applied
in its examination. Jets, aigrettes, luminous sectors, and other
appendages are often involved with the nucleus and outlying coma, and
they form a complicated structure well deserving further study. A good
deal of mystery still surrounds these appearances; their curious forms
and vagaries have yet to be explained.
Stars are frequently observed through the head of a comet, which
apparently, however, exercises no influence in dimming their lustre.
But the stars are commonly seen behind the envelopes or comæ, and very
rarely through the nucleus. Nothing is better calculated to exhibit
the transparent and tenuous character of comets than observations of
this kind, and observers should seek for further opportunities of
making them. If the motion of a comet is obviously carrying it in the
direction of one of the stars in the field, the observer may determine
for himself the approximate time of conjunction by noting the distance
between the star and comet and allowing for the motion of the latter.
He will then know when to come to his telescope and witness the
phenomenon. Should it appear probable that the comet’s nucleus will
pass over the star, he should commence his watch some time before it
occurs; he may then make comparisons before the star is involved in
the outlying nebulosity, and trace the whole event from beginning to
end. Any changes in the light or aspect of either star or comet would
then be manifested. The comet of 1847 is said to have passed centrally
over a 5th mag. star, but the latter was unaffected. Encke’s Comet on
one occasion interposed itself directly over one of a pair of 10th
mag. stars, but their relatively equal brilliancy suffered no change.
Encke’s Comet, however, has no stellar nucleus. The latter feature
is so bright and compact as displayed in many other comets, that its
transit over a small star must have some effect either in obliterating
it altogether, or in detracting from its lustre.
Visible evidences of rotation seem to have been suspected in certain
comets, but this has never been substantiated on sufficient grounds.
The circumstance is one, however, which should be remembered. During a
series of observations the observer who notes the details of structure
with particular regard to position may discover similar traces, and
possibly learn something of the cause. The nucleus of a bright comet
should always be examined with a moderately high power, so that any
variations or peculiarities of form may be detected.
_Nomenclature of Comets._—It must be confessed that no perfectly
satisfactory method has yet been devised as regards the naming of
comets. The plan of affixing Roman numbers progressively for each
year, according to date of perihelion passage, answers pretty well,
though a little confusion is sometimes caused by prematurely affixing
the number, especially when two comets are discovered successively,
the first of which is a long time before perihelion, and the second
considerably after it. Until a comet can be safely assigned its
catalogue place, it is preferable to refer to it by the name of the
discoverer and date of discovery. This is more distinctive than the
common method of lettering comets according to the epochs of their
detection. As to periodical comets, it is not difficult to find some
inconsistencies in their names. In the case of Halley’s Comet (1682,
discovered by Flamsteed) and Encke’s Comet (1819 I., discovered by
Pons), it was most fitting that they should be known to posterity by
the names of the two able computers whose investigations first revealed
to us comets of long and short period respectively. Under ordinary
circumstances the name of the discoverer is applied to a comet as a
means of convenient reference, and perhaps as a suitable recognition
of the patient labours of the man who first announced it to the world.
The plan seems to have been to name comets after those fortunate
persons who sighted them at the particular apparition during which
periodicity was determined. Thus Tuttle’s Comet (1858 I.) had been seen
as long before as 1790 by Mechain, and Biela’s (1826 I.) was previously
observed in 1772 by Montaigne, and in 1805 by Pons. It is, however,
strange that a comet found by Pons in 1819 (III. of that year), and
which Encke showed to be revolving in an ellipse with a periodic time
of 5½ years, should be called after Winnecke, who rediscovered it in
1858. To Pons the real priority belongs, though Winnecke deserves
much praise for redetecting and identifying this body after it had
effected six unobserved returns to perihelion. It is also curious to
find that the comet of short period discovered by Swift in 1880 is
called “Tempel’s 3rd Comet” in Galle’s catalogue (1885), from the fact
that Tempel found it at a previous return (1869), when, however, its
period was not ascertained. There is little doubt that the title justly
belongs to Swift. Tempel himself called it “Swift’s Comet.” One plan
should in fairness to observers be consistently adhered to. If comets
are to be called after their original discoverers, then Biela’s Comet
should be known as “Montaigne’s,” Tuttle’s as “Mechain’s,” &c.
