296 The Lunar Mare Serenitatis. 



are very low ; but close to the terminator, where the sun is 

 only 1° or 2° high, he found that they cast measurable shadows, 

 especially from their more rapid slopes. The faintness, how- 

 ever, of the terminator rendered it difficult to make them out 

 well ; even a keen eye requires practice to distinguish them ; 

 and years elapsed before he was able to measure the length of 

 the shadow, and the distance of the ridge, with complete 

 certainty. The effect of " penumbra "* is sensibly felt here, and 

 should be adverted to by the student. . It is evident that the 

 true terminator, or circle, dividing the lunar globe into two 

 equal hemispheres, must be found where the centre of the sun 

 is in the lunar horizon. But the upper hemisphere of the sun 

 casts a considerable amount of light beyond that line, gradually 

 decreasing till the whole solar globe is invisible, but, owing to 

 the variety or absence of a lunar atmosphere, retaining a per- 

 ceptible illuminating power to the last ; and hence the true 

 terminator is carried forward beyond the hemisphere by a 

 ".. penumbra," or border of about 8", of continually increasing 

 duskiness, which may be traced on level ground by an experi- 

 enced eye. A similar effect may sometimes be noticed in 

 softening the brightness of mountain tops at a distance beyond 

 the terminator, and the existence of this penumbra is occa- 

 sionally apparent in other situations, and under higher angles 

 of illumination. Most of the shadow on the moon is indeed 

 black midnight, from the absence of atmospheric diffusion ; but 

 a practised eye will occasionally detect an incomplete illumina- 

 tion, derived from the partial visibility of the solar globe. 

 In some instances no doubt a grey tint, as seen from the 

 distance of the earth, may result from the presence of innumer- 

 able and un distinguishable specks of black shadow thickly 

 scattered over a roughened surface ; but in other cases we are 

 able to trace the half-shadow, or rather half-light, resulting 

 from the effect of a rising or setting sun ; and Schr. thought 

 he could perceive a want of sharpness at the points of long 

 outstretched shadows arising from this cause. 



The ridges of the M. Serenitatis, as the illumination in- 

 creases, are converted into white streaks, and the other irre- 

 gularities of surface become luminous, so that on one occasion, 

 when he speaks of the magnificence of the scene, he could 



* This term, in its strict optical meaning, would signify the whole space 

 illuminated by anything less than the entire disc of the sun, instead of, as here, 

 the portion enlightened by his upper risible hemisphere, in advance of the true 

 terminator. The w7iole optical penumbra on the moon subtends about 16" at 

 the distance of the earth, but of course only its darker portion is perceptible, for 

 the same reason that in solar eclipses the diminution of light is not sensible till 

 a great part of the disc is obscured ; and the extent of this risible portion will 

 no doubt vary according to the power of the telescope and the sensitiveness of 

 the eye. 



