_Only one Hemisphere visible._—In discussing the nature and appearances of the lunar formations, it must be distinctly understood that our remarks apply to those visible on the side invariably turned towards the Earth. For, in point of fact, there is a considerable expanse of the lunar disk never perceptible from the Earth at all. This is occasioned by the circumstance that the Moon rotates upon her axis in precisely the same time as she revolves around the Earth, and is therefore enabled to present the same side towards us on all occasions. A slight tilting (called libration) takes place, so that we are allowed a glimpse of fragments of the side normally invisible, and its analogous aspect leads us to suppose that there is no great distinction between the features of the Moon’s visible and invisible hemispheres. From exact computations it appears that we are enabled to see a proportion of 59/100 of the surface, and that the remaining 41/100 are permanently beyond our reach.
_Earthshine._—A few mornings before new moon, and on a few evenings after it, the whole outline of the dark portion of the lunar globe may be distinctly perceived. A feeble illumination like twilight pervades the opaque part, and this is really earthlight thrown upon our satellite, for near the times of new moon the Earth appears at her brightest (her disk being fully illuminated) as seen from the Moon. The French term for this light is _la lumière cendrée_, or “the ashy light.” The appearance is often popularly referred to in our own country as “the old Moon in the new Moon’s arms.” Some of the old observers remarked that the waning Moon showed this earthlight more strongly than the new Moon.
_Telescopic Observations of the Lunar Surface._—Our telescopes give by far the most pleasing view of the Moon when she is in a crescent shape. At such a period the craters and mountains, with their dark shadows, are splendidly displayed. A good view is also obtainable with the Moon at first or last quarter, or when the disk is gibbous. But the full Moon is decidedly less attractive; for the shadows have all disappeared, and the various formations have quite lost their distinctive character. The disk is enveloped in a flood of light, causing glare, and though there is a large amount of detail, including systems of bright rays, many differences of tint, and bright spots, yet the effect is altogether less satisfactory than at the time of a crescent phase.