y pass in review under the sun, must be
observed when suitable conditions of illumination prevail, if we wish to
appreciate their true character and significance.

As a general rule, lunar objects are best seen when they are at no great
distance from "the terminator," or the line dividing the illumined from
the unillumined portion of the spherical surface. This line is constantly
changing its position with the sun, advancing slowly onwards towards the
east at a rate which, roughly speaking, amounts to about 30.5 min. in an
hour, or passing over 10 deg. of lunar longitude in about 19 hrs. 40
mins. When an object is situated on this line, the sun is either rising
or setting on the neighbouring region, and every inequality of the
surface is rendered prominent by its shadow; so that trifling variations
in level and minor asperities assume for the time being an importance to
which they have no claim. If we are observing an object at lunar sunrise,
a very short time, often only a few minutes, elapses before the confusion
caused by the presence of the shadows of these generally unimportant
features ceases to interfere with the observation, and we can distinguish
between those details which are really noteworthy and others which are
trivial and evanescent. Every formation we are studying should be
observed, and drawn if possible, under many different conditions of
illumination. It ought, in fact, to be examined from the time when its
loftiest heights are first illumined by the rising sun till they
disappear at sunset. This is, of course, practically impossible in the
course of one lunation, but by utilising available opportunities, a
number of observations may be obtained under various phases which will be
more or less exhaustive. It cannot be said that much is known about any
object until an attempt has been made to carry out this plan. Features
which assume a certain appearance at one phase frequently turn out to be
altogether different when viewed under another; important details
obscured by shadows, craters masked by those of neighbouring objects, or
by the shadows of their own rims, are often only revealed when the sun
has attained an altitude of ten degrees or more. In short, there is
scarcely a formation on the moon which does not exemplify the necessity
of noting its aspect from sunrise to sunset. Regard must also be had to
libration, which affects to a greater or less degree every object;
carrying out of the range of ob