matter weighing six pounds at the surface of the earth would weigh almost exactly one pound at the surface of the moon. The moon circuits once round the earth in 27d. 7h. 43m. 11.5s. This is the time in which, viewed from the earth, she seems to complete one circuit round the stellar heavens, and is therefore called a sidereal month. But as the earth is all the time travelling the same way round the sun, the lunar month is longer. Thus, suppose S (fig. 14) to be the sun, E the earth at the beginning of a lunar month, M_{1} M_{2} M_{3} M_{4} the moon's path, and M_{1} the moon's place on the line joining E and S. If the earth remained at rest while the moon went round the path M_{1} M_{3}, then after completing one circuit the moon would again be at M_{1} on the line joining E and S, or it would be new moon again. But the earth is moving onwards along the arc EE´ of her circuit round the sun. So that when the moon has completed one circuit she is at M_{4} (E´m_{1} drawn parallel to EM_{1}) and has still to travel some distance before she gets round to M´ on the line joining S and E´. The lunation, or interval between successive new moons, has an average duration of 29d. 12h. 44m. 38s., exceeding a sidereal month by 2d. 5h. [Illustration: Fig. 14.--Explaining the difference between a sidereal lunar month and a common lunar month or lunation.] It would not, however, be correct to regard the earth as the true centre of the moon's motion. The moon is in reality a planet circling round the sun, but largely perturbed by the attraction of its companion planet the earth. If the moon's path in the course of a year were carefully drawn to scale, or, better, were modelled by means of a fine wire, it would scarcely be distinguishable from a similar picture or model of the earth's path round the sun. Or thus, the entire width of the moon's track is about 477,636 miles, while the diameter of the orbit along which she and the earth both travel is nearly 104,000,000 miles, or 385 times as great. If we draw then a circle 3-85/100 inches in diameter to represent the earth's path round the sun, somewhat eccentrically placed, and the circular line is 1-100th of an inch wide, the moon's track would be fairly represented by a curve touching alternately the inside and the outside edge of this circular line, at equidistant points dividing the circle into about 24¾ parts.