The great comet of 1858, known generally at Donati's, was a magnificent object. Not only was it a fine comet in itself, but it seems as if everything conspired to enable it to be well seen from the earth. It was a most conspicuous object in the northern sky during the early evenings in October, and was nearest us shortly after the passage of perihelion. Its tail reached a length of 60 degrees and swept as a broad curved plume across the heavens. It was visible for several months, which enabled it to be care fully observed, and there is no doubt but that it moves in an elliptic orbit. Its period is about 2,000 years.
The great comet of 1861 was discovered by Tebbutt in Australia. It appeared suddenly in the northem hemisphere at the end of June and, according to Sir John Herschel, was the brightest comet of the century up to that time. It was not long visible.
The great comet of 1882 was remarkable on many accounts. In the first place it was shown to be one of a group of five comets all follow ing nearly in the same orbit. The four others were those of 1668, 1843, 1880 and 1887. The last was comparatively inconspicuous, but the others were all notable. The nucleus of the comet of 1882 moreover showed a tendency to fall apart, five different condensations in it be ing visible at once. Schmidt, Barnard and Brooks all noticed small companion nebulosities in the immediate vicinity of the comet and mov ing with it. It was visible for about nine months— an unusually long interval, and in this time it traversed 340 degrees of its orbit. It clearly follows an elliptical path with a period which lies between 772 and 1,000 years. The tail was very extraordinary. It measured about 200,000,000 miles in length, and was strongly forked at the end. Moreover it was accompanied by a clearly seen but filmy sheath of light, extending three or four degrees toward the sun. The whole comet was surrounded by an elliptical envelope of enormous dimensions and of a still more filmy nature. That this most remarkable object was not more generally observed by the public is due undoubtedly to the fact that it was visible only in the early morning hours. It was certainly the brightest comet of the century, and it was so bright that it was watched in perihelion passage until in contact with the sun's limb, when it suddenly disappeared, being of too filmy a nature to be detected upon the sun's surface. For three days it was visible to the naked eye with the sun above the horizon. There have been so far 14 different comets placed on record which were bright enough to be seen under these cir cumstances.
Turning now to the periodic comets; by far the finest is Halley's. Our first record of it is chief reason for its relative faintness in 1835 is that the material which formed the tail has been largely used up by its frequent visits to the sun, and that it will thus never again present the brilliancy of its past appearances. An ac count of its last appearance is given below.
The following table of the elements of the orbits of the periodic comets has been con densed and brought up to date from Holden's tables of periodic comets: in the year 11 s.c. Since then it has appeared regularly every 76 or 77 years, in all 25 times. We have a drawing of it as it appeared in 684, and another drawing of it in 1066. During its
latter appearance it was considered to be the precursor of the conquest of England by Wil liam of Normandy. It was a magnificent object in 1145, and again in 1223, when it was sup posed to foretell the death of Philip Augustus of France. In 1456 it was a superb object ap pearing shortly after the Turks had taken Con stantinople. In 1682 it was observed by Halley, who computed its orbit, and showed that it was identical with the great comets of 1531 and 1607, and predicted its return for the end of 1758 or the beginning of 1759. This is the first time that the return of a comet was ever pre dicted, and although Halley knew that he could not expect to live to see his prediction fulfilled (he was born in 1656), yet he left a somewhat plaintive appeal to posterity, that in case the comet should reappear, it should be remembered that this first prediction was made by an Eng lishman. The comet appeared Christmas day 1758. At its next appearance, in 1835, the comet was a fine object, but did not correspond in brilliancy to the descriptions of some of its earlier apparitions. This may perhaps he ex plained by the fact that it was but two years after a sun-spot minimum. Its next appearance was in 1910, which was also unfortunately very near a sun-spot minimum. But very likely the The question is often asked the astronomer, what would happen if the earth should come into collision with a comet? As far as the tail is concerned, that is probably a frequent occur rence. It happened twice during the last cen tury, in the case of the comets of 1819 II and 1861 II, and it probably occurred also during the apparition of Halley's comet in 1910, but in no case were any particular effects observed. Should we come into contact with the head of a small comet there would undoubtedly be a fine meteoric display. That is in fact what prob ably happened in 1833, and also on a number of previous occasions, notably in 472, 902, 1029, 1202 and 1799, although the comet itself was not luminous. If the comet were moving rapidly with regard to the earth, as in these cases, the meteors would be consumed at a great altitude in our atmosphere, and no harm would be done. If the comet were moving in the same direction as ourselves, however, and at about the same speed, the matter would then become more serious, as many of the meteors might reach the earth's surface.
The self-luminous, and therefore visible comet, which has come nearest to the earth, as far as we are aware, was that of 1770 I (Lexell's). It approached within 1,400,000 miles, or six times the distance of the moon. Its head appeared about four times the diameter of the moon, but its mass was imperceptibly small. Should the earth strike the nucleus of a great comet, like that, for instance, of 1858, it is impossible to foretell what would happen, but possibly the temperature and shock would be such that all life within several thousand miles of the point of contact would become extinct. Considering the vast recesses of celestial space, however, and the insignificant size of our earth, the probability of such an encounter is almost infinitely small.
For a more detailed account of the indi vidual peculiarities of the more interesting comets, see Clerke's 'History of Astronomy' ; Chambers' 'Story of the Comets' (1910).