Comets
We learn from Pliny that comets were classified in ancient times,
according to their peculiar forms, into twelve classes, of which the
principal were: _Pogonias_, bearded; _Lampadias_, torch-like; _Xiphias_,
sword-like; _Pitheus_, tun-like; _Acontias_, javelin-like; _Ceratias_,
horn-like; _Disceus_, quoit-like; and _Hippias_, horse-mane-like.[184]
Of the numerous comets mentioned in astronomical records, comparatively
few have been visible to the naked eye. Before the invention of the
telescope (1610) only those which were so visible _could_, of course, be
recorded. These number about 400. Of the 400 observed since then, some 70
or 80 only have been visible by unaided vision; and most of these now
recorded could never have been seen without a telescope. During the last
century, out of 300 comets discovered, only 13 were very visible to the
naked eye. Hence, when we read in the newspapers that a comet has been
discovered the chances are greatly against it becoming visible to the
naked eye.[185]
Although comparatively few comets can be seen without a telescope, they
are sometimes bright enough to be visible in daylight! Such were those of
B.C. 43, A.D. 1106, 1402, 1532, 1577, 1744, 1843, and the "great September
comet" of 1882.
If we except the great comet of 1861, through the tail of which the earth
is supposed to have passed, the comet which came nearest to the earth was
that of 1770, known as Lexell's, which approached us within two millions
of miles, moving nearly in the plane of the ecliptic. It produced,
however, no effect on the tides, nor on the moon's motion, which shows
that its mass must have been very small. It was computed by Laplace that
if its mass had equalled that of the earth, the length of our year would
have been shortened by 2 hours 47 minutes, and as there was no perceptible
change Laplace concluded that the comet's mass did not exceed 1/5000th of
the earth's mass. This is the comet which passed so near to Jupiter that
its period was reduced to 5-1/2 years. Owing to another near approach in
1779 it became invisible from the earth, and is now lost.[186] Its
identity with the recently discovered eighth satellite of Jupiter has been
suggested by Mr. George Forbes (see under "Jupiter"). At the near approach
of Lexell's comet to the earth in 1770, Messier, "the comet ferret,"
found that its head had an apparent diameter of 2-1/2°, or nearly five
times that of the moon!
Another case of near approach to the earth was that of Biela's comet at
its appearance in 1805. On the evening of December 9 of that year, the
comet approached the earth within 3,380,000 miles.[187]
The comet of A.D. 1106 is stated to have been seen in daylight close to
the sun. This was on February 4 of that year. On February 10 it had a tail
of 60° in length, according to Gaubil.[188]
The comet of 1577 seems to have been one of the brightest on record.
According to Tycho Brahé, it was visible in broad daylight. He describes
the head as "round, bright, and of a yellowish light," with a curved tail
of a reddish colour.[189]
The comet of 1652 was observed for about three weeks only, and Hevelius
and Comiers state that it was equal to the moon in apparent size! This
would indicate a near approach to the earth. An orbit computed by Halley
shows that the least distance was about 12 millions of miles, and the
diameter of the comet's head rather less than 110,000 miles, or about 14
times the earth's diameter.
According to Mr. Denning, "most of the periodical comets at perihelion are
outside the earth's orbit, and hence it follows that they escape
observation unless the earth is on the same side of the sun as the
comet."[190]
It was computed by M. Faye that the _volume_ of the famous Donati's comet
(1858) was about 500 times that of the sun! On the other hand, he
calculated that its _mass_ (or quantity of matter it contained) was only a
fraction of the earth's mass. This shows how almost inconceivably tenuous
the material forming the comet must have been--much more rarefied, indeed,
than the most perfect vacuum which can be produced in an air-pump. This
tenuity is shown by the fact that stars were seen through the tail "as if
the tail did not exist." A mist of a few hundred yards in thickness is
sufficient to hide the stars from our view, while a thickness of thousands
of miles of cometary matter does not suffice even to dim their brilliancy!
