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in placing the moon at an average distance of a little less than two hundred and forty thousand miles. This distance is obtained by direct measure of the parallax, as will be explained hereafter, and also by calculating how far off the moon must be in order that, being projected into space, it may describe an orbit around the earth in the time that it actually does perform its round. The orbit is elliptic, so that the actual distance varies. Sometimes it is ten or fifteen thousand miles less, at other times as much more, than the average.

The diameter of the moon's globe is a little more than one fourth that of the earth; more exactly, it is two thousand one hundred and sixty miles. The most careful measures show no deviation from the globular form except that the surface is very irregular.

The moon accompanies the earth in its revolution round the sun. To some the combination of the two motions seems a little complex; but it need not offer any real difficulty. Imagine a chair standing in the centre of a railway car in rapid motion, while a person is walking around it at a distance of three feet. He can go round and round without varying his distance from the chair and without any difficulty arising from the motion of the car. Thus the earth moves forward in its orbit, and the moon continually revolves around it without greatly varying its distance from us.

The actual time of the moon's revolution around the earth is twenty-seven days eight hours; but the time