Some years ago a Belgian astronomer, M. Stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very conditions just detailed. To various portions of the inner surface of a large dome he attached pairs of electric lights; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down! He does not, however, seem to have sought for the cause in the vaulted expanse. On the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us estimate objects as larger when in front than when overhead.
In connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. Even skilled observers find themselves in error in attempting to do so. This seems to bear out the writer's contention that the form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments.
Before leaving this question, it should also be mentioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. The full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc.
Plate VIII. The Moon
From a photograph taken at the Paris Observatory by M.P. Puiseux.
([Page 197])
The moon is just too far off to allow us to see the actual detail on her surface with the naked eye. When thus viewed she merely displays a patchy appearance,[15] and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term "Man in the Moon." An examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye.
Even with an ordinary opera-glass, an observer will be able to note a good deal of detail upon the lunar disc. If it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "Cain and his Dog," the "Man in the Moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations ([see Plate VIII.], p. 196). The dark areas retain even to the present day their ancient name of "seas," for Galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of Dreams."[16] The improved telescopes of later times showed, however, that they were not really seas (there is no water on the moon), but merely areas of darker material.
The crater-like formations above alluded to are the "lunar mountains." A person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. But once he has realised this novel point of view, he will no doubt marvel at the formations which lie scattered as it were at his feet. The type of lunar mountain is indeed in striking contrast to the terrestrial type. On our earth the range-formation is supreme; on the moon the crater-formation is the rule, and is so-called from analogy to our volcanoes. A typical lunar crater may be described as a circular wall, enclosing a central plain, or "floor," which is often much depressed below the level of the surface outside. These so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic proportions. For instance, the central plain of one of them, known as Ptolemæus,[17] is about 115 miles across, while that of Plato is about 60. The walls of craters often rise to great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. Nevertheless, a person posted at the centre of one of the larger craters might be surprised to find that he could not see the encompassing crater-walls, which would in every direction be below his horizon. This would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth.