COMETS.
At the time of Herschel the ancient superstitions in regard to comets
had to a great extent vanished, thanks mainly to the return of Halley’s
comet in 1758. Yet, although comets had ceased to be objects of terror,
no explanation or rational theory of their nature was put forward until
the appearance of the great comet of 1811. This comet was visible from
March 26, 1811, to August 17, 1812, a period of 510 days. It was one of
the most magnificent comets ever seen, its tail being 100 millions of
miles in length and its head 127,000 miles in diameter. This wonderful
phenomenon was the subject of much investigation, particularly by
Olbers, the great German astronomer.
Heinrich Wilhelm Matthias Olbers was born at Arbergen, a village near
Bremen, October 11, 1758. His father was a clergyman who, in addition to
considerable mathematical powers, was an enthusiastic lover of
astronomy. At the age of thirteen young Olbers became deeply interested
in that science. While taking an evening walk in the month of August, he
observed the Pleiades, and determined to find out to which constellation
they belonged. He therefore bought some books on astronomy, along with a
few charts of the sky, and he began to study the science with much
enthusiasm. He read every book he could lay his hands on, and a few
months sufficed to make him acquainted with all the constellations.
In 1777, when in his nineteenth year, Olbers entered the University of
Göttingen to study medicine, and at the same time he learned much
regarding mathematics and astronomy from the mathematician Kaestner.
When twenty-one years of age he observed the stars at Göttingen, and
devised a method of calculating the orbits of comets, the idea coming to
him while he was attending at the bedside of a fellow-student who had
taken ill. “Although not made public until 1797,” writes Miss Clerke,
“‘Olbers’ method’ was then universally adopted, and is still regarded as
the most expeditious and convenient in cases where absolute rigour is
not required. By its introduction, not only many a toilsome and
thankless hour was spared, but workers were multiplied and encouraged in
the pursuit of labours more useful than attractive.”
Towards the end of 1781 he returned to Bremen, settled as a medical
doctor, and continued in practice for about forty-one years. But
although he had adopted perhaps the most toilsome profession, his love
of science prevailed, and night after night he explored the heavens with
untiring zeal. He never slept more than four hours, and the upper part
of his house in the Sandgasse, in Bremen, was fitted up with
astronomical instruments. The largest telescope which he possessed was a
refractor 3¾ inches in aperture. He remained in active practice till
1823, when he retired, and was enabled to devote more attention to his
beloved science. He died on March 2, 1840, at the advanced age of
eighty-one.
Miss Clerke says of Olbers, “Night after night, during half a century
and upwards, he discovered, calculated, or observed the cometary
visitants of northern skies.” He was the discoverer of the comet of
1815, known as Olbers’ comet. It moves round the Sun in a period of over
seventy years, and returned to perihelion in 1887, forty-seven years
after the death of its discoverer. The great comet of 1811 was the
subject of a memoir which Olbers published the following year, and in
which he originated the “electrical repulsion” theory of comets’ tails.
Even after the fulfilment of Halley’s great prediction, comets were
still looked upon with profound awe, and the popular fear regarding them
was still prevalent. Olbers, however, showed that the tails of comets
resulted from purely natural causes. He regarded the Sun as possessed of
a repulsive as well as an attractive force, and considered the tails to
be vapours repelled from the nucleus of the comet by the Sun. He
calculated that in the comet of 1811 the particles of matter expelled
from the head reached the tail in eleven minutes, with a velocity
comparable to that of light. The theory of electrical repulsion, since
elaborated by other observers, is now generally accepted among
astronomers. No other hypothesis represents in such a complete manner
the formation and growth of the luminous appendages of the celestial
bodies so picturesquely called “pale-winged messengers” as that put
forward by the physician of Bremen.
Some years after Olbers’ famous theory was given to the world, a great
advance was made in cometary astronomy by another great German
astronomer, his friend and pupil Encke. The son of a Hamburg clergyman,
Johann Franz Encke was born in that city in 1791, and died in 1865 at
Spandau. After taking part in the war against Napoleon, he was in 1822
appointed director of the Gotha Observatory, being called to Berlin in
1825. In early life he was the pupil of Olbers and Gauss, and his
investigations and discoveries formed an epoch in astronomy. His most
famous discovery related to the little comet which bears his name. The
comet was discovered by _J. L. Pons_ (1761-1831) at Marseilles, although
it had previously been seen by Méchain and Caroline Herschel. In 1819
Encke computed the orbit of the comet, and boldly announced that it
would reappear in 1822, its period being about 3¼ years, or 1208 days.
In 1822 the comet, true to Encke’s prediction, returned to perihelion,
and was observed at Paramatta in Australia, the perihelion passage
taking place within three hours of the time predicted by Encke. As Miss
Clerke remarks, “The importance of this event will be better understood
when it is remembered that it was only the second instance of the
recognised return of a comet; and that it, moreover, established the
existence of a new class of celestial bodies, distinguished as comets of
short period.”
