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 5½ 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 2½°, 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!