The student should pick out upon the map those features which he has learned to know in the photograph (Fig. 55)--the Apennines, Copernicus, and the continuation of the Apennines, extending into the dark part of the moon. [Illustration: FIG. 57.--Mare Imbrium. Photographed by G. W. RITCHEY.] 102. SIZE OF THE LUNAR FEATURES.--We may measure distances here in the same way as upon a terrestrial map, remembering that near the edges the scale of the map is very much distorted parallel to the moon's diameter, and measurements must not be taken in this direction, but may be taken parallel to the edge. Measuring with a millimeter scale, we find on the map for the diameter of the crater Copernicus, 2.1 millimeters. To turn this into the diameter of the real Copernicus in miles, we measure upon the same map the diameter of the moon, 79.7 millimeters, and then have the proportion-- Diameter of Copernicus in miles : 2,163 :: 2.1 : 79.7, which when solved gives 57 miles. The real diameter of Copernicus is a trifle over 56 miles. At the eastern edge of the moon, opposite the Apennines, is a large oval spot called the Mare Crisium (Latin, _ma-re_ = sea). Measure its length. The large crater to the northwest of the Apennines is called Archimedes. Measure its diameter both in the map and in the photograph (Fig. 55), and see how the two results agree. The true diameter of this crater, east and west, is very approximately 50 miles. The great smooth surface to the west of Archimedes is the Mare Imbrium. Is it larger or smaller than Lake Superior? Fig. 57 is from a photograph of the Mare Imbrium, and the amount of detail here shown at the bottom of the sea is a sufficient indication that, in this case at least, the water has been drawn off, if indeed any was ever present. [Illustration: FIG. 58.--Mare Crisium. Lick Observatory photographs.] Fig. 58 is a representation of the Mare Crisium at a time when night was beginning to encroach upon its eastern border, and it serves well to show the rugged character of the ring-shaped wall which incloses this area. With these pictures of the smoother parts of the moon's surface we may compare Fig. 59, which shows a region near the north pole of the moon, and Fig. 60, giving an early morning view of Archimedes and the Apennines. Note how long and sharp are the shadows. [Illustration: FIG. 59.--Illustrating the rugged character of the moon's surface.--NASMYTH and CARPENTER.]