At the time of the appearance of the great comet of 1843, it was doubtful
whether the comet had transited the sun's disc. But it is now known, from
careful calculations by Prof. Hubbard, that a transit really took place
between 11{h} 28{m} and 12{h} 29{m} on February 27, 1843, and might have
been observed in the southern hemisphere. The distance of this remarkable
comet from the sun at its perihelion passage was less than that of any
known comet. A little before 10 p.m. on February 27, the comet passed
within 81,500 miles of the sun's surface with the enormous velocity of
348 miles a second! It remained less than 2-1/4 hours north of the
ecliptic, passing from the ascending to the descending node of its orbit
in 2{h} 13{m}·4.[191] The great comet of 1882 transited the sun's disc on
Sunday, September 17, of that year, the ingress taking place at 4{h} 50{m}
58{s}, Cape mean time. When on the sun the comet was absolutely invisible,
showing that there was nothing solid about it. It was visible near the sun
with the naked eye a little before the transit took place.[192] This great
comet was found by several computors to have been travelling in an
elliptic orbit with a period of about eight centuries. Morrison found 712
years; Frisby, 794; Fabritius, 823; and Kreutz, 843 years.[193]
The great southern comet of 1887 may be described as a comet without a
head! The popular idea of a comet is a star with a tail. But in this case
there was no head visible--to the naked eye at least. Dr. Thome of the
Cordoba Observatory--its discoverer--describes it as "a beautiful
object--a narrow, straight, sharply defined, graceful tail, over 40° long,
shining with a soft starry light against a dark sky, beginning apparently
without a head, and gradually widening and fading as it extended
upwards."[194]
The great southern comet of 1901 had five tails on May 6 of that year. Two
were fairly bright, and the remaining three rather faint. Mr. Gale saw a
number of faint stars through the tails. The light of these seem to have
been "undimmed." Mr. Cobham noticed that the stars Rigel and [Greek: b]
Eridani shone through one of the faint tails, and "showed no perceptible
difference."[195]
Prof. W. H. Pickering says that "the head of a comet, as far as our
present knowledge is concerned, seems therefore to be merely a meteor
swarm containing so much gaseous material that when electrified by its
approach to the sun it will be rendered luminous" (_Harvard Annual_, vol.
xxxii. part ii. p. 295) "... if the meteors and their atmospheres are
sufficiently widely separated from one another, the comet may be brilliant
and yet transparent at the same time."
In the case of Swift's comet of 1892 some periodical differences of
appearance were due, according to Prof. W. H. Pickering, to a rotation of
the comet round an axis passing longitudinally through the tail, and he
estimated the period of rotation at about 94 to 97 hours. He computed that
in this comet the repulsive force exerted by the sun on the comet's tail
was "about 39·5 times the gravitational force."[196]
The comet known as 1902_b_ approached the planet Mercury within two
millions of miles on November 29 of that year. Prof. O. C. Wendell, of
Harvard Observatory, made some observations on the transparency of this
comet. He found with the aid of a photometer and the 15-inch telescope of
the observatory that in the case of two faint stars over which the comet
passed on October 14, 1902, the absorption of light by the comet was
insensible, and possibly did not exceed one or two hundredths of a
magnitude,[197] an amount quite imperceptible to the naked eye, and shows
conclusively how almost inconceivably rarefied the substance of this comet
must be.
The comet known as Morehouse (1908_c_) showed some curious and wonderful
changes. Mr. Borelly found that five tails are visible on a photographic
plate taken on October 3, 1908, and the trail of an occulted star
indicates a slight absorption effect. According to M. L. Rabourdin, great
changes took place from day to day, and even during the course of an hour!
Similar changes were recorded by G. M. Gauthier; and Prof. Barnard, who
photographed the comet on 30 nights from September 2 to October 13, states
that the photographs of September 30 "are unique, whilst the
transformation which took place between the taking of these and the taking
of the next one on October 1 was very wonderful."[198] The spectrum
showed the lines of cyanogen instead of the hydrocarbon spectrum shown by
most comets.
Prof. Barnard has suggested that all the phenomena of comets' tails cannot
be explained by a repulsive force from the sun. Short tails issuing from
the comet's nucleus at considerable angles with the main tail point to
eruptive action in the comet itself. The rapid changes and distortions
frequently observed in the tails of some comets suggest motion through a
resisting medium; and the sudden increase of light also occasionally
observed points in the same direction.[199]
It was computed by Olbers that if a comet having a mass of 1/2000th of the
earth's mass--which would form a globe of about 520 miles in diameter and
of the density of granite--collided with the earth, with a velocity of 40
miles a second, our globe would be shattered into fragments.[200] But that
any comet has a solid nucleus of this size seems very doubtful; and we may
further say that the collision of the earth with _any_ comet is highly
improbable.
It seems to be a common idea that harvests are affected by comets, and
even "comet wines" are sometimes spoken of. But we know that the earth
receives practically no heat from the brightest comet. Even in the case of
the brilliant comet of 1811, one of the finest on record, it was found
that "all the efforts to concentrate its rays did not produce the
slightest effect on the blackened bulb of the most sensitive thermometer."
