And yet we must believe that change does take place on the Moon even now, because during the 354 hours of its long day the Sun beats down with full force on the unprotected surface, and during the equally long night that surface is exposed to the cold of outer space. Every part of the surface must be exposed in turn to an extreme range of temperature, and must be cracked, torn, and riven by alternate expansion and contraction. Apart from this slow, wearing process, and a very few rather doubtful cases in which a minute alteration of some surface detail has been suspected, our sister planet, the Moon, shows herself as changeless and inert, without any appreciable trace of air or water or any sign {62} of life--a dead world, with all its changes and activities in the past. MARS, after the Moon, is the planet whose surface we can study to best advantage. Its orbit lies outside that of the Earth, so that when it is nearest to us it turns the same side to both the Sun and Earth, and we see it fully illuminated. Mercury and Venus, on the contrary, when nearest us are between us and the Sun, and turn their dark sides to us. When fully illuminated they are at their greatest distance, and appear very small, and, being near the Sun, are observed with difficulty. These three are intermediate in size between the Moon and the Earth. In early telescopic days it was seen that Mars was an orange-coloured globe with certain dusky markings upon it, and that these markings slowly changed their place--that, in short, it was a world rotating upon its axis, and in a period not very different from that of the Earth. The rotation period of Mars has indeed been fixed to the one-hundredth part of a second of time; it is 24 h. 37 m. 22.67 s. And this has been possible because some of the dusky spots observed in the seventeenth century can be identified now in the twentieth. Some of the markings on Mars, like our own continents and seas, and like the craters on the Moon, are permanent features; and many charts of the planet have been constructed.