The admission of the possibility of change does not, however, imply anything like fitness for human habitation. The moon, to use Beer and Mädler's oft-quoted phrase, is 'no copy of the earth'; and the conditions of her surface differ widely from anything that we are acquainted with. The extreme rarity of her atmosphere must render her, were other conditions equally favourable, an ideal situation for an observatory. From her surface the stars, which are hidden from us in the daytime by the diffused light in our air, would be visible at broad noonday; while multitudes of the smaller magnitudes which here require telescopic power would there be plain to the unaided eye. The lunar night would be lit by our own earth, a gigantic moon, presenting a surface more than thirteen times as large as that which the full moon offers us, and hanging almost stationary in the heavens, while exhibiting all the effects of rapid rotation upon its own axis. Those appendages of the sun, which only the spectroscope or the fleeting total eclipse can reveal to us, the corona, the chromosphere, and the prominences, would there be constantly visible. Our astronomers who are painfully wrestling with atmospheric disturbance, and are gradually being driven from the plains to the summits of higher and higher hills in search of suitable sites for the giant telescopes of to-day, may well long for a world where atmospheric disturbance must be unknown, or at least a negligible quantity. [Footnote *: See drawings by Colonel Markwick with 2-3/4-inch refractor, of the eclipse of August 30, 1905, 'The Total Solar Eclipse, 1905,' British Astronomical Association, pp. 59, 60.] CHAPTER VIII MARS