William F. Denning.

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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



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