200 Lunar Delineation. 



sion is very simple and attainable. Libration in longitude 

 may be represented by the interval of time since, or until, the 

 nearest perigee or apogee (or both may be specified if each is 

 distant) : if these intervals do not greatly differ in the draw- 

 ings to be compared, the change in the direction of aspect E. 

 or W. can have no material effect. Libration in latitude is 

 correlative with the latitude of the moon at the epoch of obser- v 

 vation ; and if this nearly corresponds, so does the position of 

 the observer's eye as regards N. and S. We have only there- 

 fore to take from the Nautical, or Dietrichseiv 's Almanac, the 

 date of the nearest perigee or apogee, or both, and the value of 

 the latitude (which however ought to be reduced by a little 

 calculation to the hour of observation), and we have all the ma- 

 terials for judging whether the circumstances of vision are 

 sufficiently similar to admit of close comparison. And it will 

 now be apparent, that if the distance and direction of the ter- 

 minator, and the amount of each libration, are not materially 

 different at two different epochs, then, and then only, will the 

 corresponding drawings admit of such a rigorous comparison 

 as not only to establish general coincidence, but to check those 

 minute details to which astronomers are now beginning to 

 attend. 



It should, however, be borne in mind, that though the form, or 

 position, or reflective quality of certain objects may render them, 

 so to speak, peculiarly sensitive to slight changes in the angles 

 of illumination and reflection (which is of course only that of 

 vision reversed), yet in the majority of cases but little variation 

 ensues, and that of a readily intelligible nature : and it is 

 fortunate for selenography that it is so, since otherwise it would 

 present a scene of such continual unsettledness as to detail as to 

 be the source of endless perplexity and uncertainty; espe- 

 cially when we consider that an exact state of mean or balanced 

 libration returns only once in three years. Schroter pointed 

 out long ago that the ordinary changes of aspect thus arising 

 lie within narrow bounds ; and experience will soon convince 

 us that, though we ought to be on our guard, and more than 

 he may have sometimes been, against special cases of apparent 

 change, yet the general aspect of things is not subject to any 

 extensive variation, beyond that gradual transformation from 

 light and shade to local colouring which attends the progress 

 of the sun to the lunar meridian. As to the special cases just 

 referred to,many such exist; some already known to astronomers, 

 to which we shall call attention as they come before us ; many 

 more remaining yet to be studied with care, not only to acquire 

 a minuter knowledge of the lunar surface, but to be able to 

 pronounce more decidedly as to the truth or groundlessness of 

 Schroter' s idea, that many seeming variations indicate the 



