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 11h 28m and 12h 29m 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 2¼ hours north of the ecliptic, passing from the ascending to the descending node of its orbit in 2h 13m·4.[191] The great comet of 1882 transited the sun’s disc on Sunday, September 17, of that year, the ingress taking place at 4h 50m 58s, 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 β 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 1902b 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 (1908c) 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.