Lunar Delineation. 199 



to tlie ecliptic), being inclined to it at an angle of 1° 28' 47".* 

 Hence results a continuous and systematic change, analogous 

 to that of the terrestrial seasons, only as inferior in extent as 

 the angle 1° 28' 47" is to 23° 27V (the inclination of the earth's 

 axis). And consequently each pole of the moon is in light 

 or darkness by turns ; and the horns are constantly approach- 

 ing, coinciding with, or departing from, the polar points; and 

 the " line of the horns/' that is, the chord of the whole termi- 

 nator, or the terminator itself when it is a straight line, seldom 

 coincides with a lunar meridian, but intersects it at the equator 

 in some small and constantly varying angle. The result of these 

 changes is that, instead of rising always due E., the sun, as 

 viewed from the moon, may sometimes rise about 1|°, or three 

 times its own apparent breadth, N., sometimes as far S. of that 

 point; and as, under such circumstances, a house standing 

 due E. and W. on the earth would see the sunrise sometimes 

 from its N., at others from its S. windows, so a line of cliffs 

 running due E. and W. on the moon would sometimes be 

 bright in the early morning, sometimes all in black shadow. To 

 eliminate such apparent discrepancies, we must obtain not 

 merely, as already specified, the distance from the terminator, 

 that is, the sun's altitude, but also the direction of the termi- 

 nator itself, which is equivalent to his azimuth. And as the 

 former datum is obtained by the position of the terminator as 

 regards some known spot as near as may be to the region we 

 are studying, so we get the latter by noting its position in the 

 same w T ay with regard to two other spots, lying as far N. and 

 S. as we can conveniently find them, from the more central 

 one already referred to. The distance and direction of the 

 terminator being thus ascertained, we can compare the angles 

 of illumination at different epochs with sufficient accuracy for 

 our present purpose. 



The varying direction of vision depends upon other con- 

 siderations, and two other distinct data are required — libration 

 in longitude, and libration in latitude. The effect of the 

 former at its maximum would be the same as if the eye of the 

 observer were shifted to points alternately 7° 55' E. or W. of 

 its normal position ; the latter at its maximum would in like 

 manner transpose it alternately 6° 47' N. or S. These two 

 changes, involving, at least, the possibility of much diversity of 

 aspect, are combined in every proportion and degree ; but 

 fortunately for amateur selenographers, their adequate expres- 



* The inclination of the moon's orbit, and its continually varying direction 

 from the motion of the nodes, have not been noticed here, because, though very 

 conspicuous to u.s, they subtend so extremely small an angle, not exceeding 50", 

 when viewed from the distance of the sun, as to exercise no perceptible influence 

 on the direction of the incident light. 



