[Sidenote: Astronomical facts imply slow secular changes.] If astronomers sought in systematic causes an explanation of these facts if, for instance, they were disposed to examine how far changes in the obliquity of the ecliptic are connected therewith--it was necessary at the outset to concede that the scale of time on which the event proceeds is of prodigious duration, this secular variation observing a slow process of only 45·7'' in a century; and hence, since the time of Hipparchus, two thousand years ago, the plane of the ecliptic has approached that of the equator by only a quarter of a degree. Or if, again, they looked to a diminishing of the eccentricity of the earth's orbit, they were compelled to admit the same postulate, and deal with thousands of centuries. Under whatever aspect, then, the theory was regarded, if once a former high temperature were admitted, and the fact coupled therewith that there has been no sensible decline within the observation of man, whether the explanation was purely geological or purely astronomical, the motion of heat in the mass of the earth is so slow, yet the change that has taken place is so great, the variations of the contemplated relations of the solar system so gradual--under whatever aspect and in whatever way the fact was dealt with, there arose the indispensable concession of countless centuries. To the astronomer such a concession is nothing extraordinary. It is not because of the time required that he entertains any doubt that the sun and his system accomplish a revolution round a distant centre of gravity in nineteen millions of years, or that the year of epsilon Lyræ is half a million of ours. He looks forward to that distant day when Sirius will disappear from our skies, and the Southern Cross be visible, and Vega the polar star. He looks back to the time when gamma Draconis occupied that conspicuous position, and the builders of the great pyramid, B.C. 3970, gave to its subterranean passage an inclination of 26° 15´, corresponding to the inferior culmination of that star. He tells us that the Southern Cross began to be invisible in 52° 30´ N., 2900 years before our era, and that it had previously attained an altitude of more than 10°. When it disappeared from the horizon of the countries on the Baltic, the pyramid of Cheops had been erected more than a thousand years.