Arago found that the year 1808, in which several comets were visible, was
a cold year, "and 1831, in which there was no comet, enjoyed a much higher
temperature than 1819, when there were three comets, one of which was very
brilliant."[201] We may, therefore, safely conclude that even a large
comet has no effect whatever on the weather.
From calculations on the orbit of Halley's comet, the next return of which
is due in 1910, Messrs. Cowell and Crommelin find that the identity of the
comet shown on the Bayeux Tapestry with Halley's comet is now "fully
established." They find that the date of perihelion passage was March 25,
1066, which differs by only 4 days from the date found by Hind. The
imposing aspect of the comet in 1066 described in European chronicles of
that time is confirmed by the Chinese Annals. In the latter records the
brightness is compared to that of Venus, and even with that of the moon!
The comparison with the moon was probably an exaggeration, but the comet
doubtless made a very brilliant show. In the Bayeux Tapestry the
inscription on the wall behind the spectators reads: "_isti mirant
stella_." Now, this is bad Latin, and Mr. W. T. Lynn has made the
interesting suggestion that some of the letters are hidden by the
buildings in front and that the real sentence is "_isti mirantur
stellam_."[202] The present writer has examined the copy of the Bayeux
Tapestry which is in the Dublin Museum, and thinks that Mr. Lynn's
suggestion seems very plausible. But the last letter of _stellam_ is
apparently hidden by the comet's tail, which does not seem very probable!
The conditions under which the comet will appear in 1910 are not unlike
those of 1066 and 1145. "In each year the comet was discovered as a
morning star, then lost in the sun's rays; on its emergence it was near
the earth and moved with great rapidity, finally becoming stationary in
the neighbourhood of Hydra, where it was lost to view."[203] In 1910 it
will probably be an evening star before March 17, and after May 11, making
a near approach to the earth about May 12. About this time its apparent
motion in the sky will be very rapid. As, however, periodical comets--such
as Halley's--seem to become fainter at each return, great expectations
with reference to its appearance in 1910 should not be indulged in.
The appearance of Halley's comet in A.D. 1222 is thus described by
Pingré--a great authority on comets--(quoting from an ancient writer)--
"In autumn, that is to say in the months of August and September, a
star of the first magnitude was seen, very red, and accompanied by a
great tail which extended towards the top of the sky in the form of a
cone extremely pointed. It appeared to be very near the earth. It was
observed (at first?) near the place of the setting sun in the month of
December."
With reference to its appearance in the year 1456, when it was of "vivid
brightness," and had a tail of 60° in length, Admiral Smyth says,[204] "To
its malign influence were imputed the rapid successes of Mahomet II.,
which then threatened all Christendom. The general alarm was greatly
aggravated by the conduct of Pope Callixtus III., who, though otherwise a
man of abilities, was a poor astronomer; for that pontiff daily ordered
the church bells to be rung at noon-tide, extra _Ave-Marias_ to be
repeated, and a special protest and excommunication was composed,
exorcising equally the Devil, the Turks, and the comet." With reference to
this story, Mr. G. F. Chambers points out[205] that it is probably based
on a passage in Platina's _Vitæ Pontificum_. But in this passage there is
no mention made of excommunication or exorcism, so that the story, which
has long been current, is probably mythical. In confirmation of this view,
the Rev. W. F. Rigge has shown conclusively[206] that no bull was ever
issued by Pope Callixtus III. containing a reference to _any_ comet. The
story would therefore seem to be absolutely without foundation, and should
be consigned to the limbo of all such baseless myths.
With reference to the appearance of Halley's comet, at his last return in
1835, Sir John Herschel, who observed it at the Cape of Good Hope, says--
"Among the innumerable stars of all magnitudes, from the ninth
downwards, which at various times were seen through it, and some
extremely near to the nucleus (though not _exactly on it_) there never
appeared the least ground for presuming any extinction of their light
in traversing it. Very minute stars indeed, on entering its brightest
portions, were obliterated, as they would have been by an equal
illumination of the field of view; but stars which before their entry
appeared bright enough to bear that degree of illumination, were in no
case, so far as I could judge, affected to a greater extent than they
would have been by so much lamp-light artificially introduced."[207]
It is computed by Prof. J. Holetschak that, early in October, 1909,
Halley's comet should have the brightness of a star of about 14-1/2
magnitude.[208] It should then--if not detected before--be discoverable
with some of the large telescopes now available.
