Through the Telescope

by James Baikie

Most of us, however, will be content to discover our comets in the columns of the daily newspaper, or by means of a post-card from some obliging friend. The intimation, in whatever way received, will generally contain the position of the comet at a certain date, given in right ascension and declination, and either a statement of its apparent daily motion, or else a provisional set of places for several days ahead. Having either of these, the comet's position must be marked down on the star-map, and the course which it is likely to pursue must be traced out in pencil by means of the data—a perfectly simple matter of marking down the position for each day by its celestial longitude and latitude as given. The observer will next note carefully the alignment of the comet with the most conspicuous stars in the neighbourhood of the particular position for the day of his observation; and, guiding his telescope by means of these, will point it as nearly as possible to that position. He may be lucky enough to hit upon his object at once, especially if it be a comparatively bright one. More probably, he will have to 'sweep' for it. In this case the telescope must be pointed some little distance below and to one side of the probable position of the comet, and moved slowly and gently along, careful watch being kept upon the objects which pass through the field, until a similar distance on the opposite side of the position has been reached. Then raise the instrument by not more than half a field's breadth, estimating this by the stars in the field, and repeat the process in the opposite direction, going on until the comet appears in the field, or until it is obvious that it has been missed. A low power should be used at first, which may be changed for a somewhat higher one when the object has been found. But in no case will the use of really high magnifiers be found advisable. It is, of course, simply impossible with the tail, for which the naked eye is the best instrument, nor can the coma bear any degree of magnification, though occasionally the nucleus may be sufficiently sharply defined to bear moderate powers. The structure of the latter should be carefully observed, with particular attention to the question of whether any change can be seen in it, or whether there seem any tendency to such a multiplication of nuclei as characterized the great comet of 1882. It is possible that the pulses of vapour sunwards from the nucleus may also be observed.

Appearance of motion, wavy or otherwise, in the tail, should also be looked for, and carefully watched if seen. Beyond this there is not very much that the ordinary observer can do; the determination of positions requires more elaborate appliances, and the spectroscope is necessary for any study of cometary constitution. It only remains to express a wish for the speedy advent of a worthy subject for operations.

We turn now to those bodies which, as has been pointed out, appear to be the débris of comets which have exhausted their cometary destiny, and ceased to have a corporate existence. Everyone is familiar with the phenomenon known as a meteor, or shooting-star, and there are few clear nights on which an observer who is much in the open will not see one or more of these bodies. Generally they become visible in the form of a bright point of light which traverses in a straight line a longer or shorter path across the heavens, and then vanishes, sometimes leaving behind it for a second or two a faintly luminous train. The shooting-stars are of all degrees of brightness, from the extremely faint streaks which sometimes flash across the field of the telescope, up to brilliant objects, brighter than any of the planets or fixed stars, and sometimes lighting up the whole landscape with a light like that of the full moon.

The prevailing opinion, down to a comparatively late date, was that shooting-stars were mere exhalations in the earth's atmosphere, arising as one author expressed it, 'from the fermentation of acid and alkaline bodies, which float in the atmosphere'; and it was also suggested by eminent astronomers that they were the products of terrestrial volcanoes, returning, after long wanderings, to their native home.

The true study of meteoric astronomy may be said to date from the year 1833, when a shower of most extraordinary splendour was witnessed. The magnificence of this display was the means of turning greater attention to the subject; and it was observed as a fact, though the importance of the observation was scarcely realized, that the meteors all appeared to come from nearly the one point in the constellation Leo. The fact of there being a single radiant point implied that the meteors were all moving in parallel lines, and had entered our atmosphere from a vast distance. Humboldt, who had witnessed a previous appearance of this shower in 1799, suggested that it might be a periodic phenomenon; and his suggestion was amply confirmed when in 1866 the shower made its appearance again in scarcely diminished splendour. Gradually other showers came to be recognised, and their radiant points fixed; and meteoric astronomy began to be established upon a scientific basis.

In 1866 Schiaparelli announced that the shower which radiates in August from the constellation Perseus follows the same track as that of Swift's comet (1862 iii.); and in the following year the great November shower from Leo, already alluded to, was proved to have a similar connection with Tempel's comet (1866 i.). The shower which comes from the constellation Lyra, about April 20, describes the same orbit as that of Comet 1861 i.; while, as already mentioned, the mysterious disappearance of Biela's comet received a reasonable explanation by its association with the other great November shower—that which radiates from the constellation Andromeda. With regard to the last-named shower, it has not only been shown that the meteors are associated with Biela's comet, but also that they separated from it subsequent to 1841, in which year the comet's orbit was modified by perturbations from Jupiter. The Andromeda meteors follow the modified orbit, and hence must have been in close association with the comet when the perturbation was exercised.

