Here may be placed an important remark on the subject of the map which they used for their observations. In the selenographic maps, where, on account of the reversal of objects by the telescope, the south is at the top and the north at the bottom, it seems natural that the east should be on the left and the west on the right. However, it is not so. If the map were turned upside down, and showed the moon as she appears, the east would be left and the west right, the inverse of the terrestrial maps. The reason of this anomaly is the following:—Observers situated in the northern hemisphere—in Europe, for example—perceive the moon in the south from them. When they look at her they turn their backs to the north, the opposite position they take when looking at a terrestrial map. Their backs being turned to the north, they have the east to the left and the west to the right. For observers in the southern hemisphere—in Patagonia, for example—the west of the moon would be on their left and the east on their right, for the south would be behind them.
Such is the reason for the apparent reversal of these two cardinal points, and this must be remembered whilst following the observations of President Barbicane.
Helped by the Mappa Selenographica of Boeer and Moedler, the travellers could, without hesitating, survey that portion of the disc in the field of their telescopes.
"What are we looking at now?" asked Michel.
"At the northern portion of the Sea of Clouds," answered Barbicane. "We are too far off to make out its nature. Are those plains composed of dry sand, as the first astronomers believed? Or are they only immense forests, according to the opinion of Mr. Waren de la Rue, who grants a very low but very dense atmosphere to the moon? We shall find that out later on. We will affirm nothing till we are quite certain."
"This Sea of Clouds is rather doubtfully traced upon the maps. It is supposed that this vast plain is strewn with blocks of lava vomited by the neighbouring volcanoes on its right side, Ptolemy, Purbach, and Arzachel. The projectile was drawing sensibly nearer, and the summits which close in this sea on the north were distinctly visible. In front rose a mountain shining gloriously, the top of which seemed drowned in the solar rays."
"That mountain is—?" asked Michel.
"Copernicus," answered Barbicane.
"Let us have a look at Copernicus," said Michel.
This mountain, situated in north latitude 9°, and east longitude 20°, rises to a height of nearly 11,000 feet above the surface of the moon. It is quite visible from the earth, and astronomers can study it with ease, especially during the phase between the last quarter and the new moon, because then shadows are thrown lengthways from east to west, and allow the altitudes to be taken.