CHAPTER THREE

THE STORY OF DONATI’S COMET

Hast thou ne’er seen the comet’s flaming flight? Th’ illustrious stranger, passing terror sheds On gazing Nations, from his fiery train Of length enormous, takes his ample round Thro’ depths of ether; coasts unnumbered Worlds, Of more than solar glory, doubles wide Heaven’s mighty cape; and then revisits Earth, From the long travel of a thousand years. —YOUNG. “Night Thoughts.”

When a comet draws near to pay its respects to its ruler, the sun, it usually assumes a splendor befitting this momentous occasion. It adorns itself with a glittering train millions of miles in length, and composed of myriads of particles reflecting the sun’s light. The head is often enveloped in a multiplicity of transparent veils, through which bright jets may be seen emanating from the star-like nucleus within. Some comets have been seen with five or six trains, spread out like that of a peacock, the camera revealing rapid and marvelous changes in their appearance, during the course of a few hours. No fair débutante, about to be presented to royalty, could vie with a comet in capriciousness regarding her raiment, nor could she equal it in splendor, even though she owned the mystic lamp of Aladdin.

The brief assumption of splendor on the part of a comet is very unlike its usual humdrum existence when it is as yet so far distant as to be not only invisible to the comet-hunter, but beyond range of the far-reaching eye of the telescope or the entrapping power of the camera. Not the slightest impression is made on the photographic plate, and as far as an observer on planet earth is concerned the comet might have ceased to exist. It is only when it begins to draw near to the sun that we are enabled to obtain a record of the marvelous changes produced in its appearance, until at its nearest approach it has sometimes been known to vibrate as though with intense excitement. For instance, in the case of Biela’s comet, concerning which a special account is given further on, it was apparently so overcome at its last appearance in 1846, that it split in two and literally went to pieces.

Quite a different story is told concerning the magnificent comet which greeted us in the summer of 1858, and was first seen at Florence on the 2d of June by Giambattista Donati, after whom it was named. At the time it was merely a nebulous mass about one-twentieth the diameter of the moon, and for some weeks it retained about the same brightness except for a gradual increase in the central star-like point, the only indication of its coming splendor. At the end of August it had increased so rapidly in brightness that by September it was visible to the unaided eye, resembling a hazy-looking star adorned with a small tail.

Gradually, as it drew nearer to the sun, it increased in size and splendor, reaching its maximum brightness in October. Its train extended over an arc of forty degrees, or eight times the distance separating Alpha and Beta—the so-called pointers in the constellation of Ursa Major, the Great Bear. Its real length was then about forty-five million miles, with a width of ten million. The nucleus varied in diameter from five hundred miles to three thousand, or nearly half that of our planet earth.

The comet was kept under accurate observation for fully nine months, and during part of that time it was visible to the naked eye. Professor G. P. Bond, the director of the Harvard College Observatory, availed himself of the opportunity thus presented, of making a series of drawings of the comet which convey an excellent idea of its changing appearance, and the delicate shadings and misty outlines of this marvelous visitant from the star-depths. These drawings are of all the more value, since it will be nearly two thousand years before Donati’s comet visits these realms again.

To go back to the earlier history of the comet, before these drawings were made, we find that its tail was not observed telescopically until seventy-three days after Donati’s discovery. It was seen on August 14, 1858, by astronomers at Copenhagen and Vienna, but not at Harvard until the 20th of that month. The brilliancy of the comet was somewhat impaired by a strong twilight and its low altitude. This may account for the fact that it was described as ruddy in hue, and concentrated, and having a mere suggestion of a tail. On August 23 the tail was still so faint as to be easily overlooked in the moonlight, the record being: “Bright, but no trace of a tail; the sky clear, but the moon nearly at full.” On August 30, according to the record in the _Times_ (London), of observations made by J. Russell Hind, “The comet was just perceptible to the naked eye; its nucleus is strongly condensed and brilliant, and the tail is thrown off in the ordinary form, without bifurcation.”

During the month of September the tail of the comet showed a tendency to curve, and by September 7 it was recorded as being very conspicuous to the naked eye. September 12 it had increased wonderfully in brilliancy, and on September 16 the first sketch was made by G. P. Bond, showing a view of it with the naked eye. The tail was now estimated as being 7° long, thus exceeding the distance (of 5°) separating the pointers. A tangent to the convex edge near the nucleus prolonged would pass through Delta in Ursa Major, and it was noticeable that this side was the brightest in all the sketches. A narrow dark channel extending from the nucleus up the axis of the tail was very remarkable, and its edges were surprisingly well defined, especially very near the nucleus. In fact, the comparatively sharp definition of the eastern edge of the tail was in marked contrast to the softness of outline on the western side. (See _Monthly Notices_, R. A. S., Vol. XIX, pp. 88–89.)