_Curiosities of Comets._—The comet of 1729, which was hardly visible to
the naked eye, has far the greatest perihelion distance (4·0435) of any
comet known. Barnard’s Comet (1885 II.) comes next with a perihelion
distance of 2·5068.
Pons’s Comet at its return in 1883-4 remained visible for nine months.
When last seen, on June 2, 1884, it was 470 millions of miles from the
Earth, and more remote in the depths of space than any other observed
comet since that of 1729. Barnard’s Comet (1889 I.), though never
visible to the naked eye, was followed from Sept. 2, 1888, to Aug. 18,
1890. Its distance from the Sun was then 6·25 (Earth’s distance = 1),
or about 580 millions of miles, which is greater than that of many of
the short-period comets at aphelia. The most prolonged visibility of
any previous comet was that of 1811 I. (510 days). But this comet of
Barnard has been retained in view 715 days.
The great comet of 1882 was watched right up to the Sun’s limb by
Messrs. Finlay and Elkin at the Cape of Good Hope on Sept. 16, 1882.
The comet was actually seen to disappear at the margin, and not a
vestige of it could be traced during its transit across the solar
disk. The nucleus of the comet was 4” in diameter, and before transit
it looked as bright as a part of the Sun’s surface; but it was quite
invisible when projected on the disk. The alleged observations by
Pastorff and Stark, which were construed into visible transits of
comets, are therefore thoroughly disproved, and will require another
interpretation.
At the time of the total solar eclipse of May 17, 1882, a bright comet
was observed near the Sun. It was a striking object visible to the
naked eye. In the photographs which were taken of the eclipse the comet
is well shown, but this body escaped subsequent observation, so that
its orbit could not be determined.
_Naked-eye Comets._—Arago mentions that twelve comets were visible to
the naked eye during the period from 1800 to 1853, but there appear
to have been certainly thirty comets fulfilling this condition, and I
believe a careful search amongst cometary records would further augment
the number. During the ten years from 1880 to 1889 inclusive there
were no less than sixteen comets perceptible to unaided vision, and a
considerable proportion of these were fine comets. It is very rarely
that two naked-eye comets are to be seen at the same epoch, as in
August 1881 and at the end of April 1886.
_Comet-seeking._—For a long time after the invention of the telescope
comet-seeking does not appear to have been undertaken in a methodical
way, and to have formed the habitual work of certain observers. But
the expected return of Halley’s Comet in 1759 roused observers to
take the initiative in a branch of practical research which in after
years was destined to prove remarkably productive. Messier, Palitzch,
and others began a system of sweeping the heavens for the predicted
comet; and it had a successful issue, for Palitzch, who did not relax
his labours even on Christmas day, alighted upon the coveted prize on
Dec. 25, 1758. Since that time a regular search after comets has been
maintained. Messier pursued it with indomitable energy through a long
period of years, and achieved many successes. It is said of him that
on one occasion he was anticipated in a discovery by Montaigne, and
he appears to have deplored the loss of the comet more than the loss
of his wife, who was lying dead at the time. A friend visited him,
and spoke a few words of sympathy in reference to his bereavement,
but Messier, in despair about the comet, exclaimed: “I had discovered
twelve—alas! that I should be robbed of the thirteenth by Montaigne!”
and his eyes filled with tears. Recollecting himself, and appreciating
the loss he had sustained in his wife, he added, “Ah, this poor woman!”
Messier encountered some serious obstacles to his favourite pursuit.
Breen, in his ‘Planetary Worlds,’ mentions that Messier, while walking
in President Saron’s garden, fell into an ice-house, and was disabled
for a time. Later on “the revolution deprived him of his little income
and every evening he was wont to repair to the house of Lalande to
replenish the supply of oil for his midnight lamp. The political storm
necessitated his removal to another neighbourhood, where he no longer
heard the clocks of forty-two churches sounding the hours during his
night-watchings.” Messier discovered all his comets with a small 2-foot
telescope of 2½ inches aperture magnifying 5 times and with a field of
4°.
