f-way round this journey, occupies, therefore, that
region of the sky which the sun itself will occupy half a year later.
Thus in winter the full moon will be found roughly to occupy the sun's
summer position in the sky, and in summer the sun's winter position. It
therefore follows that the full moon in winter time is high up in the
heavens, while in summer time it is low down. We thus get the greatest
amount of full moonlight when it is the most needed.

The great French astronomer, Laplace, being struck by the fact that the
"lesser light" did not rule the night to anything like the same extent
that the "greater light" ruled the day, set to work to examine the
conditions under which it might have been made to do so. The result of
his speculations showed that if the moon were removed to such a distance
that she took a year instead of a month to revolve around the earth; and
if she were started off in her orbit at full moon, she would always
continue to remain full--a great advantage for us. Whewell, however,
pointed out that in order to get the moon to move with the requisite
degree of slowness, she would have to revolve so far from the earth that
she would only look one-sixteenth as large as she does at present, which
rather militates against the advantage Laplace had in mind! Finally,
however, it was shown by M. Liouville, in 1845, that the position of a
_perennial full moon_, such as Laplace dreamed of, would be
unstable--that is to say, the body in question could not for long remain
undisturbed in the situation suggested (see Fig. 16, p. 191).

[Illustration: Various positions of Laplace's "Moon" with regard to the
earth and sun during the course of a year.

The same positions of Laplace's "Moon," arranged around the earth, show
that it would make only one revolution in a year.

FIG. 16.--Laplace's "Perennial Full Moon."]

There is a well-known phenomenon called _harvest moon_, concerning the
nature of which there seems to be much popular confusion. An idea in
fact appears to prevail among a good many people that the moon is a
harvest moon when, at rising, it looks bigger and redder than usual.
Such an appearance has, however, nothing at all to say to the matter;
for the moon always _looks_ larger when low down in the sky, and,
furthermore, it usually looks red in the later months of the year, when
there is more mist and fog about than there is in summer. What
astronomers actually term the harvest moon is, indee