ON MAPPING THE SURFACE OF THE MOON. 243 



20. Wot visible. There is, ho-wever, some indication of depression or 

 excavation on the interior slope of IVA?^, just south-west of the 

 promontory stretching towards the crater-row 13-19. 



In this manner every object may be sought for, and either identified, or if 

 it be beyond the power of the telescope to show it, recorded as such, and so 

 far as the pair of subzones extends, the capability of the telescope to reach 

 only certain objects determined. These may be arranged — for the ielescope 

 employed — in classes of conspicuous, easy, difficult, veiy difficult, and invi- 

 sible; and in thus becoming fully acquainted with the objects in the paii" of 

 subzones he has selected, tlie observer will find that he has employed his 

 time to great advantage ; indeed he will be surprised that, by regularly exa- 

 mining his portion to identify the objects already on record, he has become 

 thoroughly acquainted with all the prominent features of the region. 



In work of this kind the observer will find it very advantageous to exa- 

 mine his subzones under similar conditions of illumination, which recur 

 roughly in periods of fifty-nine days, but more acciu'atcly in fifty-nine days 

 eighty-eight minutes; and these periods -uill afford an interval of about eight 

 months, during which, in eveiy alternate month, should the sky be clear, he 

 will see the objects similarly illuminated (very nearly). The numbers in the 

 sixth column, headed " days elapsed of the Julian Period," on p. xx of each 

 month in the Nautical Almanac, wUl facilitate the computation of these 

 periods. Thus, if on January 13, 1867, at 6 p.m., he notices the Terminator 

 or Light boundary to pass over any particular spot, 5Q'^ V" 30™ afterwards 

 the Terminator wiU very nearly be on the same spot. Now the day elapsed 

 of the Julian Period (written short D. J. P.) on 



January 13, 1867, is 2402980 6 



Add ^. . 59 1 30 



March 13, 1867 2403039 7 30 



Upon referring to the column for March it will be found that on the 13th, 

 at 7'' 30" P.M., the same phase will recur ; and in a similar manner it may 

 readily be found that on May 11th, 9'' 0", July 9th, 10'^ 30", and September 6th, 

 12'' 0", the Terminator -will be nearly in the same part of the disk, so that on 

 those evenings the observer can resume his examination or other kind of 

 observations under nearly similar cii'cumstances. 



Corrections. — ^When he has become familiar with his ground, the ob- 

 server may proceed to examine the map and catalogue critically. For ex- 

 ample, he may carefully ascertain if the relative position of the objects on 

 the portion of the map furnished him be correct, and whether the alignments 

 generally agree ; for this purpose, a pair of cross wires in the field of the eye- 

 piece, by which he can readily determine objects in tlie same line, wiU be 

 found useful. It is important to notice particularly the outlines of objects, 

 as should he be satisfied after observing them under differing circiunstances 

 of light and libration * that correction is needed, such correction should be 

 made on the map either in ink or a different colour from that of the map. 



New Objects. — As the Terminator passes over the portions of the sub- 



* Libration, in a popular sense, signifies the displacement of an object as regards its 

 mean position on the moon's surface, in which it can be observed only once in three years. 

 When the moon has north latitude, all objects appear north of their mean places, and 

 ■when she has south latitude south of them. When the moon is passing from Apogee 

 to Perigee they are west, and when she is passing from Perigee to Apogee they arc east 

 of their mean positions. 



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