rough body
of the moon, shown in section (Fig. 59), illuminates the whole
cavity at _a_, part of the one at _b_, casts a long shadow from the
mountain at _c_, and touches the tip of the one at _d_, which appears
to a distant observer as a point of light beyond the terminator,
As the moon revolves the conical cavity, _a_ is illuminated on
the forward side only; the light creeps down the backward side
of cavity _b_ to the bottom; mountain _c_. comes directly under
the sun and casts no shadow, and mountain _d_ casts its long shadow
over the plain. Knowing the time of revolution, and observing the
change of [Page 156] illumination, we can easily measure the height
of mountain and depth of crater. An apple, with excavations and
added prominences, revolved on its axis toward the light of a
candle, admirably illustrates the crescent light that fills either
side of the cavities and the shadows of the mountains on the plain.
Notice in Fig. 58 the crescent forms to the right, showing cavities
in abundance.

[Illustration: Fig. 60.--Lunar Crater "Copernicus," after Secchi.]

The selenography of one side of the moon is much better known to
us than the geography of the earth. Our maps of the moon are far
more perfect than those of the earth; and the photographs of lunar
objects by Messrs. Draper and De la Rue are wonderfully perfect,
[Page 157] and the drawings of Padre Secchi equally so (Fig. 60).
The least change recognizable from the earth must be speedily
detected. There are frequently reports of discoveries of volcanoes
on the moon, but they prove to be illusions. The moon will probably
look the same to observers a thousand years hence as it does to-day.

This little orb, that is only 1/81 of the mass of the earth, has
twenty-eight mountains that are higher than Mont Blanc, that "monarch
of mountains," in Europe.

_Eclipses._

[Illustration: Fig. 61.--Eclipses; Shadows of Earth and Moon.]

It is evident that if the plane of the moon's orbit were to correspond
with that of the earth, as they all lie in the plane of the page
(Fig. 61), the moon must pass between the centres of the earth
and sun, and exactly behind the earth at every revolution. Such
successive and total darkenings would greatly derange all affairs
dependent on light. It is easily avoided. Venus does [Page 158] not
cross the disk of the sun at every revolution, because of the
inclination of the plane of its orbit to that of the earth (see Fig.
41, p. 107)