The total area of the moon's surface is about equal to that of North and South America. The actual surface visible at any one time is about equal to North America.
The famous lunar observer, Schröter, thought that the moon had an atmosphere, but estimated its height at only a little over a mile. Its density he supposed to be less than that of the vacuum in an air-pump. Recent investigations, however, seem to show that owing to its small mass and attractive force the moon could not retain an atmosphere like that of the earth.
Prof. N. S. Shaler, of Harvard (U.S.A.), finds from a study of the moon (from a geological point of view) with the 15-inch refractor of the Harvard Observatory, that our satellite has no atmosphere nor any form of organic life, and he believes that its surface "was brought to its present condition before the earth had even a solid crust."[80]
There is a curious illusion with reference to the moon's apparent diameter referred to by Proctor.[81] If, when the moon is absent in the winter months, we ask a person whether the moon's diameter is greater or less than the distance between the stars [Greek: d] and [Greek: e], and [Greek: e] and [Greek: z] Orionis, the three well-known stars in the "belt of Orion," the answer will probably be that the moon's apparent diameter is about equal to each of these distances. But in reality the apparent distance between [Greek: d] and [Greek: e] Orionis (or between [Greek: e] and [Greek: z], which is about the same) is more than double the moon's apparent diameter. This seems at first sight a startling statement; but its truth is, of course, beyond all doubt and is not open to argument. Proctor points out that if a person estimates the moon as a foot in diameter, as its apparent diameter is about half a degree, this would imply that the observer estimates the circumference of the star sphere as about 720 feet (360° × 2), and hence the radius (or the moon's distance from the earth) about 115 feet. But in reality all such estimates have no scientific (that is, accurate) meaning. Some of the ancients, such as Aristotle, Cicero, and Heraclitus, seem to have estimated the moon's apparent diameter at about a foot.[82] This shows that even great minds may make serious mistakes.