224 The Lunar Eclipse of September 13. 



at 9.43; for at that time the Moon entered the penumbra of the 

 earth's shadow ; yet, three-quarters of an hour after this, no 

 diminution of luminosity in the Moon could be detected. 



At length, at about lOh. 30m., a slight shade was seen 

 stealing gently over the Moon's disk, commencing at the limb 

 in the N.E. quadrant. This shading steadily increased in 

 intensity until eleven o'clock. At this time a darkening of the 

 limb in the N.E. became distinctly perceptible. Three minutes 

 earlier the Moon had entered the umbra of the Earth's shadow ; 

 still, though I was watching most anxiously at the telescope, I 

 could not detect the exact instant at which this, the second 

 stage of the eclipse, began. 



The darkness kept creeping over the disk, veiling first one 

 and then another well-known object, until it had covered about 

 one-third of the Moon's surface. When it had reached thus 

 far, many of the craters on the dark limb became distinctly 

 visible. I noticed also that some of the ray-streaks projected 

 within the shadow. At the maximum of eclipse, which occurred 

 at 12.26, three-quarters of the disk seemed darkened; the 

 exact amount was 0.693, the whole disk being 1. 



The full disk of the Moon could at all times be dimly made 

 out, the edge of its disk, even the portion most deeply 

 immersed in the shadow, being much brighter than the other 

 portions in shadow, and being, in fact, encircled by a narrow 

 line of light. An observation in close relation to this was made 

 during a late solar eclipse by a very careful observer, Captain 

 Noble. This gentleman noticed that the limb of the Moon, 

 though not the body, was visible, extending some distance 

 beyond the solar disk. "What is the meaning of this peculiar 

 appearance ? It seems the very reverse of what we might 

 expect to obtain. At the time of an eclipse the Moon is in 

 opposition, and it is, of course, at the full. Under these 

 circumstances we might expect that the centre of the disk 

 would be the most luminous, and that there would be a degra- 

 dation of light towards the edges.* 



The planets Mars, Saturn, and Jupiter also exhibit an 

 increased luminosity at the edges of their disks. This is 

 accounted for by supposing that they are surrounded by 

 clouds. The whole matter would be explained if we could 

 suppose that any vapour exists in the atmosphere of the Moon. 

 Everything, except the appearance I have been describing, 

 points to the contrary conclusion. 



A peculiarity of the light reflected from the Moon's surface 



* This singular action of the edge of the disk nppears to extend to the actinic 

 rajs. In one of Mr. "Warren De La Kue's exquisite photographs of the full Moon, 

 I noticed that some of the grey plains were continued to the edge of the disk. 

 Frequently they almost touched the edge, but the extreme edge was alwavs 

 marked by a narrow line of light. 



