Both these delicate objects have since been repeatedly observed, both in Europe and America, even with comparatively small instruments. At the opposition of 1884, indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at Washington. But the Lick equatoreal showed them, July 18, 1888, when their brightness was only 0·12 its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions.
The names chosen for them were taken from the Iliad, where "Deimos" and "Phobos" (Fear and Panic) are represented as the companions in battle of Ares. In several respects, they are interesting and remarkable bodies. As to size, they may be said to stand midway between meteorites and satellites. From careful photometric measures executed at Harvard in 1877 and 1879, Professor Pickering concluded their diameters to be respectively six and seven miles.[1011] This is on the assumption that they reflect the same proportion of the light incident upon them that their primary does. But it may very well be that they are less reflective, in which case they would be more extensive. The albedo of Mars is put by Müller at 0·27; his surface, in other words, returns 27 per cent. of the rays striking it. If we put the albedo of his satellites equal to that of our moon, 0·17, their diameters will be increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one, being the larger. Mr. Lowell, however, formed a considerably larger estimate of their dimensions.[1012] It is interesting to note that Deimos, according to Professor Pickering's very distinct perception, does not share the reddish tint of Mars.
Deimos completes its nearly circular revolutions in thirty hours eighteen minutes, at a distance from the surface of its ruling body of 12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours thirty-nine minutes twenty-two seconds, at a distance of only 3,760 miles. This is the only known instance of a satellite circulating faster than its primary rotates, and is a circumstance of some importance as regards theories of planetary development. To a Martian spectator the curious effect would ensue of a celestial object, seemingly exempt from the general motion of the sphere, rising in the west, setting in the east, and culminating twice, or even thrice a day; which, moreover, in latitudes above 69° north or south, would be permanently and altogether hidden by the intervening curvature of the globe.
The detection of new members of the solar system has come to be one of the most ordinary of astronomical events. Since 1846 no single year has passed without bringing its tribute of asteroidal discovery. In the last of the seventies alone, a full score of miniature planets were distinguished from the thronging stars amid which they seem to move; 1875 brought seventeen such recognitions; their number touched a minimum of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped to six in 1889, and sprang up with the aid of photography to twenty-seven in 1892. That high level has since, on an average, been maintained; and on January 1, 1902, nearly 500 asteroids were recognised as revolving between the orbits of Mars and Jupiter. Of these, considerably more than one hundred are claimed by one investigator alone—Dr. Max Wolf of Heidelburg; M. Charlois of Nice comes second with 102; while among the earlier observers Palisa of Vienna contributed 86, and C. H. F. Peters of Clinton (N. Y.), whose varied and useful career terminated July 19, 1890, 52 to the grand total.
The construction by Chacornac and his successors at Paris, and more recently by Peters at Clinton, of ecliptical charts showing all stars down to the thirteenth and fourteenth magnitudes respectively, rendered the picking out of moving objects above that brightness a mere question of time and diligence. Both, however, are vastly economised by the photographic method. Tedious comparisons of the sky with charts are no longer needed for the identification of unrecorded, because simulated stars. Planetary bodies declare themselves by appearing upon the plate, not in circular, but in linear form. Their motion converts their images into trails, long or short according to the time of exposure. The first asteroid (No. 323) thus detected was by Max Wolf, December 22, 1891.[1014] Eighteen others were similarly discovered in 1892, by the same skilful operator; and ten more through Charlois's adoption at Nice of the novel plan now in exclusive use for picking up errant light-specks. Far more onerous than the task of their discovery is that of keeping them in view once discovered—of tracking out their paths, fixing their places, and calculating the disturbing effects upon them of the mighty Jovian mass. These complex operations have come to be centralised at Berlin under the superintendence of Professor Tietjen, and their results are given to the public through the medium of the Berliner Astronomisches Jahrbuch.
The cui bono? however, began to be agitated. Was it worth while to maintain a staff of astronomers for the sole purpose of keeping hold over the identity of the innumerable component particles of a cosmical ring? The prospect, indeed, of all but a select few of the asteroids being thrown back by their contemptuous captors into the sea of space seemed so imminent that Professor Watson provided by will against the dereliction of the twenty-two discovered by himself. But the fortunes of the whole family improved through the distinction obtained by one of them. On August 14, 1898, the trail of a rapidly-moving, star-like object of the eleventh magnitude imprinted itself on a plate exposed by Herr Witt at the Urania Observatory, Berlin. Its originator proved to be unique among asteroids. "Eros" is, in sober fact,
'one of those mysterious stars Which hide themselves between the Earth and Mars,'
divined or imagined by Shelley.[1015] True, several of its congeners invade the Martian sphere at intervals; but the proper habitat of Eros is within that limit, although its excursions transcend it. In other words, its mean distance from the sun is about 135, as compared with the Martian distance of 141 million miles. Further, its orbit being so fortunately circumstanced as to bring it once in sixty-seven years within some 15 millions of miles of the earth, it is of extraordinary value to celestial surveyors. The calculation of its movements was much facilitated by detections, through a retrospective search,[1016] of many of its linear images among the star-dots on the Harvard plates.[1017] The little body—which can scarcely be more than twenty miles in diameter—shows peculiarities of behaviour as well as of position. Dr. von Oppolzer, in February, 1901,[1018] announced it to be extensively and rapidly variable. Once in 2 hours 38 minutes it lost about three-fourths of its light,[1019] but these fluctuations quickly diminished in range, and in the beginning of May ceased altogether.[1020] Evidently, then, they depend upon the situation of the asteroid relatively to ourselves; and, so far, events lent countenance to M. André's eclipse hypothesis, since mutual occultations of the supposed planetary twins could only take place when the plane of their revolutions passed through the earth, and this condition would be transitory. Yet the recognition in Eros of an "Algol asteroid" seems on other grounds inadmissible;[1021] nor until the phenomenon is conspicuously renewed—as it probably will be at the opposition of 1903—can there be much hope of finding its appropriate rationale.