tion. S represents the Sun, E the Earth, and _mn_ the orbit of
the Moon. It is obvious that whilst the Moon is moving from _m_ to _n_
it becomes immersed in the Earth's shadow. But before actually reaching
the shadow the Moon passes through a point in its orbit at which it
begins to lose the full light of the Sun. This is the entrance into the
"penumbra" (or "Partial shade"). Similarly, after the eclipse, when the
Moon has emerged from the full shadow it does not all at once come into
full sunshine, but again passes through the stage of penumbral
illumination,[112] and under such circumstances (to speak in the style
of Old "Oireland") the invisible Moon is very often not invisible, and
the part partially eclipsed is often not eclipsed, and when the Moon is
totally eclipsed it is frequently still visible. Of course the general
idea involved in all cases of a body passing into the shadow of another
body is that the body which so passes disappears, because all direct
light is cut off from it. In the case, however, of a lunar eclipse this
state of things is not always literally accomplished, and very often
some residual light reaches the Moon (of course from the Sun) with the
result that traces of the Moon may often be discerned. The laws which
govern this matter are very ill-understood. The fact remains that if we
examine a series of reports of observed eclipses of the Moon extending
over many centuries (and records exist which enable us to do this) we
shall find that in some instances when the Moon was "totally" eclipsed
in the technical sense of that word, it was still perfectly visible,
whilst during other eclipses it absolutely and entirely disappeared from
view. Such eclipses are sometimes spoken of as "black" eclipses of the
Moon, but the phrase is not a happy one. Many instances of both kinds
will be found mentioned in the chapter on historical lunar
eclipses.[113]

[Illustration: FIG. 14.--CONDITIONS OF ECLIPSES OF THE MOON.]

The different conditions of eclipses of the Moon are illustrated by Fig.
14 which must be studied with the aid of the remarks made in a former
chapter concerning the apparent movements of the Sun and Moon and their
nodal passages. Suffice it to state here that in Fig. 14 AB represents
the ecliptic, and CD the Moon's path. The three black circles are
imaginary sections of the Earth's shadow as cast when the Earth is in
three successive positions in the ecliptic. If when the Earth's shadow
is