It remains to describe some of the most remarkable comets of which we have any record, but of which we have no means of determining with certainty whether they move in ellipses, parabolas, or hyperbolas.
In the year 466 B.C., a large comet appeared simultaneously with the famous fall of meteoric stones near Ægospotamos. The former was supposed by the ancients to have had some agency in producing the latter phenomenon. Another of extraordinary magnitude appeared in the year 373 B.C. This comet was so bright as to throw shadows, and its tail extended one-third of the distance from the horizon to the zenith. The years 156, 136, 130, and 48, before our era, were also signalized by the appearance of very large comets. The apparent magnitude of the first of these is said to have equaled that of the sun itself; while its light was sufficient to diminish sensibly the darkness of the night. The second is said to have filled a fourth part of the celestial hemisphere. The comet of 130 B.C., sometimes called the comet of Mithridates, because of its appearance about the time of his birth, is said to have rivaled the sun in splendor.
In A.D. 178 a large comet was visible during a period of nearly three months. Its nucleus had a remarkably red or fiery appearance, and the greatest length of its tail exceeded 60°. The most brilliant comets of the sixth century were probably those of 531 and 582. The train of the latter, as seen in the west soon after sunset, presented the appearance of a distant conflagration.
Great comets appeared in the years 975, 1264, and 1556. Of these, the comet of 1264 had the greatest apparent magnitude. It was first seen early in July, and attained its greatest brilliancy in the latter part of August, when its tail was 100° in length. It disappeared on the 3d of October, about the time of the death of Pope Urban IV., of which event the comet, in consequence of this coincidence, was considered the precursor. These comets, on account of the similarity of their elements, were believed by many astronomers to be the same, and to have a period of about 300 years. In the case of identity, however, another reappearance should have occurred soon after the middle of the nineteenth century. As no such return was observed, we may conclude that the comets were not the same, and that their periods are wholly unknown.
The comet discovered on the 10th of November, 1618, was one of the largest in modern times; its tail having attained the extraordinary length of 104°. The comet of 1652, so carefully observed by Hevelius, almost equaled the moon in apparent magnitude. It shone, however, with a lurid, dismal light. The tail of the comet of 1680 was 90° in length. This body is also remarkable for its near approach to the sun; its least distance from the solar surface having been only 147,000 miles. It will always be especially memorable, however, for having furnished Newton the data by means of which he first showed that comets in their orbital motions are governed by the same principle that regulates the planetary revolutions.
Of all the comets which appeared during the eighteenth century, that which passed its perihelion on the 7th of October, 1769, had the greatest apparent magnitude. It was discovered by Messier on the 8th of August, and continued to be observed till the 1st of December. On the 11th of September the length of its tail was 97°. The comet discovered on the 26th of March, 1811, is in some respects the most remarkable on record. It was observed during a period of 16 months and 22 days,—the longest period of visibility known. On account of its situation with respect to the earth, the apparent length of its tail was much less than that of some other comets; its true length, however, was at one time 120,000,000 miles; and Sir William Herschel found that on the 12th of October the greatest circular section of the tail was 15,000,000 miles in diameter. The same astronomer found the diameter of the head of the comet to be 127,000 miles, and that of the envelope at least 643,000. As a general thing, the length of a comet-train increases very rapidly as the body approaches the sun. But the perihelion distance of the comet of 1811 was considerably greater than the distance of the earth from the sun; while its nearest approach to the earth was 110,000,000 miles. Its true magnitude, therefore, has probably not been surpassed by any other observed; and had its perihelion been very near the sun, it must have exhibited an appearance of terrific grandeur. This comet has an elliptic orbit, and its period, according to Argelander, is 3065 years.
The great comet of 1861 was discovered on the 13th of May, by Mr. John Tebbut, Jr., of New South Wales. In this country, as well as in Europe, it was first generally observed on the evening of June 30,—19 days after its perihelion passage. Sir John Herschel, who observed it in Kent, England, remarks that it far exceeded in brilliancy any comets he had ever seen, not excepting those of 1811 and 1858. According to Father Secchi, of the Collegio Romano, the length of its tail was 118°. This, with a single exception,[8] is the greatest on record. The computed orbit is elliptical; the period, 419 years.