And there are many nations at the present hour who merely know the 2 face of the sky and do not yet understand why the moon is obscured in an eclipse. It is but recently indeed that science brought home to ourselves certain knowledge on the subject. The day will yet come when the progress of research through long ages will reveal to sight the mysteries of nature that are now concealed. A single lifetime, though it were wholly devoted to the study of the sky, does not suffice for the investigation of problems of such complexity. And then we never make a fair division of the few brief years of life as between study and vice. It must, therefore, require long successive ages to unfold all. The day will yet come when posterity will be amazed that we 3 remained ignorant of things that will to them seem so plain. The five planets are constantly thrusting themselves on our notice; they meet us in all the different quarters of the sky with a positive challenge to our curiosity. Yet it is but lately we have begun to understand their motions, to realise what their morning and evening settings mean, what their turnings when they move straight toward us, why they are driven back from us. We have learned but a few years ago whether Jupiter would rise or set, or whether he would retrograde--the term that has been applied to his retirement from us. People have been found bold enough 4 to say to us: You are mistaken in thinking that any star ever stops or wheels in its course. The heavenly bodies may not stand or turn away. All advance; once the signal is given they start on their race. Their career will end only with their existence. This eternal creation has motions that suffer no recall. Should they once be arrested, they will encounter obstacles in front which are meantime held in place by the ordered, regular march of the universe. XXVI What then is the reason, you may ask, for the apparent retrogression 1 of some heavenly bodies? The appearance of slowness in their motion is caused by their encountering the sun, as well as by the character of their paths and the position of their orbits, which are at certain periods calculated to deceive the eye. Ships in the same way moving under full sail seem withal to be stationary. Men will some day be able to demonstrate in what regions comets have their paths, why their course is so far removed from the other stars, what is their size and constitution. Let us be satisfied with what we have discovered, and leave a little truth for our descendants to find out.