uantity of water has been poured in. This is an
example of refraction. The rays of light coming from the coin ought
_not_ to reach the eye, on account of the basin's side being in the way;
yet by the action of the water they are _refracted_, or bent over its
edge, in such a manner that they do.

CHAPTER XVI

THE MOON

What we call the moon's "phases" are merely the various ways in which we
see the sun shining upon her surface during the course of her monthly
revolutions around the earth (see Fig. 14, p. 184). When she passes in
the neighbourhood of the sun all his light falls upon that side which is
turned away from us, and so the side which is turned towards us is
unillumined, and therefore invisible. When in this position the moon is
spoken of as _new_.

As she continues her motion around the earth, she draws gradually to the
east of the sun's place in the sky. The sunlight then comes somewhat
from the side; and so we see a small portion of the right side of the
lunar disc illuminated. This is the phase known as the _crescent_ moon.

As she moves on in her orbit more and more of her illuminated surface is
brought into view; and so the crescent of light becomes broader and
broader, until we get what is called half-moon, or _first quarter_, when
we see exactly one-half of her surface lit up by the sun's rays. As she
draws still further round yet more of her illuminated surface is brought
into view, until three-quarters of the disc appear lighted up. She is
then said to be _gibbous_.

Eventually she moves round so that she faces the sun completely, and
the whole of her disc appears illuminated. She is then spoken of as
_full_. When in this position it is clear that she is on the contrary
side of the earth to the sun, and therefore rises about the same time
that he is setting. She is now, in fact, at her furthest from the sun.

[Illustration: Direction from which the sun's rays are coming.

Various positions and illumination of the mooon by the sun during her
revolution around the earth.

The corresponding positions as viewed from the earth, showing the
consequent phases.

FIG. 14.--Orbit and Phases of the Moon.]

After this, the motion of the moon in her orbit carries her on back
again in the direction of the sun. She thus goes through her phases as
before, only these of course are _in the reverse order_. The full phase
is seen to give place to the gibbous, and this in turn to the half-moon
and to t