In 1825 the comet was again observed by Valz, passing perihelion on
September 16, and in 1828 it was seen by Struve. Encke now made a very
remarkable discovery. Determining its period with great accuracy, in
1832 he found that his comet returned to perihelion two and a half hours
before the predicted time. As this repeatedly happened, Encke put
forward the theory that the acceleration was due to the existence of a
resisting medium in the neighbourhood of the Sun, too rarefied to retard
the planetary motions, but quite dense enough to make the comet’s path
smaller, and to eventually precipitate it on the Sun. The theory was
widely accepted, but after 1868 the acceleration began to decrease,
diminishing by one-half; besides, no other comet is thus accelerated,
and the hypothesis has accordingly been abandoned.
The second comet recognised as periodic was that discovered on February
27, 1826, by an Austrian officer, _Wilhelm von Biela_ (1782-1856), and
ten days later by the French observer, _Gambart_ (1800-1836), both of
whom, in computing its orbit, noticed a remarkable similarity to the
orbits of comets which appeared in 1772 and 1805. Accordingly, they
concluded it to be periodic, with a period of between six and seven
years. The comet returned in 1832. In 1828 Olbers had published certain
calculations showing that portions of the comet would sweep over the
part of the Earth’s orbit a month later than the Earth itself. This gave
rise to a panic that the comet would destroy the Earth, which did not
subside till it was announced by Arago that the Earth and the comet
would at no time approach to within fifty million miles of each other.
The comet returned again in the end of 1845. It was kept well in view by
astronomers in Europe and America. On December 19, 1846, Hind noticed
that the comet was pear-shaped, and ten days later it had divided in
two. The two comets returned again in 1852 and were well observed; but
they were never seen again, at least as comets. Their subsequent history
belongs to meteoric astronomy.
A comet discovered by Faye at Paris in 1843 was found to have a period
of seven and a half years. It has returned regularly since its
discovery, true to astronomical prediction. Its motion was particularly
investigated for traces of a resisting medium, by _Didrik Magnus Axel
Möller_ (1830-1896), director of the Lund Observatory, who reached a
negative conclusion.
In 1835 Halley’s comet returned to perihelion, and was attentively
studied by the most famous astronomers of the age. It was particularly
studied by Sir John Herschel and by Bessel, who assisted in developing
Olbers’ theory of electrical repulsion. But the most brilliant comet of
the century was that which suddenly appeared on February 28, 1843, in
the vicinity of the Sun. This great comet, whose centre approached the
Sun within 78,000 miles, rushed past its perihelion at the speed of 366
miles a second. The comet’s tail reached the length of 200 millions of
miles. The comet of 1843 was however outshone, not in brilliance but as
a celestial spectacle, by the great comet discovered on June 2, 1858, by
_Giovanni Battista Donati_ (1826-1873) at Florence, and since known by
his name. It became visible to the naked eye on August 19, and was
telescopically observed until March 4, 1859. There was abundance of
time, therefore, to study the comet, which was exhaustively observed by
G. P. Bond at Harvard. His observations convinced him that the light
from Donati’s comet was merely reflected sunshine, and this was
generally accepted. Another great comet appeared in 1861. Like that of
1843, its appearance was sudden, being observed after sunset on June 30,
1861, when, says Miss Clerke, “a golden yellow planetary disc, wrapt in
dense nebulosity, shone out while the June twilight in these latitudes
was still in its first strength.” On the same evening the Earth and the
Moon passed through the tail of the great comet. The vast majority of
people never knew that such a phenomenon had taken place, and even the
astronomers only noticed a singular phosphorescence in the sky—a proof
of the extreme tenuity of comets.
The first application of the spectroscope to the light of comets was
made by Donati in 1864. The spectrum was found to consist of three
bright bands, but Donati was unable to identify them. However, his
observation gave the death-blow to the theory that comets shone by
reflected light alone, for it implied the existence of glowing gas in
them. On the appearance in 1868 of the periodic comet discovered by
_Friedrich August Theodor Winnecke_ (1835-1897), the spectrum was
examined by Huggins, who identified the bright bands with the spectrum
of hydrocarbon. This was confirmed in regard to Coggia’s comet of 1874
by Huggins himself, and also Brédikhine and Vogel. The hydrocarbon
spectrum is characteristic of comets, and has been recognised in all
those spectroscopically studied.
The time had now come for a more complete theory of comets than that of
Olbers. The theory of electrical repulsion was developed in 1871 by
Zöllner, whose principle of investigation is thus described by Miss
Clerke: “The efficacy of solar electrical repulsion relatively to solar
attraction grows as the size of the particle diminishes.” If the
particle is small enough, it will obey the repulsive, and not the
attractive, power of the Sun. Zöllner considered that the smallest
particles of comets obeyed the repulsive power, and thus formed the
tails of comets. The development of a complete cometary theory is due,
however, to the genius of a Russian astronomer. Theodor Alexandrovitch
Brédikhine, born in 1831 at Nicolaieff, was employed at Moscow
Observatory from 1857 to 1890, when he was promoted to the position of
director at Pulkowa. He resigned in 1895, and spent his last years in St
Petersburg, where he died on May 14, 1904. From the beginning of his
astronomical career he was devoted to the study of comets and their
tails, but it was the appearance of Coggia’s comet in 1874 which marked
the commencement of his most important observations. In that year, on
making certain calculations regarding the hypothetical repulsive force
exerted by the Sun on various comets, he reached the conclusion that the
values representing the intensity of the repulsion fell into three
classes. This was the first hint of a classification of cometary tails.
