The first day of the week is related to the greatest body in the
heavens--the sun--and accordingly we call that day Sun-day. The second
day of the week is similarly called after the next most important
celestial body--the moon--and though we do not actually say Moon-day,
we do say Monday, which is very nearly the same. In French, too, we
have _lune_ for moon, and _Lundi_ signifies our Monday. The other days
of the week also have names derived from the heavens, but of these we
shall speak hereafter. We are now going to talk about the moon.
We can divide the objects in this room into two classes. There are the
bright faces in front of me, and there are the bright electric lights
above. The electric lights give light, and the faces receive it. I can
see both lights and faces; but I see the electric lamps by the light
which they themselves give. I see the faces by the illumination which
they have received from the electric lights. This is a very simple
distinction, but it is a very important one in Star-land. Among all
these bodies which glitter in the heavens there are some which shine
by their own light, like the lamps. There are others only brilliant by
reflected light, like the faces. It seems impossible for us to confuse
the brightness of a pleasant face with the beam from a pretty lamp, but
it is often not very easy to distinguish in the heavens between a body
which shines by its own light and a body which merely shines by some
other light reflected from it. I think many people would make great
mistakes if asked to point out which objects on the sky were really
self-luminous and which objects were merely lighted up by other bodies.
Astronomers themselves have been sometimes deceived in this way.
The easiest example we can give of bodies so contrasted is found in the
case of the sun and the moon. Of course, as we have already seen, the
sun is the splendid source of light which it scatters all around. Some
of that light falls on our earth to give us the glories of the day;
some of the sunbeams fall on the moon, and though the moon has itself
no more light than earth or stones, yet when exposed to a torrent of
sunbeams, she enjoys a day as we do. One side of her is brilliantly
lighted; and this it is which renders our satellite visible.
Hence we explain the marked contrast between the sun and the moon. The
whole of the sun is always bright; while half of the moon is always in
darkness. When the bright side of the moon is turned directly towards
us, then, no doubt, we see a complete circle, and we say the moon
is full. On other occasions a portion only of the bright surface is
directed to us, and thus are produced the beautiful crescents and
semicircles and other phases of the moon.
[Illustration: FIG. 28.--To show that the Moon is lighted by Sunbeams.]
A simple apparatus (illustrated in Fig. 28) will explain their various
appearances. The large india-rubber ball there shown represents the
moon, which I shall illuminate by a beam from the electric light. The
side of the ball turned towards the light is glowing brilliantly, and
from the right side of the room you see nearly the whole of the bright
side. To you the moon is nearly full. From the centre of the room you
see the moon like a semicircle, and from the left it appears a thin
crescent of light. I alter the position of the ball with respect to the
lamp, and now you see the phases are quite changed. To those on my left
our mimic moon is now full; to those on my right the moon is almost
new, or is visible with only a slender crescent. From the centre of the
room the quarter is visible as before. We can also show the same series
of changes by a little contrivance of Figs. 29 and 30.
Thus every phase of the moon (Fig. 31), from the thinnest beautiful
crescent of light that you can just see low in the west after sunset
up to the splendor of the full moon, can be completely accounted for
by the different aspects of a globe, of which one-half is brilliantly
illuminated.
[Illustration: