ON MAPPING THE SURFACE Ol'' THE MOON. 9 



measuring the distance from the centre, which in the ease of lannc is erinal 

 to 29° 5-i' 40", therefore GllGwx secant 29° 54' 40' = 7056-6 English feet, 

 which subtend an angle of l"-0 at the mean position of Linne; hut as this 

 mean jjositiou maj' be either at perigee or apogee, ■where the value of l"-0 

 may be greater or less, the above is the value at nicaii distance, which is 

 never contemporaneous with mean libration. See lieport, 1S6G, p. 245. 



The apparent sizes of objects on the moon's disk are affected both by dis- 

 tance and libration. The former presents them alternately to the eye under 

 larger and smaller angles, according as the moon is nearer to or further from 

 us. The latter alters their positions on the disk, sometimes bringing them 

 nearer to the apparent centre, at others removing them to a greater distance 

 from it. Approximately, distance may aficct the measurements of objects to 

 an extent of about a 9th part of the greatest measures at the epoch of mean 

 libration ; for as mean libration may occur when the moon is in perigee, 

 a measure taken at the time of apogee, when the moon is in a state of mean 

 hbration, will be less by about the 9th part of a measure taken at perigee 

 mean libration. 



Libration atfects the measures of objects by presenting them imder larger 

 or smaller angles, according as they are nearer to or further from the centre 

 of the apparent disk ; thus an object of 6116-7 English feet in diameter, 

 occupying the centre of the disk nt mean distance, would subtend an angle 

 of l"-0. At mean Libration, moon in Perigee, an object of the same ex- 

 tent would subtend an angle of l"-059 + ; moon in Apogee 0"-941. In the 

 first case a similar object at an angular distance of 19° 54' 40" would appear 

 foreshortened in a radial direction, the longer axis at right angles to a radius 

 measuring l"-059, the shorter axis 0"-996 ; the shorter axis of an object of 

 the same diameter at a chstance of 29° 54' 40" on the same radius would 

 measure only 0"-918 ; the difference, 0"-07S, is the change of angle arising 

 from the displacement of such an object by libration (about the epoch of 

 Perigee) equal to an arc on a radius of the moon's apparent surface of 10°, i. e. 

 between 20° and 30° ; on the oi)posite side of the orbit it is less. 



Table I. 



Estimations and measures of the extent of the white spot on the Crater 

 Linne reduced to 29° 54' 40" = angular distance from the moon's centre. 



Authoritv. 



Schmidt 



Birt 



Bii-t 



Birt 



Bii-t 



Schmidt 



Birt 



Buckingham 



Wolf 



Birt 



Hugging . . . 

 Huggins . . . 



Birt 



Birt 



Birt 



Ejioch. 



Date. 



Seconds. 



i866 

 i866 

 i866 

 1866 

 iS66 

 i866 

 1S67 

 1867 

 186- 

 1867 

 1867 

 1867 

 1867 

 1867 

 1867- 



•794- 

 ■953 

 •g6l 

 964 

 ■969 

 986 

 036 

 •197 



443 

 515 

 S18 

 518 

 S18 

 520 

 528 



1866, Oct. 18 

 1866, Dec. 15 



1566, „ 18 

 1866, „ 19 



1866, ,, 2T 



1866, „ 27 



1567, Jan. 14 

 1S67, Mar. 14 

 1S67, June 12 



1867, July 8 

 1S67, „' 9 

 1867, ,, 9 

 1867, „ 9 

 1867, ,, 10 

 1867, „ 13 



6-90 

 ii-6i* 



7'07* 

 7-32* 



675* 

 i-8i 



7-95* 

 6 -co* 

 4-50 



5"33* 

 7-85* 

 6-14* 

 7'oo* 

 5-36* 

 6-31* 



Eng. feet. 



Fr. metres. 



4868S 

 81932 

 49871 

 51652 

 47644 

 127S9 

 56105 

 42340 



317SS 

 37626 

 55423t 



43314$ 

 49426 



37848 

 44528 



14840 

 24972 

 15201 



15744 

 14522 



3898 

 17100 

 12905 



9679 

 11468 

 16893 

 13202 

 15065 

 11536 

 13572 



