effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its orbit about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at C, and appears on the side opposite the sun, in the phase called full moon.  At this point the moon’s orbit about the sun has a shorter radius of curvature than the earth’s.  In traveling from C to D the moon still moves more rapidly than the earth, and, having passed it, appears at D to the left of the earth, in the phase called third quarter.  Now, the earth being behind the moon, the effect of its attraction combined with the sun’s tends to retard the moon in its orbit about the sun, with the result that the moon moves again less rapidly than the earth, and the latter overtakes it, so that, upon reaching E, the two are once more in the same relative positions that they occupied at A, and it is again new moon.  Thus it will be seen that, although the real orbit of the moon has the sun for its center of revolution, nevertheless, in consequence of the attraction of the earth, combined in varying directions with that of the sun, the moon, once every month, makes a complete circuit of our globe.

The above explanation should not be taken for a mathematical demonstration of the moon’s motion, but simply for a graphical illustration of how the moon appears to revolve about the earth while really obeying the sun’s attraction as completely as the earth does.

There is no other planet that has a moon relatively as large as ours.  The moon’s diameter is 2,163 miles.  Its volume, compared with the earth’s, is in the ratio of 1 to 49, and its density is about six tenths of the earth’s.  This makes its mass to that of our globe about as 1 to 81.  In other words, it would take eighty-one moons to counterbalance the earth.  Before speaking of the force of gravity on the moon we will examine the character of the lunar surface.

To the naked eye the moon’s face appears variegated with dusky patches, while a few points of superior brilliance shine amid the brighter portions, especially in the southern and eastern quarters, where immense craters like Tycho and Copernicus are visible to a keen eye, gleaming like polished buttons.  With a telescope, even of moderate power, the surface of the moon presents a scene of astonishing complexity, in which strangeness, beauty, and grandeur are all combined.  The half of the moon turned earthward contains an area of 7,300,000 square miles, a little greater than the area of South America and a little less than that of North America.  Of these 7,300,000 square miles, about 2,900,000 square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, maria—­i.e., “seas.”  Whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges