the
_variation_, and _secular acceleration_, are other perturbations of the
lunar motion, which depend directly or indirectly on the attractive
influence of the Sun and the motion of the Earth in her orbit.

As the plane of the Moon's orbit is inclined at an angle of rather more
than 5 deg. to the ecliptic, it follows that the orb, in its journey round
the Earth, intersects this great circle at two points called the
'Nodes.' When crossing the ecliptic from south to north the Moon is in
its ascending node, and when crossing from north to south in its
descending node. In December the Moon reaches the most northern point of
its course, and in June the southernmost. Consequently we have during
the winter nights the greatest amount of moonlight, and in summer the
least. In the evenings the moonlight is least in March and greatest in
September, when we have what is called the Harvest Moon.

The telescopic appearance of the Moon is very interesting and beautiful,
especially if the orb is observed when waxing and waning. As no aqueous
vapour or cloud obscures the lunar surface, all its details can be
perceived with great clearness and distinctness. Indeed, the topography
of the Moon is better known than that of the Earth, for the whole of its
surface has been mapped and delineated with great accuracy and
precision. The Moon is in no sense a duplicate of its primary, and no
analogy exists between the Earth and her satellite. Evidence is wanting
of the existence of an atmosphere surrounding the Moon; no clouds or
exhalations can be perceived, and no water is believed to exist on the
lunar surface. Consequently there are no oceans, seas, rivers, or lakes;
no fertile plains or forest-clad mountains, such as are found upon the
Earth. Indeed, all the conditions essential for the support and
maintenance of organic life by which we are surrounded appear to be
nonexistent on the Moon. Our satellite has no seasons; its axial
rotation is so slow that one lunar day is equal in length to fourteen of
our days; this period of sunshine is succeeded by a night of similar
duration. The alternation of such lengthened days and nights subjects
the lunar surface to great extremes of heat and cold.

When viewed with a telescope, the surface of the Moon is perceived to
consist of lofty mountain chains with rugged peaks, numerous extinct
volcanoes called crater mountains, hills, clefts, chasms, valleys, and
level plains--a region of desolation, prese