198 Lunar Delineation. 



nated and viewed under precisely the same angle at the 

 corresponding hour in every successive lunation ; and in that 

 case any apparent would certainly indicate a corresponding 

 physical alteration. But since none of these three conditions of 

 identity are fulfilled, variations of aspect, especially among the 

 minuter details, are of continual occurrence. And while these 

 doubtless enable us to become better acquainted with the 

 character of some objects, as a countenance is better under- 

 stood under slight differences of position, than when gazed at 

 in the unvaried fixity of a portrait, still, all such accidental 

 causes of variation have to be allowed for, and, as far as pos- 

 sible, eliminated, before we can pronounce upon the probability 

 of physical change. 



In order then to be able to compare with rigorous accuracy 

 any two drawings of the same lunar object, presenting general 

 similarity with difference in detail, it would be necessary to 

 ascertain by calculation the altitude and azimuth both of the 

 sun and of the eye of the observer, as viewed from the spot 

 in question, at the epoch of observation, and this would involve 

 a number of troublesome computations. But, fortunately, for 

 general purposes the end may be sufficiently attained, in most 

 cases, in a much simpler way. First, we have to consider the 

 conditions of illumination, as to vertical angle and lateral 

 direction. The vertical angle depends upon the distance from 

 the terminator, that distance being equal to the altitude of the 

 sun at the place. It will be sufficient therefore for our purpose 

 if we record the position of the terminator as regards some 

 conspicuous feature near the observed region, specifying for 

 instance that it bisects the ring of a known crater, or that one- 

 third, or three-quarters of the ring are enlightened, or that 

 its summit is just touched by the sun in the night-side, or that 

 it has advanced by its own diameter, or so many parts of its 

 own diameter, within the boundary line. This degree of 

 accuracy will in general be quite sufficient to ascertain the 

 vertical angle of illumination, or elevation Of the sun above the 

 horizon of the feature we are observing. But the lateral direc- 

 tion of the illumination has also to be attended to ; in other 

 words, the azimuth, as well as altitude of the sun, as viewed 

 from the lunar spot. For though it is the difference of seasons 

 upon the earth that causes the sun to have such widely different 

 bearings by compass at equal altitudes at different times of our 

 year ; and though, in a popular sense, it may be said that the 

 moon has no seasons, yet this is not strictly true. It would 

 bo so if she revolved in the plane of the ecliptic, on an axis 

 at right angles to that plane, for then jbhe sun would rise and 

 set on the same point of the lunar compass throughout the 

 lunar year. But her axis is not perpendicular to her orbit (nor 



