Until then many people did not know how the distance between the earth and the moon had been calculated.  This fact was taken advantage of to explain to them that it was done by measuring the parallax of the moon.  If the word “parallax” seemed new to them, they were told it was the angle formed by two straight lines drawn from either extremity of the earth’s radius to the moon.  If they were in doubt about the perfection of this method, it was immediately proved to them that not only was the mean distance 234,347 miles, but that astronomers were right to within seventy miles.

To those who were not familiar with the movements of the moon, the newspapers demonstrated daily that she possesses two distinct movements, the first being that of rotation upon her axis, the second that of revolution round the earth, accomplishing both in the same time—­that is to say, in 27-1/3 days.

The movement of rotation is the one that causes night and day on the surface of the moon, only there is but one day and one night in a lunar month, and they each last 354-1/3 hours.  But, happily, the face, turned towards the terrestrial globe, is lighted by it with an intensity equal to the light of fourteen moons.  As to the other face, the one always invisible, it has naturally 354 hours of absolute night, tempered only by “the pale light that falls from the stars.”  This phenomenon is due solely to the peculiarity that the movements of rotation and revolution are accomplished in rigorously equal periods, a phenomenon which, according to Cassini and Herschel, is common to the satellites of Jupiter, and, very probably to the other satellites.

Some well-disposed but rather unyielding minds did not quite understand at first how, if the moon invariably shows the same face to the earth during her revolution, she describes one turn round herself in the same period of time.  To such it was answered—­“Go into your dining-room, and turn round the table so as always to keep your face towards the centre; when your circular walk is ended you will have described one circle round yourselves, since your eye will have successively traversed every point of the room.  Well, then, the room is the heavens, the table is the earth, and you are the moon!”

And they go away delighted with the comparison.

Thus, then, the moon always presents the same face to the earth; still, to be quite exact, it should be added that in consequence of certain fluctuations from north to south and from west to east, called libration, she shows rather more than the half of her disc, about 0.57.