The announcement of a new comet is always read with avidity by amateurs, and their first desire is to see it for themselves. This they may readily do by marking its place on a star-map or globe, and noting its relative place amongst the stars near. The telescope should then be directed towards the point indicated, and if the comet is not presented in the field, the instrument should be moved a little so that the surrounding region may be examined. If failure still attends the effort, the observer should point the telescope a few degrees E. or W. of the suspected point, and then carefully sweep over the place of the comet. It will then be picked up, unless it is too faint for his aperture. The first announcement of a comet generally gives the position at discovery, and the daily rate and direction of motion. The latter must of course be allowed for when the search is instituted.
The physical aspects of comets are as diversified as they are variable. No two comets are exactly alike, nor does the same comet exhibit a permanency of detail. Of course, when these objects are enormously distant, and barely visible, many of them appear to present similar characteristics; but under the closer and more expanded views obtainable near perihelion the resemblance vanishes, and every comet is seen to possess features peculiar to itself. To trace these features, and to record them by delineation and description, forms one of the most interesting branches in which amateurs may engage. Much has been learnt of previous comets by successively noting their transitions of form and brightness, and the same scrupulous attention should be given to future comets.
The tails of comets are not always turned away from the Sun. Indeed, the contrary effect is sometimes produced. Occasionally there is a duple tail, the largest branch of which follows the normal direction, while the other is turned towards the Sun. Forms of this character require close watching from night to night. Is the sunward tail developed suddenly? and has it a fairly durable existence? Instances of singular curvature should also be noted. The tails are seldom perfectly straight, especially those attached to naked-eye comets, and decided changes affect their visible outlines at very short intervals. In large comets the space over which the tail extends should be sketched upon a star-map on successive evenings; its changes of position and curve will then be manifested by comparisons, and its increasing or decreasing length will also be apparent. Dark rifts, like shadows, often run lengthwise through the tail, and occasion a fan-like appearance analogous to that which distinguished the great comet of 1744 and gave it a sextuple tail.
The light of comets sometimes fluctuates in a very extraordinary manner, and too rapidly and irregularly to be consistent with theory. In this respect, Pons’s Comet, at its last return in 1883-4 presented an eccentric behaviour. Bigourdan found that during the nineteen days from Sept. 5 to 24, 1883, the increase in the comet’s brilliancy exceeded by thirty or forty times that resulting from reflected light alone! This increase appears to have been due to a sudden outburst on Sept. 22, which occurred some time within the four hours preceding midnight. Dr. Müller, of Potsdam, witnessed a further outburst on Jan. 1, 1884, within 1-3/4 hour; and the extent of this was accurately determined by means of a photometer. He found an augmentation of seven tenths of a magnitude in the brightness of the comet, and an equally sudden fall to its previous lustre. While these fluctuations were in progress, he noticed variations in the shape of the nucleus not less remarkable than its variations in light. Those who observe future comets will do well, therefore, to be on the alert for similar phenomena. The apparent brightness of the nuclei and alterations of shape or size should be recorded on every night when observations are feasible.
As a comet approaches the Sun its material apparently contracts, while with increasing distance from that luminary it expands. Usually the nucleus is extremely small and bright, and it often looks like a star shining through nebulosity. High powers must therefore be applied in its examination. Jets, aigrettes, luminous sectors, and other appendages are often involved with the nucleus and outlying coma, and they form a complicated structure well deserving further study. A good deal of mystery still surrounds these appearances; their curious forms and vagaries have yet to be explained.