According to the computations of Messrs. Cowell and Crommelin, the comet
should enter Pisces from Aries in January, 1910. "Travelling westward
towards the star [Greek: g] Piscium until the beginning of May, and then
turning eastward again, it will travel back through the constellations
Cetus, Orion, Monoceros, Hydra, and Sextans." From this it seems that
observers in the southern hemisphere will have a better view of the comet
than those in northern latitudes. The computed brightness varies from 1 on
January 2, 1910, to 1112 on May 10. But the actual brightness of a comet
does not always agree with theory. It is sometimes brighter than
calculation would indicate.
According to Prof. O. C. Wendell, Halley's comet will, on May 12, 1910,
approach the earth's orbit within 4·6 millions of miles; and he thinks
that possibly the earth may "encounter some meteors," which are presumably
connected with the comet. He has computed the "radiant point" of these
meteors (that is, the point from which they appear to come), and finds its
position to be R.A. 22{h} 42{m}·9, Decl. N. 1° 18'. This point lies a
little south-west of the star [Greek: b] Piscium.
According to Dr. Smart, the comet will, on June 2, "cross the Equator
thirteen degrees south of Regulus, and will then move slowly in the
direction of [Greek: ph] Leonis. The comet will be at its descending node
on the ecliptic in the morning of May 16, and the earth will pass through
the node on the comet's orbit about two and a half days later. The
comet's orbit at the node is about 13 million miles within that of the
earth. Matter repelled from the comet's nucleus by the sun with a velocity
of about 216,000 miles per hour, would just meet the earth when crossing
the comet's orbit plane. Matter expelled with a velocity of 80,000 miles
per hour, as in the case of Comet Morehouse, would require seven days for
the journey. Cometary matter is said to have acquired greater velocities
than this, for (according to Webb, who quotes Chacornac) Comet II., 1862,
shot luminous matter towards the sun, with a velocity of nearly 2200 miles
per second. It is therefore possible that matter thrown off by the comet
at the node may enter our atmosphere, in which case we must hope that
cyanogen, which so often appears in cometary spectra, may not be
inconveniently in evidence."[209]
Cyanogen is, of course, a poisonous gas, but cometary matter is so
rarefied that injurious effects on the earth need not be feared.
If we can believe the accounts which have been handed down to us, some
very wonderful comets were visible in ancient times. The following may be
mentioned:--
B.C. 165. The sun is said to have been "seen for several hours in the
night." If this was a comet it must have been one of extraordinary
brilliancy.[210]
B.C. 146. "After the death of Demetrius, king of Syria, the father of
Demetrius and Antiochus, a little before the war in Achaia, there appeared
a comet as large as the sun. Its disc was first red, and like fire,
spreading sufficient light to dissipate the darkness of night; after a
little while its size diminished, its brilliancy became weakened, and at
length it entirely disappeared."[211]
B.C. 134. It is recorded that at the birth of Mithridates a great comet
appeared which "occupied the fourth part of the sky, and its brilliancy
was superior to that of the sun." (?)[212]
B.C. 75. A comet is described as equal in size to the moon, and giving as
much light as the sun on a cloudy day. (!)[213]
A.D. 531. In this year a great comet was observed in Europe and China. It
is described as "a very large and fearful comet," and was visible in the
west for three weeks. Hind thinks that this was an appearance of Halley's
comet,[214] and this has been confirmed by Mr. Crommelin.
A.D. 813, August 4. A comet is said to have appeared on this date, of
which the following curious description is given: "It resembled two moons
joined together; they separated, and having taken different forms, at
length appeared like a man without a head." (!)[215]
A.D. 893. A great comet is said to have appeared in this year with a tail
100° in length, which afterwards increased to 200°![216]
A.D. 1402. A comet appeared in February of this year, which was visible in
daylight for eight days. "On Palm Sunday, March 19, its size was
prodigious." Another comet, visible in the daytime, was seen from June to
September of the same year.
When the orbit of the comet known as 1889 V was computed, it was found
that it had passed through Jupiter's system in 1886 (July 18-21). The
calculations show that it must have passed within a distance of 112,300
miles of the planet itself--or less than half the moon's distance from the
earth--and "its centre may possibly have grazed the surface of
Jupiter."[217]
Sir John Herschel thought that the great comet of 1861 was by far the
brightest comet he had ever seen, those of 1811 and 1858 (Donati's) not
excepted.[218] Prof. Kreutz found its period of revolution round the sun
to be about 409 years, with the plane of the orbit nearly at right angles
to the plane of the ecliptic.