Meanwhile he carefully studied the tails of comets both from direct
observation and from drawings.
In 1877 he wrote: “I suspect that comets are divisible into groups, for
each of which the repulsive force is perhaps the same.” Subsequent
investigations led Brédikhine to divide the tails of comets into three
types. The first type consists of long, straight tails, pointed directly
away from the Sun, represented by the tails of the great comets of 1811,
1843, and 1861. In the second type, represented by Donati’s and Coggia’s
comets, the tails, although pointed away from the Sun, appear
considerably curved. In the third type the tails are, to quote Miss
Clerke, “short, strongly-bent, brushlike emanations, and in bright
comets seem to be only found in combination with tails of the higher
classes.”
In 1879 Brédikhine fully developed his cometary theory. Assuming the
reality of the repulsive force, he concluded that to produce tails of
the first type, the repulsion requires to be twelve times greater than
the solar attraction; the production of tails of the second type
necessitates a repulsive force about equal to gravity; while the force
producing third-type tails has only one-fourth the power of gravitation.
It was concluded that the tails are formed by particles of matter
repelled from the comet by the repulsive force of the Sun, and in tails
of the first type the velocity with which these particles leave the body
of the comet is four or five miles a second. Brédikhine reached the
conclusion that the Sun’s repulsive force is invariable, and that the
different types of tails are formed by the same force acting on
different elements. The numbers 12, 1, and ¼, are inversely proportional
to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour.
Here, then, was the key to the mystery. Brédikhine pointed out that in
all probability the first-type tails are formed of hydrogen, the second
of hydrocarbon, and the third of iron, with a mixture of sodium and
other elements.
Within a few years of the publication of Brédikhine’s theory, five
bright comets made their appearance, and there was abundant chance of
testing the theory spectroscopically. In 1882 Well’s comet was
particularly studied at Greenwich by Maunder, who discerned a
sodium-line in its spectrum. The magnificent comet which appeared in
1882 was spectroscopically studied at Dunecht in Aberdeenshire by _Ralph
Copeland_ (1837-1905), Astronomer-Royal of Scotland, who identified in
its spectrum the prominent iron-lines as well as the sodium-line. These
observations were certainly confirmatory of Brédikhine’s theory. It
should also be stated, however, that several comets have shown, in
addition to the hydrocarbon spectrum, that of reflected sunlight, which
proves that the light we receive from comets is of a compound nature.
The comet which appeared in 1880 was announced by _Benjamin Apthorp
Gould_ (1824-1896) to be a return of the great comet of 1843.
Calculations by Gould, Copeland, and Hind revealed a close similarity
between the elements of the two orbits. Eventually it had to be admitted
that the comets were separate bodies travelling in the same orbit. Then,
two years later, the great September comet of 1882 was found to revolve
in the same orbit as those of 1668, 1843, and 1880. Four years later,
another comet, discovered in 1887, was found to move in the same path.
Closely allied to this subject is the existence of “comet families,”
demonstrated by Hoek of Utrecht in 1865, and mentioned in our chapter on
the Outer Planets. These comets are found to be dependent on the
planets, Jupiter, Saturn, Uranus, and Neptune, each possessing a
comet-group. Various theories have been advanced to account for the
existence of these groups. One of these theories is that the comets have
been captured by the various planets, who have forced them into their
present orbits. A mathematical study by _Jean Pierre Octave Callandrean_
(1852-1904) shows that the large number of comets possessed by the
various planets may be explained by the disintegration of large comets
into small ones. The capture theory, it must be remembered, is purely
hypothetical, and must not be regarded as anything but a theory. All
that we really know is the existence of comet-families, and of comets
moving in the same orbits.
The first photograph of a comet was that of Donati’s, taken in 1858 by
Bond. In 1881 Tebbutt’s comet was photographed in England by Huggins,
and in America by _Henry Draper_ (1837-1882), while in 1882 Gill secured
excellent photographs of the great September comet. The first
photographic discovery of a comet was made by Barnard in 1892. Since
then photography has been much used in cometary astronomy. No bright
comets have appeared since 1882,—if we except the comet of 1901, only
seen in the southern hemisphere,—although several have been just visible
to the naked eye, among them Swift’s comet of 1892 and Perrine’s in the
autumn of 1902. Telescopic comets, however, are very numerous, and a
year never passes without one or more being discovered. The ordinary
periodic comets, such as Encke’s, Faye’s, and others, are very faint,
and are becoming fainter at each return—a clear proof that comets die,
as Kepler said three centuries ago. This brings us to the subject of the
next chapter, Meteoric Astronomy.