The actual diameter of the moon is about 2163 miles, which is somewhat more than one-quarter the diameter of the earth. For a satellite, therefore, she seems very large compared with her primary, the earth; when we consider that Jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of Jupiter. Furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year. Seen from space, therefore, she would not give the impression of a circling body, as other satellites do. Her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its orbit. In view of all this, some astronomers are inclined to regard the earth and moon rather as a "double planet" than as a system of planet and satellite. When the moon is full she attracts more attention perhaps than in any of her other phases. The moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. The sun _appears_ to go once entirely round the sky in the course of a year, and the moon performs the same journey in the space of about a month. The moon, when full, having got half-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.