Neighbourhood of the Lunar Spot, Mare Crisium. 365 



amount of this libration in longitude may be 7° 55' each way. The 

 other kind of lib ration, that in latitude, is of a less simple nature, 

 and its effect is not so readily allowed for. It arises from the 

 fact, that the plane of the moon's equator is coincident neither 

 with that of the earth's orbit nor its own. Were all these 

 identical, the moon's equator would always pass as an imaginary 

 straight line across the centre of the disc, and the poles would 

 stand exactly in the limb. But, as the moon's orbit is inclined 

 to the ecliptic at a mean 5° 8' 49", the lunar globe is carried, 

 during each revolution, alternately above and below the level of 

 the eye ; and hence the equator is sensibly straight only when 

 the moon is in its node or passage across the ecliptic. At all 

 other times it is projected, either upwards or downwards, into 

 a narrow semi-ellipse of continually-varying dimensions cor- 

 responding with the moon's latitude, and each polar region in 

 turn comes more into sight, or passes away into the invisible 

 hemisphere. Such would have been the case from the inclination 

 of the orbit, even had the plane of the moon's equator been co- 

 incident with, or parallel to, that of the ecliptic ; but, in addition 

 to this, it is inclined to it at an angle of 1° 28' 47" ; so that 

 the whole change in projection, in a N. and S. direction, may 

 amount at a maximum 6° 47' on either side of the mean 

 position. This has, of course, the same effect as the libration 

 in longitude, in proportion to its amount, on the perspective 

 foreshortening of the surface, though not, like that, in an E. 

 and W., but in a N. and S. direction. On the contrary, it has 

 no influence on the actual length of the shadows ; though, from 

 its allowing us to see sometimes more, sometimes less, of the fore- 

 shortened interior of a crater or base of a mountain, there may be 

 a little change in their visible extent. It has, however, an effect 

 of some importance on the direction in which the shadows fall. 

 We have been referring only to the angle under which we 

 view the surface — in other words, the angle of reflected light ; 

 but the angle of incident light has also to be considered, since 

 it also varies, though to a less degree. The deviation of the 

 moon on either side of the ecliptic, is, in this case, less im- 

 portant, the 380-fold distance of the sun rendering its angular 

 amount there so much less than it subtends at the earth ; but 

 the inclination of the lunar equator to the ecliptic, as seen from 

 the sun, produces the full effect of its angular value ; and the 

 moon's axis being tilted at one time towards, at another from 

 the sun, the direction of the incident light will be varied in the 

 same way, though by no means in so great a degree, as in the 

 different seasons upon the earth ; the sun will not rise and set 

 at precisely the same points of the lunar horizon, or attain ex- 

 actly the same meridian altitude ; and hence, the appearance of 

 objects may be much varied, especially of such as lie nearly in 



