OP ASPECT, ROTATION, AND LIBRATION. 153

hoar at which the moon rose on the preceding night(at least in our latitudes, and everywhere save in veryhigh latitudes), but the difference will be much greaterin one case than in the other.

Now these are the extreme cases : the ecliptic cannever cross the horizon at a greater angle than e B H',or at a less angle than e' B H'. Accordingly—stillassuming that the moon moves in the ecliptic—weshall have the greatest possible difference betweenthe hours of rising when the moon is on the eclipticplaced as at e B M, and the least possible differencewhen she is on the ecliptic placed as e'EM'; and ifthe moon is “ full ” or nearly so, when in one or otherof these positions, the peculiarity will be very note-worthy. In one case, we shall have a remarkableretardation in the hours of rising on successive days,and in the other as remarkably small a difference.Now the full moon is in or near the former position inspring, for then the new moon is, with the sun, at ornear the ascending node of the ecliptic, and thereforethe full moon at or near the descending node. Ac-cordingly in spring the difference between the hoursat which the full moon rises on successive nights isconsiderable. It amounts, in fact, on the average, inour latitudes to about an hour and twenty minutes,*

* There is a table in Ferguson’s Astronomy which seems toimply differently, since he gets 1 h. 16 m. as the greatest possibledifference between the hours of successive rising or setting of themoon, when the inclination of her orbit to the ecliptic is taken intoaccount; and this value has been carefully reproduced in our text-