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.
[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.
103. The moon's atmosphere.—Upon the earth the sun casts no shadows so sharp and black as those of [Fig. 60], because his rays are here scattered and reflected in all directions by the dust and vapors of the atmosphere ([§ 51]), so that the place from which direct sunlight is cut off is at least partially illumined by this reflected light. The shadows of [Fig. 60] show that upon the moon it must be otherwise, and suggest that if the moon has any atmosphere whatever, its density must be utterly insignificant in comparison with that of the earth. In its motion around the earth the moon frequently eclipses stars (occults is the technical word), and if the moon had an atmosphere such as is shown in [Fig. 61], the light from the star A must shine through this atmosphere just before the moon's advancing body cuts it off, and it must be refracted by the atmosphere so that the star would appear in a slightly different direction (nearer to B) than before. The earth's atmosphere refracts the starlight under such circumstances by more than a degree, but no one has been able to find in the case of the moon any effect of this kind amounting to even a fraction of a second of arc. While this hardly justifies the statement sometimes made that the moon has no atmosphere, we shall be entirely safe in saying that if it has one at all its density is less than a thousandth part of that of the earth's atmosphere. Quite in keeping with this absence of an atmosphere is the fact that clouds never float over the surface of the moon. Its features always stand out hard and clear, without any of that haze and softness of outline which our atmosphere introduces into all terrestrial landscapes.