“This leads us next to inquire, ‘What is the actual size of the moon?’ When we know the distance of any body from the eye it is not difficult to determine its size. The diameter of the moon is 2,163 miles. The face of the full moon contains 7,300,000 square miles. It is a little larger than the continent of South America. For a reason that we will speak of presently, the moon always keeps the same side toward us no matter in what part of its orbit it may be. Consequently we always see the same features of her surface and, except through inference, we do not know what exists on the other side of the lunar globe. Of the 7,300,000 square miles of surface which the moon presents to us, about 2,900,000 are occupied by those dark gray patches which you see so plainly spotting her face, and which were once supposed to be seas. The remaining 4,400,000 square miles consist of a very rough, broken country, ridged with gigantic mountains and containing hundreds of enormous craters, and mountain-ringed valleys, which are so vast that one hesitates to call them, what many of them seem evidently to be, extinct volcanoes. A single explosion of a volcano of the dimensions of some of these lunar monsters would shake the whole earth to its center!”
“Please stop a moment,” my friend laughingly interrupted. “So many merciless facts, chasing one at the heels of another, are as bad as the books on your science that I have tried to read. Give my imagination time to overtake you.”
“Very well,” I said, “then relieve your attention a little while by regarding the face of the moon. Do you perceive the portrait of the Moon Maiden there?”
“I believe I do, although I never noticed it before. It is in profile, is it not?”
“Yes, and it occupies all the central portion of the western half of the disk. Take the opera glass and you will see it more clearly.”
“Really, I find her quite charming,” said my companion, after gazing for a minute through the glass. “But what a coquette! Look at the magnificent jewel she wears at her throat, and the parure of pearls that binds her hair!”
“Yes,” I replied, “and no terrestrial coquette ever wore gems so unpurchasable as those with which the Moon Maiden has decked herself. That flaming jewel on her breast is a volcano, with a crater more than fifty miles across! Tycho, astronomers call it. Observe with the glass how broad rays shoot out from it in all directions. They are among the greatest mysteries of lunar scenery. And the string of brilliants in her hair consists of a chain of mountains greater than the Alps—the lunar Apennines. They extend more than 450 miles, and have peaks 20,000 feet high, which gleam like polished facets.”
“Truly,” said my companion, smiling, “these gigantesque facts of yours rather tend to dissipate the romantic impression that I had conceived of the Moon Maiden.”
“No doubt,” I replied. “It is only distance that lends her enchantment. But we must not disregard the facts. Her hair, you perceive, is formed by some of the vast gray plains of which I spoke a few minutes ago. She is like a face in the clouds—approach her, or change the point of view and she disappears or dissolves into something else.
“Now, to return to my preliminaries, upon which I must insist. Knowing the distance and the size of the moon, the next question relates to her motions. You are aware that she travels around the earth about once every month. There are two ways in which we measure the length of time that the moon takes for each revolution. First, regarding the face of the sky as a great dial, with the stars for marks upon it, we notice the time that elapses between two successive conjunctions of the moon with the same star. In the interval she has gone completely around the earth and come back to the starting point. This is called the moon’s sidereal revolution, and it occupies, on the average, twenty-seven days, seven hours, forty-three minutes, twelve seconds. Every twenty-four hours the moon advances among the stars, from west to east, about 13° 11´.