Dr. Olbers, of Bremen, was another diligent student in this field. He
did not effect many discoveries, but, from an upper apartment of his
house, he observed nearly all the comets which appeared during half a
century.
During the first twenty-seven years of the present century, Pons
discovered the majority of the comets that were seen. He was a
door-keeper at the Observatory at Marseilles, and owing to the teaching
and encouragement he received from Thulis, the director, he achieved
phenomenal success as a comet-hunter.
Discoveries of comets have rarely been effected in England. This
is chiefly to be assigned to two circumstances. First, because the
labour involved in seeking for these bodies has never perhaps been
pursued to an equal degree and with the same tenacity as it formerly
was in France, and as it has recently been in the United States; and
second, because the cloud-laden skies of England oppose the successful
prosecution of a research in which a clear atmosphere is eminently
desirable.
Though comet-seeking does not always produce new discoveries, it is
certainly entertaining to those engaged in it; for one of the most
agreeable diversions of telescopic work is to scan the firmament with a
large-field comet-eyepiece, which exhibits the most pleasing views of
star-groups, coloured stars, nebulæ, and telescopic meteors.
The operation of sweeping for comets is attractive from other aspects,
though it undoubtedly needs close application, patience, and much
caution. The possibility of seeing a comet in the field at any time
proves a constant source of allurement to the observer, and sustains
his enthusiasm. The glimpsing of a nebulous object, and the expectation
(before it has been identified) that it _may_ prove a comet, induces
a little excitement which pleasantly relieves the monotony that might
otherwise be attached to a sedulous research of this nature; and it
is one in which amateurs may suitably engage with a fair prospect of
success. Instruments of great power, refinement, and expense are not
required. It is rather a work calling for the exercise of patience and
acute perception, and for that tireless servitude which those only who
have an inborn love for it can maintain.
_English Weather and Comet-seeking._—Only two new comets having been
discovered in England during the last forty years some people regard
our climate as in a great measure responsible for this. But the opinion
seems to be erroneous. The lack of discoveries has arisen from want
of effort as much as from want of opportunity. The best weather for
comet-seeking is when the atmosphere is very transparent, and the
stars are lucid and sparkling. Haze, fog, or cloud of any kind offers
a serious hindrance. A thoroughly good night for planetary work is
not usually good for cometary observation, because sharp definition
is not so requisite as a very clear sky. A little fog or thin cloud,
which will often improve planetary images, utterly obliterates a small
telescopic comet. The air is sometimes very pure and dark after storms,
and the stars remarkably bright; it is then that the best opportunities
are afforded for comet-hunting. Any systematic and regular work like
this may be pursued in this country with every prospect of success by
an observer who will persevere in it. From some statistics printed in
the ‘Science Observer,’ Boston, it appears that during the seven months
from May to November, 1882, Lewis Swift was comet-seeking during 300
hours. I have no English results of the same kind, but my meteoric
observations will supply a means of comparison. From June to November,
1887 (six months), I was observing during 217 hours, and for a nearly
similar period during the last half of 1877, though in each year work
was only attempted with the Moon absent. My result for 1887 averages
36 hours per month, which is little less than the average derived
from the comet-seeking records above quoted. It is therefore fair to
suppose that as much may be done here as in some regions of the United
States. Mr. W. R. Brooks wrote me in 1889, saying: “We have much cloudy
weather in this part of America. While in other portions of the country
clear weather abounds, it is not so in this section, where much of
my work has been done. This is a most fertile section—the beautiful
lake region of N.Y.,—but it is for this reason a cloudy belt. It is
far different in Colorado and California. In the latter place, at the
Lick Observatory, I hear they have 300 clear nights in the year—a
paradise for the astronomical observer! My former site, the Red House
Observatory, Phelps, N.Y., is only six miles from Geneva, and hence
in the same cloudy region.” Prof. Swift also referred to the subject
of weather in a letter to me dated July 30, 1889, where he says: “I
arrived home, after a five weeks’ visit to the Lick Observatory, on
March 1, and have not had half a dozen first-class nights since—not
in thirty years have I seen such prolonged rainy and cloudy weather.”
