Venus was once supposed to possess a satellite. But belief in its existence has died out. No one, indeed, has caught even a deceptive glimpse of such an object during the last 125 years. Yet it was repeatedly and, one might have thought, well observed in the seventeenth and eighteenth centuries. Fontana "discovered" it in 1645; Cassini—an adept in the art of seeing—recognised it in 1672, and again in 1686; Short watched it for a full hour in 1740 with varied instrumental means; Tobias Mayer in 1759, Montaigne in 1761; several astronomers at Copenhagen in March, 1764, noted what they considered its unmistakable presence; as did Horrebow in 1768. But M. Paul Stroobant,[883] who in 1887 submitted all the available data on the subject to a searching examination, identified Horrebow's satellite with θ Libræ, a fifth-magnitude star; and a few other apparitions were, by his industry, similarly explained away. Nevertheless, several withstood all efforts to account for them, and together form a most curious case of illusion. For it is quite certain that Venus has no such conspicuous attendant.
The third planet encountered in travelling outward from the sun is the abode of man. He has in consequence opportunities for studying its physical habitudes altogether different from the baffling glimpse afforded to him of the other members of the solar family.
Regarding the earth, then, a mass of knowledge so varied and comprehensive has been accumulated as to form a science—or rather several sciences—apart. But underneath all lie astronomical relations, the recognition and investigation of which constitute one of the most significant intellectual events of the present century.
It is indeed far from easy to draw a line of logical distinction between items of knowledge which have their proper place here, and those which should be left to the historian of geology. There are some, however, of which the cosmical connections are so close that it is impossible to overlook them. Among these is the ascertainment of the solidity of the globe. At first sight it seems difficult to conceive what the apparent positions of the stars can have to do with subterranean conditions; yet it was from star measurements alone that Hopkins, in 1839, concluded the earth to be solid to a depth of at least 800 or 1,000 miles.[884] His argument was, that if it were a mere shell filled with liquid, precession and nutation would be much larger than they are observed to be. For the shell alone would follow the pull of the sun and moon on its equatorial girdle, leaving the liquid behind; and being thus so much the lighter, would move the more readily. There is, it is true, grave reason to doubt whether this reasoning corresponds with the actual facts of the case;[885] but the conclusion to which it led has been otherwise affirmed and extended.
Indications of an identical purport have been derived from another kind of external disturbance, affecting our globe through the same agencies. Lord Kelvin (then Sir William Thomson) pointed out in 1862[886] that tidal influences are brought to bear on land as well as on water, although obedience to them is perceptible only in the mobile element. Some bodily distortion of the earth's figure must, however, take place, unless we suppose it of absolute or "preternatural" rigidity, and the amount of such distortion can be determined from its effect in diminishing oceanic tides below their calculated value. For if the earth were perfectly plastic to the stresses of solar and lunar gravity, tides—in the ordinary sense—would not exist. Continents and oceans would swell and subside together. It is to the difference in the behaviour of solid and liquid terrestrial constituents that the ebb and flow of the waters are due.
Six years later, the distinguished Glasgow professor suggested that this criterion might, by the aid of a prolonged series of exact tidal observations, be practically applied to test the interior condition of our planet.[887] In 1882, accordingly, suitable data extending over thirty-three years having at length become available, Mr. G. H. Darwin performed the laborious task of their analysis, with the general result that the "effective rigidity" of the earth's mass must be at least as great as that of steel.[888]
Ratification from an unexpected quarter has lately been brought to this conclusion. The question of a possible mobility in the earth's axis of rotation has often been mooted. Now at last it has received an affirmative reply. Dr. Küstner detected, in his observations of 1884-85, effects apparently springing from a minute variation in the latitude of Berlin. The matter having been brought before the International Geodetic Association in 1888, special observations were set on foot at Berlin, Potsdam, Prague, and Strasbourg, the upshot of which was to bring plainly to view synchronous, and seemingly periodic fluctuations of latitude to the extent of half a second of arc. The reality of these was verified by an expedition to Honolulu in 1891-92, the variations there corresponding inversely to those simultaneously determined in Europe.[889] Their character was completely defined by Mr. S. C. Chandler's discussion in October, 1891.[890] He showed that they could be explained by supposing the pole of the earth to describe a circle with a radius of thirty feet in a period of fourteen months. Confirmation of this hypothesis was found by Dr. B. A. Gould in the Cordoba observations,[891] and it was provided with a physical basis through the able co-operation of Professor Newcomb.[892] The earth, owing to its ellipsoidal shape, should, apart from disturbance, rotate upon its "axis of figure," or shortest diameter; since thus alone can the centrifugal forces generated by its spinning balance each other. Temporary causes, however, such as heavy falls of snow or rain limited to one continental area, the shifting of ice-masses, even the movements of winds, may render the globe slightly lop-sided, and thus oblige it to forsake its normal axis, and rotate on one somewhat divergent from it. This "instantaneous axis" (for it is incessantly changing) must, by mathematical theory, revolve round the axis of figure in a period of 306 days. Provided, that is to say, the earth were a perfectly rigid body. But it is far from being so; it yields sensibly to every strain put upon it; and this yielding tends to protract the time of circulation of the displaced pole. The length of its period, then, serves as a kind of measure of the plasticity of the globe; which, according to Newcomb's and S. S. Hough's independent calculations,[893] seems to be a little less than that of steel. In an earth compacted of steel, the instantaneous axis would revolve in 441 days; in the actual earth, the process is accomplished in 428 days. By this new path, accordingly, astronomers have been led to an identical estimate of the consistence of our globe with that derived from tidal investigations.
Variations of latitude are intrinsically complex. To produce them, an incalculable interplay of causes must be at work, each with its proper period and law of action.[894] All the elements of the phenomenon are then in a perpetual state of flux,[895] and absorb for their continual redetermination, the arduous and combined labours of many astronomers. Nor is this trouble superfluous. Minute in extent though they be, the shiftings of the pole menace the very foundations of exact celestial science; their neglect would leave the entire fabric insecure. Just at the beginning of the present century they reached a predicted minimum, but are expected again to augment their range after the year 1902. The interesting suggestion has been made by Mr. J. Halm that such fluctuations are, in some obscure way, affected by changes in solar activity, and conform like them to an eleven-year cycle.[896]