The Moon. 463 



see in terrestrial effects those sharp and vehement contrasts 

 of light and shade which strike us so much in the moon. The 

 presence of a highly reflective atmosphere, even when unvaried 

 by clouds, which diffuse much light, modifies the depth of our 

 shadows, in proportion as it illuminates and colours what would 

 otherwise be the blackness of our sky. On the moon that 

 vaporous envelope is, if not altogether wanting, far too rare to 

 produce any such result. 



Here much general light is interwoven with our darkness, 

 and the direct sunshine is not the only medium of illumination ; 

 air-tints and reflections soften all our shadows by day, and 

 twilight encroaches far upon the regions of night : there (save 

 only when the keen eye of Dawes, sheltered by his own con- 

 tracted eye-piece, detects in the black interior of some colossal 

 crater a feeble reverberation from its illuminated wall) every- 

 thing not in direct sunshine is involved in absolute midnight. 

 Half-tones, the cause of so much beauty upon the earth, do not 

 exist upon the moon, excepting where a narrow dusky fringe 

 along the terminator, or the fainter beam that first breaks in 

 upon the floor of a crater, or touches some mountain's peak 

 amidst the darkness of the night-side, shows that a portion of 

 the solar disc alone is visible. But the student who has mas- 

 tered these peculiar characteristics has still a task remaining — 

 to allow for the difference of aspect introduced by what is called 

 " libration.'" Were the moon to travel round us with a perfectly 

 equable velocity, in an orbit coincident with the ecliptic, re- 

 volving at the same time on an axis perpendicular to its orbit, 

 we should always see exactly the same hemisphere, 'with, an 

 invariable arrangement of the spots relatively to the centre and 

 the edges of the disc ; and precisely the same effects of light 

 and shade would recur at corresponding periods in every luna- 

 tion. But none of these conditions are fulfilled. The moon's 

 orbit is an ellipse, a form necessarily involving inequality of 

 speed in its different parts ; and since her rotation upon her 

 axis is perfectly equable, this alternate acceleration and retar- 

 dation in her orbit, urging her beyond, or keeping her behind, 

 the place in the sky which she would have occupied with an 

 equable motion, has the effect of making her spots appear to 

 " librate," or swing to and fro, alternately in advance of and 

 behind their mean position. This motion from E. to W., and 

 from W. to E., is called the " libration in longitude." The 

 inclination of the moon's orbit to the ecliptic, and of her axis 

 to her orbit, combine to produce another kind of libration, "in 

 latitude," which shifts the spots in a similar way upwards and 

 downwards; and as in the previous case we see alternately a 

 little way round the mean E. or W. limb, so in tins, we catch, a 

 little more of the regions beyond the arctic or antarctic pole. 



