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The Plinius rills form an especially interesting system, and under favourable conditions may be seen in their entirety with a good 4 inch refractor, about the time when the morning terminator passes through Julius Caesar.
A fine rill-valley curves round the outer slope of the W. wall, just below its crest, which is an easy object in a 8 1/2 inch reflector when the opposite border is on the morning terminator, and could doubtless be seen in a smaller instrument; and there is an especially brilliant crater on the S. border, which is not visible till a somewhat later stage of sunrise.
The notable crater-row on the W., often utilised by observers for testing the steadiness of the air and the definition of their telescopes, should be examined when it is on the morning terminator, at which time Webb's homely comparison, "a mole-run with holes in it," will be appreciated, and its evident connection with the E. side of Stadius clearly made out.
The great Alpine valley, though first described by Schroter, is said to have been discovered on September 22, 1727, by Bianchini, but it is very unlikely that an object which is so prominent when near the terminator was not often remarked before this.
It should be observed about the time when the morning terminator lies on the W. border of the Mare Humorum, as at this phase the best view is obtained of the two deep parallel terrace valleys which run round the bright inner slope of the E. wall, of the crater-row against which they abut on the S.E., and of the massive W. glacis, with its spurs and depressions.
His steady watch over them showed the invariability of their position with regard to the terminator; and this is as much as to say that the regions of day and night do not shift on the surface of the planet. In other words, she keeps the same face always turned towards the sun. Moreover, since her orbit is nearly circular, libratory effects are very small.
When near the morning terminator, the region on the N. is seen to be traversed by some very remarkable ridges and markings; one cutting across the N. wall appears to represent a "fault." On the S. is a large polygonal enclosure formed by low ridges. On the W., towards Timocharis, is a brilliant mountain 3000 feet high, a beautiful little object under a low sun.
No map shows this cleft, though it is obvious enough; and, when the E. wall of Hevel is on the morning terminator, the notches made by it in the border of Lohrmann are easily detected. Capt. Noble, F.R.A.S., aptly compares two of the crossed clefts to a pair of scissors, the craters at which they terminate representing the oval handles.
Though almost every trace of Stadius disappears under a high light, I have had little difficulty in seeing portions of the border and some of the included details when the morning terminator had advanced as far as the E. wall of Herodotus, and the site was traversed by innumerable light streaks radiating from Copernicus.
As a general rule, lunar objects are best seen when they are at no great distance from "the terminator," or the line dividing the illumined from the unillumined portion of the spherical surface.
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