* * * * *
On November 9, 1795, Sir William Herschel saw the comet of that year pass
centrally over a small double star of the 11th and 12th magnitudes, and
the fainter of the two components remained distinctly visible during the
comet's transit over the star. This comet was an appearance of the comet
now known as Encke's.[219] Struve saw a star of the 10th magnitude through
nearly the brightest part of Encke's comet on November 7, 1828, but the
star's light was not dimmed by the comet.
Sir John Herschel saw a cluster of stars of the 16th or 17th magnitude
through Biela's comet, although the interposed cometary matter must have
been at least 50,000 miles in thickness.[220]
Bessel found that on September 29, 1835, a star of the 10th magnitude
shone with undimmed lustre through the tail of Halley's comet within 8
seconds of arc of the central point of the head. At Dorpat (Russia) Struve
saw the same star "in conjunction only 2"·2 from the brightest point of
the comet. The star remained continuously visible, and its light was not
perceptibly diminished whilst the nucleus of the comet seemed to be almost
extinguished before the radiance of the small star of the 9th or 10th
magnitude."[221]
Webb says--
"Donati saw a 7 mg. star enlarged so as to show a sensible disc, when
the nucleus of comet III., 1860, passed very near it. Stars are said
to have started, or become tremulous, during occultations by comets.
Birmingham observed the comet of Encke illuminated by a star over
which it passed, August 23, 1868; and Klein, in 1861, remarked an
exceptional twinkling in 5 mg. stars involved in the tail."[222]
The comet of 1729 had the greatest perihelion distance of any known
comet;[223] that is, when nearest to the sun, it did not approach the
central luminary within four times the earth's distance from the sun!
Barnard's comet, 1889 I., although it never became visible to the naked
eye, was visible with a telescope from September 2, 1888, to August 18,
1890, or 715 days--the longest period of visibility of any comet on
record. When last seen it was 6-1/4 times the earth's distance from the
sun, or about 580 millions of miles,[224] or beyond the orbit of Jupiter!
Messier, who was called "the comet ferret," discovered "all his comets
with a small 2-foot telescope of 2-1/4 inches aperture, magnifying 5
times, and with a field of 4°."[225]
It is a very curious fact that Sir William Herschel, "during all his
star-gaugings and sweeps for nebulæ, never discovered a comet;"[226] that
is an object which was afterwards _proved_ to be a comet. Possibly,
however, some of his nebulæ which are now missing, may have been really
comets.
Sir William Herschel found the diameter of the head of the great comet of
1811 to be 127,000 miles. The surrounding envelope he estimated to be at
least 643,000 miles, or about three-fourths of the sun's diameter.
On a drawing of the tails of the great comet of 1744 given in a little
book printed in Berlin in that year, no less than 12 tails are shown!
These vary in length and brightness. A copy of this drawing is given in
_Copernicus_.[227] The observations were made by "einen geschichten
Frauenzimmer," who Dr. Dreyer identifies with Christian Kirch, or one of
her two sisters, daughters of the famous Gottfried and Maria Margaretta
Kirch (_Idem_, p. 107). Dr. Dreyer thinks that the drawing "seems to have
been carefully made, and not to be a mere rough sketch as I had at first
supposed" (_Idem_, p. 185).
The tails of some comets were of immense length. That of the comet of 1769
had an absolute length of 38 millions of miles. That of 1680, 96 million
of miles, or more than the sun's distance from the earth. According to Sir
William Herschel, the tail of the great comet of 1811 was over 100
millions of miles in length. That of the great comet of 1843--one of the
finest in history--is supposed to have reached a length of 150 millions of
miles![228]
In width the tails of comets were in some cases enormous. According to Sir
William Herschel, the tail of the comet of 1811 had a diameter of 15
millions of miles! Its volume was, therefore, far greater than that of the
sun![228]
According to Hevelius the comet of 1652 was of such a magnitude that it
"resembled the moon when half full; only it shone with a pale and dismal
light."[229]
Halley's comet at its next appearance will be examined with the
spectroscope for the first time in its history. At its last return in
1835, the spectroscope had not been invented.
For the great comet of 1811, Arago computed a period of 3065 years; and
Encke found a period of 8800 years for the great comet of 1680.[230]
The variation in the orbital velocity of some comets is enormous. The
velocity of the comet of 1680 when passing round the sun (perihelion) was
about 212 miles a second! Whereas at its greatest distance from the sun
(aphelion) the velocity is reduced to about 10 feet a second!