By September 27 the length of the tail as observed with the naked eye was about 9° or 10°. It was curved, convex toward the star Cor Caroli, being much better defined on the side near the star than on the concave side. The narrow dark stripe in the axis of the tail was still very marked, and the outline of the tail could be traced from the nucleus halfway to Delta in Ursa Major, and a degree or so further. It was now strongly curved and its upper outline well defined and bright as compared with the inner. A straight ray or secondary tail could be seen faintly suggested on the eastern side and reaching northward from the main tail.

By October 3 a marvelous change had taken place in the appearance of the comet. The train had increased in length and brightness, extending nearly as far as Eta in Ursa Major, and the straight ray or secondary train was still very much in evidence. It was supplemented by another slender ray, as shown in drawings made by Professor Bond on October 4 and 5, but it had vanished by October 6, although its position was indicated, for that date, in the faint suggestion of a ray between the main tail and the outer or secondary tail. The bright star to the left of the nucleus of the comet is Arcturus (in the constellation Boötes), over which the comet passed without perceptibly diminishing its brightness, thus showing of what airy texture the train of a comet is composed. Was it not Sir John Herschel who said that a comet could be easily packed in a portmanteau, and in the recent edition of _The Vault of Heaven_ Sir Richard Gregory gives the following unique illustration of the insignificance of the whole mass of a comet:

[Illustration:

COMET OF DONATI

Photograph taken October 10, 1858, at Harvard College Observatory ]

“Suppose we could take a comet, head, tail and all, and put it in one pan of a balance, and we could carve out from the air which surrounds us an object of the same size to put in the other pan, we should find that our aërial body weighed four or five thousand times more than the comet. But though a comet as a whole is lighter than air, it must not be concluded that comets consist solely of gases in a state of extreme tenuity. The head may be, and very probably is, composed of a large number of small but solid bodies; nevertheless, when a comet is taken in its entirety, the mean density is extremely low.”

By October 10, the comet was receding from the neighborhood of Ursa Major, drifting across the constellation of Boötes. On this date the comet made its nearest approach to the earth. Its train now resembled that of a widely opened fan, but its outline was already growing dim. It showed strange alterations of dark and bright bands, resembling the streamers which are sometimes seen to break up the continuous outline of an auroral arch. The extreme length of the tail was nearly 64°, the greatest extent observed during the apparition of the comet. The secondary tail was still visible, but extremely faint.

October 11, the dark stripe in the tail had almost vanished, the secondary tail was no longer to be seen, and the main tail was curved like an ostrich plume. Its length was now judged to be about 30°, and the nucleus had somewhat diminished in brightness. By October 15 the comet was considerably fainter and smaller, as seen with the naked eye, and it was bent southward like a sail wafted by a celestial breeze. After the middle of October, the comet was best seen from the southern hemisphere, and the last glimpse obtained in the northern hemisphere was on October 25, when it was at an altitude of 3°, the sky fortunately being very clear. The nucleus was still bright, but the tail was only 1° long.

Its course was then followed by Maclear, Royal Astronomer at the Cape of Good Hope, who reported that on December 23 the comet was merely a faint nebulous body, about 90″ in diameter, with a slight central condensation of light and no trace of a tail. Thus, it vanished in the remote depths of space, in the same undecorated condition as when it first made its eventful début to gladden the eyes of mortals on planet earth. Its visit lasted but one hundred and seventy-seven days, from the time of its first appearance until it took its departure along a track which will not bring it within our ken again until nearly two thousand years have rolled away.

BIOGRAPHICAL NOTE.—G. B. Donati, the Italian astronomer, was born at Pisa, in 1826. At the age of twenty-six, he obtained a post in the observatory at Florence, and there by his superior abilities, acquirements, and unwearied application to duty soon gained a high reputation among the men of science of his native country. He became known to the world in 1858, by his discovery of the magnificent comet called by his name. In 1864 he was appointed director of the observatory in which he had worked so efficiently for twelve years. He then undertook the arduous task of superintending the erection of a new and more convenient observatory on the site of Arcetri, near Florence. All difficulties were conquered, the new observatory was in working condition, and the director had entered upon a new series of observations when his labors were suddenly cut short by death. He died at his home in Arcetri, September 29, 1873.