Scientific astronomy, as distinguished from mere star-gazing, can hardly claim an age of more than 3000 years. It is less than this since Pythagoras, Aristarchus and others explained that the earth moved around a fixed sun. Yet the really significant figure for our present purpose is not so much the time since men began to make conjectures about the structure of the universe, as the time since they began to unravel its true structure by the help of ascertained fact. The important length of time is that which has elapsed since that evening in 1610 when Galileo first turned his telescope on to Jupiter—a mere three centuries or so. We begin to grasp the true significance of these round-number estimates when we re-write them in tabular form. We have: Age of earth about 2,000,000,000 years Age of life on earth ” 300,000,000 ” Age of man on earth ” 300,000 ” Age of astronomical science ” 3,000 ” Age of telescopic astronomy ” 300 ” When the various figures are displayed in this form we see what a very recent phenomenon astronomy is. Its total age is only a hundredth part of the age of man, only a hundred-thousandth part of the time that life has inhabited the earth. During 99,999 parts out of the 100,000 of its existence, life on earth was hardly concerned about anything beyond the earth. But whereas the past of astronomy is to be measured on the human time-scale, a hundred generations or so of men, there is every reason to expect that its future will be measured on the astronomical time-scale. We shall discuss the probable future stretching before the human race in a later chapter. For the moment it is not unreasonable to suppose that this future will probably be terminated by astronomical causes, so that its length is to be measured on the astronomical time-scale. As the earth has already existed for 2000 million years, it is _à priori_ reasonable to suppose that it will exist for at least something of the order of 2000 million years yet to come, and humanity and astronomy with it. Actually we shall find reasons for expecting it to last far longer than this. But if once it is conceded that its future life is to be estimated on the astronomical time-scale, no matter in what exact way, we see that astronomy is still at the very opening of its existence. This is why its message can claim no finality—we are not describing the mature convictions of a man, so much as the first impressions of a new-born babe which is just opening its eyes. Even so they are better than the idle introspective dreamings in which it indulged before it had learned to look around itself and away from itself.