Stars are frequently observed through the head of a comet, which apparently, however, exercises no influence in dimming their lustre. But the stars are commonly seen behind the envelopes or comæ, and very rarely through the nucleus. Nothing is better calculated to exhibit the transparent and tenuous character of comets than observations of this kind, and observers should seek for further opportunities of making them. If the motion of a comet is obviously carrying it in the direction of one of the stars in the field, the observer may determine for himself the approximate time of conjunction by noting the distance between the star and comet and allowing for the motion of the latter. He will then know when to come to his telescope and witness the phenomenon. Should it appear probable that the comet’s nucleus will pass over the star, he should commence his watch some time before it occurs; he may then make comparisons before the star is involved in the outlying nebulosity, and trace the whole event from beginning to end. Any changes in the light or aspect of either star or comet would then be manifested. The comet of 1847 is said to have passed centrally over a 5th mag. star, but the latter was unaffected. Encke’s Comet on one occasion interposed itself directly over one of a pair of 10th mag. stars, but their relatively equal brilliancy suffered no change. Encke’s Comet, however, has no stellar nucleus. The latter feature is so bright and compact as displayed in many other comets, that its transit over a small star must have some effect either in obliterating it altogether, or in detracting from its lustre.
Visible evidences of rotation seem to have been suspected in certain comets, but this has never been substantiated on sufficient grounds. The circumstance is one, however, which should be remembered. During a series of observations the observer who notes the details of structure with particular regard to position may discover similar traces, and possibly learn something of the cause. The nucleus of a bright comet should always be examined with a moderately high power, so that any variations or peculiarities of form may be detected.
Nomenclature of Comets.—It must be confessed that no perfectly satisfactory method has yet been devised as regards the naming of comets. The plan of affixing Roman numbers progressively for each year, according to date of perihelion passage, answers pretty well, though a little confusion is sometimes caused by prematurely affixing the number, especially when two comets are discovered successively, the first of which is a long time before perihelion, and the second considerably after it. Until a comet can be safely assigned its catalogue place, it is preferable to refer to it by the name of the discoverer and date of discovery. This is more distinctive than the common method of lettering comets according to the epochs of their detection. As to periodical comets, it is not difficult to find some inconsistencies in their names. In the case of Halley’s Comet (1682, discovered by Flamsteed) and Encke’s Comet (1819 I., discovered by Pons), it was most fitting that they should be known to posterity by the names of the two able computers whose investigations first revealed to us comets of long and short period respectively. Under ordinary circumstances the name of the discoverer is applied to a comet as a means of convenient reference, and perhaps as a suitable recognition of the patient labours of the man who first announced it to the world. The plan seems to have been to name comets after those fortunate persons who sighted them at the particular apparition during which periodicity was determined. Thus Tuttle’s Comet (1858 I.) had been seen as long before as 1790 by Mechain, and Biela’s (1826 I.) was previously observed in 1772 by Montaigne, and in 1805 by Pons. It is, however, strange that a comet found by Pons in 1819 (III. of that year), and which Encke showed to be revolving in an ellipse with a periodic time of 5½ years, should be called after Winnecke, who rediscovered it in 1858. To Pons the real priority belongs, though Winnecke deserves much praise for redetecting and identifying this body after it had effected six unobserved returns to perihelion. It is also curious to find that the comet of short period discovered by Swift in 1880 is called “Tempel’s 3rd Comet” in Galle’s catalogue (1885), from the fact that Tempel found it at a previous return (1869), when, however, its period was not ascertained. There is little doubt that the title justly belongs to Swift. Tempel himself called it “Swift’s Comet.” One plan should in fairness to observers be consistently adhered to. If comets are to be called after their original discoverers, then Biela’s Comet should be known as “Montaigne’s,” Tuttle’s as “Mechain’s,” &c.
Curiosities of Comets.—The comet of 1729, which was hardly visible to the naked eye, has far the greatest perihelion distance (4·0435) of any comet known. Barnard’s Comet (1885 II.) comes next with a perihelion distance of 2·5068.
Pons’s Comet at its return in 1883-4 remained visible for nine months. When last seen, on June 2, 1884, it was 470 millions of miles from the Earth, and more remote in the depths of space than any other observed comet since that of 1729. Barnard’s Comet (1889 I.), though never visible to the naked eye, was followed from Sept. 2, 1888, to Aug. 18, 1890. Its distance from the Sun was then 6·25 (Earth’s distance = 1), or about 580 millions of miles, which is greater than that of many of the short-period comets at aphelia. The most prolonged visibility of any previous comet was that of 1811 I. (510 days). But this comet of Barnard has been retained in view 715 days.