Now Mr. Brooks has discovered 13 comets in 7 years, Prof. Swift has
found 8 comets (1862-1890), and in addition to these has detected
more than 700 new nebulæ, all of the latter since 1883. From this it
appears conclusively that if such extensive and valuable results can
be obtained, notwithstanding frequently bad weather, then English
observers may prove equally successful, the important factor being that
similar energy and ability direct their labours.
_Aperture and Power required._—Opinions are divided as to the most
suitable aperture and power for this work. Any telescope of from
4-to 10-inches aperture may be employed in it. A low power (30 to
50) and large field (50′ to 90′) eyepiece are imperative; and the
instrument, to be really effective, should be mounted to facilitate
sweeping either in a vertical or horizontal direction. A reflector on
an alt-azimuth stand is a most convenient form for vertical sweeps.
The defining-capacity of the telescope need not necessarily be perfect
to be thoroughly serviceable, the purpose being to distinguish faint
nebulous bodies, and not details of form. Far more will depend upon
the observer’s aptitude and persistency than upon his instrumental
means, which ought to be regarded as a mere adjunct to his powers
and not a controlling influence in success, for the latter lies in
himself. Very large instruments are not often used, because of their
necessarily restricted fields. Moreover, a small instrument, apart
from its advantage in this respect, is worked with greater facility
and expedition. This is important, especially when the observer is
to examine the region in the immediate neighbourhood of the Sun. He
has then a very brief interval for the attainment of his purpose,
and a small telescope must be used on account of its large field,
its ready manipulation, and its general effectiveness on objects at
low altitudes. The case is somewhat different when the search is to
be conducted in regions far removed from the Sun’s place; for here
the comets are in general faint, and there is time for the work to
be deliberately and critically performed. Large instruments are to
be recommended for these districts as capable of revealing fainter
objects, though they are troublesome in several respects. They show
large numbers of nebulæ, especially if the observer is exploring the
region of Virgo, Coma Berenices, or Ursa Major; and he will have great
difficulty in identifying them and in feeling his way with certainty.
These complications are inseparable from the work, and, though
chiefly affecting large apertures, should not always be shunned; for
a telescope capable of displaying very faint nebulæ is also capable
of showing faint comets. Many comets have eluded discovery by the
inadequate reach of the instruments in the hands of comet-seekers;
and the statement recently made that there are only about one hundred
nebulæ liable to be mistaken for comets is not accurate, because comets
in certain positions are of the last degree of faintness, and there is
no identifying them from small nebulæ except by means of their motion.
Mr. Brooks says:—“Medium magnifying powers, with necessarily
moderate-sized fields, are better than very low powers and large
fields. While with the latter a large amount of the sky can be swept
over in a given time, the work is not so well done, and a faint comet
would be easily swept over and not seen. A small region, _thoroughly
worked_, is far more likely to be successful. This gives a feeling of
satisfaction with the work performed, even with negative results. In
support of this I may remark that, during all the years I have engaged
in comet-seeking, not a single comet has been discovered by another
astronomer in a region of the heavens that I had just previously
searched; so that I have never had occasion to feel that I had swept
over a comet and missed seeing it. Aside from the obvious requirement
of good eyesight, capable of detecting exceedingly faint objects,
a good telescope of at least moderate aperture, and a familiarity
attained by experience with the large number of nebulæ resembling
telescopic comets, the comet-seeker, to be successful, must possess
in a high degree the qualities of patience, perseverance, energy, and
enthusiasm. I have the highest admiration for the man or woman who
discovers a comet, because I know of the hard and thorough work which
the success implies.”
Mr. Brooks’s experience and success in this branch give weight to
his suggestions, and there can be little doubt that his commendation
of moderate powers is fully justified. I believe he usually sweeps
with a power of 40 (field of 1° 20′) on the 10-1/8-inch equatoreal
of his observatory. Speaking for myself, I find powers of 32 (field
1-1/4°) and 40 (field 1°) perform very satisfactorily on my 10-inch
With-Browning reflector, having frequently tried them on faint
nebulæ and comets. Sometimes I employ a power of 60, field 50′; but
for ordinary purposes this is too high. It is a good plan to sweep
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.”