The four outstanding meteor radiants are those named, but there are very many others. Mr. Denning, to whom this branch of science owes so much, estimates the number of distinct radiants known at about 4,400; and it seems likely that every one of these showers, some of them, of course very feeble, represents some comet deceased. The history of a meteor shower would appear to be something like this: When the comet, whose executor it is, has but recently deceased, it will appear as a very brilliant periodic shower, occurring on only one or two nights exactly at the point where the comet in its journeying would have crossed the earth's track, and appearing only at the time when the comet itself would have been there. Gradually the meteors get more and more tailed out along the orbit, as runners of unequal staying powers get strung out over a track in a long race, until the displays may be repeated, with somewhat diminished splendour, year after year for several years before and after the time when the parent comet is due. At last they get thinly spread out over the whole orbit, and the shower becomes an annual one, happening each year when the earth crosses the orbit of the comet. This has already happened to the Perseid shower; at least 500,000,000 miles of the orbit of Biela's comet are studded with representatives of the Andromedes; and the Leonid shower had already begun to show symptoms of the same process at its appearance in 1866. Readers will remember the disappointment caused by the failure of the Leonid shower to come up to time in 1899, and it seems probable that the action of some perturbing cause has so altered the orbit of this shower that it now passes almost clear of the earth's path, so that we shall not have the opportunity of witnessing another great display of the Leonid meteors.

So far as is known, no member of one of these great showers has ever fallen to the earth. There are two possible exceptions to this statement, as in 1095 a meteor fell to the ground during the progress of a shower of Lyrids, and in 1885 another fell during a display of the Andromedes. In neither case, however, was the radiant point noted, and unless it was the same as that of the shower the fall of the meteor was a mere coincidence. It seems probable that this is the case, and the absence of any evidence that a specimen from a cometary shower has reached the earth points to the extreme smallness of the various members of the shower and also to the fine division of the matter of the original comet.

In addition to the meteors originating from systematic showers, there are also to be noted frequent and sometimes very brilliant single meteors. Specimens of these have in many instances been obtained. They fall into three classes—'Those in which iron is found in considerable amount are termed siderites; those containing an admixture of iron and stone, siderolites; and those consisting almost entirely of stone are known as aerolites' (Denning). The mass of some of these bodies is very considerable. Swords have been forged out of their iron, one of which is in the possession of President Diaz of Mexico, while diamonds have been found in meteoric irons which fell in Arizona. It may be interesting to know that, according to a grave decision of the American courts, a meteor is 'real estate,' and belongs to the person on whose ground it has fallen; the alternative—that it is 'wild game,' and the property of its captor—having been rejected by the court. So far as I am aware, the legal status of these interesting flying creatures has not yet been determined in Britain.

The department of meteoric astronomy is one in which useful work can be done with the minimum of appliances. The chief requisites are a good set of star-maps, a sound knowledge of the constellations, a straight wand, and, above all, patience. The student must make himself familiar with the constellations (a pleasant task, which should be part of everyone's education), so that when a meteor crosses his field of view he may be able to identify at once with an approach to accuracy its points of appearance and disappearance. It is here that the straight wand comes into play. Mr. Denning advises the use of it as a means of guiding the eye. It is held so as to coincide with the path of the meteor just seen, and will thus help the eye to estimate the position and slope of the track relatively to the stars of the constellations which it has crossed. This track should be marked as quickly as possible on the charts. Mere descriptions of the appearance of meteors, however beautiful, are quite valueless. It is very interesting to be told that a meteor when first seen was 'of the size and colour of an orange,' but later 'of the apparent size of the full moon, and surrounded by a mass of glowing vapour which further increased its size to that of the head of a flour-barrel'; but the description is scarcely marked by sufficient precision of statement for scientific purposes. The observer must note certain definite points, of which the following is a summary: (1) Date, hour, and minute of appearance. (2) Brightness, compared with some well-known star, planet, or, if exceptionally bright, with the moon. (3) Right ascension and declination of point of first appearance. (4) The same of point of disappearance. (5) Length of track. (6) Duration of visibility. (7) Colour, presence of streak or train, and any other notable features. (8) Radiant point.