50 Lunar Atmosphere, 



feeble and obscure phenomenon, which we ourselves have never 

 succeeded in distinguishing from the effect of the earth's light 

 upon the moon, might easily, if it were more than a simple o]>- 

 tical illusion, be explained in many other ways than by having 

 recourse to a lunar atmosphere. The sun's diameter alone must 

 occasion a small twilight with a mean breadth on the moon of 

 2,29 miles, and geocentrically must subtend an angle of 9" ; 

 and which may, towards the margin, when the moon shews only 

 a very small crescent, extend to many minutes. The inequali- 

 ties of the surface, too, must occasion a somewhat more extend- 

 ed twilight ; the sides of illuminated mountains must reflect a 

 part of their light upon the valleys below, and we may, per- 

 haps, thus explain the phenomenon observed by Schroeter in the 

 moon''s southern horn, and which has hardly been noticed in any 

 other point. The mountain Doerfel,* which is probably the 

 most elevated in the moon, thus projects its light over a widely 

 extended space on the two halves of the moon's surface ; these 

 points may be considered as isolated islands of light, which rise 

 conspicuously above the extremity of the crescent, and in this 

 locality affect both the phases and the libration. 



The equal distinctness with which all parts of the moon are 

 perceived, those near its edges as much as those in the centre, 

 is another positive reason against the existence of any medium 

 of imperfect transparency on its surface : for all the outlines in 

 the different regions, when observed at the same time, and un- 

 der equivalent circumstances as to light, are distinguished alike 

 distinctly ; and towards the edges they do not present other 

 difficulties in their representation and delineation, than those 

 arising from their being fore- shortened. Attentive observations 

 of the spots of Mars and Jupiter, have led us to recognise, that, 

 towards the edges of these planets, every object becomes indeter- 

 minate, and disappears; we have ourselves experienced this, 

 ^nd in all probability it arises from the atmosphere of these 



■ This mountain is among the number of those which are situated towards 

 the limit of the visible part of the moon, and whose height cannot be determined 

 by the length of their shadows, but only by the approximative method of the 

 projection which their profile forms upon the margin of the disk. Two measure- 

 jnents of this kind gave MM. de Beer and Miidler 3800 toises as its height. 



