A Century's Progress in Astronomy :: CHAPTER VII. COMETS.

CHAPTER VII. 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.

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