464 Occultations. 



These changes, it is true, are of no great extent, never exceed- 

 ing 7° 55' from B. to W., or 6° 47' from N. to S:,* and their 

 influence is not important upon the central parts of the disc ; 

 but they keep the limb in an unsettled state, and are continually- 

 altering the perspective of the parts adjacent to it, so that in 

 this situation a region will have an entirely different aspect at 

 the same age of the moon in different lunations. The libration 

 in latitude may also sometimes occasion a slight deviation in the 

 direction of the shadows, which may not be unimportant in the 

 case of objects lying E. and W., and assuming an altered 

 appearance as the shadow falls on the N. or S. side. The 

 larger features, indeed, are liable to no misapprehension, keep- 

 ing, under all circumstances, their own determinate character ; 

 but as to minuter details, it would hardly be supposed, except 

 from actual experience, how great an amount of apparent change 

 may sometimes depend upon very small deviations in the angles 

 of incident and reflected light. The landscape artist, however, 

 who has dehneated the same subject repeatedly from slightly 

 altered points of view, and in the varying light of successive 

 hours, will require no further or closer illustration of these 

 transformations. Such are the difficulties which beset the study 

 of the moon, though not without some degree of compensation, 

 since, if these discrepancies at one time perplex us, at another 

 time they unravel perplexities ; and the play of light and shade 

 which may occasionally bring a familiar object before us in " a 

 questionable shape," will now and then clear up, beyond a 

 question, the nature of one previously doubtful. 



The subject will be resumed in a future number. A pres- 

 sure on our space compels the postponement of our list of 

 "Double Stars." 



OCCULTATIONS. 



There will be only four occultations during this month at 

 convenient hours. Jan. 1st., k 1 Tauri, h\ mag. (one of the 

 components of a wide double star) will disappear at lOh. 

 3m., and reappear at llh. 8m. — Jan. 9th, 55 Leonis, 6 mag., 

 occulted when the moon rises, will emerge at lOh. 12m. — Jan. 

 26th, 27 Arieiis, 6 mag., will be hidden from llh. 31m. till 

 12h. 8m. — Jan. 27th, 8 Arietis, 4^ mag., will disappear at 

 5h. 10m., and reappear at 6h. 28m. 



* These quantities are somewhat variously given ; the value here adopted is 

 from Beer and Madler. 



There is a third kind of libration called tbe parallactic, arising from the diffe- 

 rent position of the point of view upon the surface of the earth, and varying with 

 the moon's altitude above the horizon, but it never exceeds 1° 1' 30". 



