The Lunar Eratosthenes and Copernicus. 275 



may be expected among such irregular surfaces, an interior 

 depression of about 8000ft. This is something entirely unpa- 

 ralleled on the earth, and, at first sight, in connection with the 

 vast extent and height of the ring, would seem to remove the 

 phenomenon out of the reach of terrestrial analogy ; when, 

 however, the very different amount of gravity on the Moon, 

 and possibly a very different degree of resistance from cohe- 

 sion, are taken into consideration, it does not seem necessary 

 to abandon the idea of volcanic action. 



On one occasion (1789, Sept. 12), when Eratosthenes was a 

 little less than its own diameter removed from the terminator, 

 Schr. observed that a zone forming the extremity of the shadow 

 which then nearly filled the cavity, being the portion which lay 

 on the interior slope of the ring, and, according to his figure, 

 amounting to nearly i of its whole extent, was noticeably less 

 dark than the rest. This he supposed to be the effect of the 

 commingling of the true shadow with the penumbra, or partial 

 darkness, which arises from the apparent breadth of the Sun, 

 and, therefore, borders every shadow in the solar system, where 

 the Sun subtends a sufficient angle to make it perceptible. 

 This is, of course, the reason why the shadows of all objects in 

 terrestrial sunshine are ill-defined in proportion to their dis- 

 tance from the body which casts them ; and this hazy-looking 

 edge, which would be of great breadth on Mercury, where all 

 the shadows would be extremely woolly, and imperceptible on 

 Uranus, where they would be almost critically sharp, would, 

 on the Moon, be sensibly equal to what we see on the Earth. 

 But it is most improbable that such could have been the cause 

 of what Schr. observed. The lunar penumbra is, indeed, ren- 

 dered visible as a narrow border* of diminished brightness 

 where the terminator passes through surfaces making a very 

 small angle with the rays of the rising or setting sun, while 

 they are fully exposed to the direction of our sight ; such as 

 the grey levels of the M. Serenitatis, Imbrium, Vaporum, and 

 others ; or flat-topped elevations of any height ; but where the 

 surface is inclined towards the incident ray, as is evidently the 

 case of the inner slope of a crater-ring after sunrise or (as in 

 this case) before sunset, the penumbra could not possibly attain 

 the projected magnitude described by Schr.f — Schmidt (who 



* Schmidt gives its theoretical breadth 8" on the terminator ; but practically 

 it will be much less, as the diminished illumination would not be perceptible till a 

 considerable portion of the solar disk was concealed. 



f On a former occasion the same observer has recorded a somewhat similar 

 but much fainter grey border along the edge of the shadow, then become very 

 narrow, within the crater Eudoxus (17). But in that case, being on the side of the 

 cavity next the Sun, instead of the opposite, the appearance might be readily 

 explained as a true penumbra, or illumination by a portion only of the solar disk, 

 apparently enlarged and rendered more visible by its falling on the gentle slope of 

 the foot of the wall. 



