The moon and the earth form a little system of their own and are bound together by a strong attraction. [Figure 159: EARTH-LIT NEW MOON. Showing a slender crescent embracing the earth-lit night portion of the moon. Photograph through 40-inch refractor by Yerkes Observatory.] This strong attraction, called the force of gravity, has resulted in a strange thing happening to both the earth and the moon, for the earth pulls so hard on the moon that it has long since destroyed the moon's axial rotation with reference to itself, and the moon pulls so hard on the earth that it moves the great fluid body of the ocean. Thus the moon has always the same face toward the earth for it cannot turn around and show us the other side, while the pulling of the earth by the moon causes the ocean's waters to rise and fall every 12 hours and 26 minutes, or twice a day. When the Greek and Roman travelers first ventured to the ocean, they could not understand why great stretches of beaches were left dry, and with equal regularity covered up; where did the water go, and why? It seemed a hopeless mystery to people who had lived around a landlocked sea where the tide, with few exceptions, rises but a few inches. When the sun and moon are in line with the earth they co-operate in their pulling and hauling at the sea. The moon is then either "new" or "full" and it is then that the highest tides occur. "The silver Moon o'er briny seas presides, And heaves huge ocean with alternate tides." --_Lucan's Pharsalia_ (ROWE'S _Trans._) At exactly new moon (the moon is called new when it lies between the earth and sun), the moon comes above the horizon at the same time as the sun and sets with it. A few days after new moon we sometimes see what is called the old moon in the young moon's arms, the dimly lighted portion being illuminated by light reflected from the day side of the earth. This earthlight at the moon is at least 50 times greater than moonlight at the earth. Its somewhat ruddy color is caused by the sunlight having passed twice through our atmosphere. The bright crescent, which is the young moon's arms enclosing the old moon, is the sun-illuminated portion of the moon. "I saw the new moon late yestere'en, Wi' the auld moon in her arms." --_Old Scotch Ballad._ After "new moon" the moon appears a little to the east of the sun as a thin semicircle, the horns of which _always_ point to the cast. The horns point to the east because the sun sets in the west and the illuminated part of the moon is on the side nearest to the sun, a fact sometimes forgotten by artists. The thin crescent of the moon gradually becomes broader as the moon moves away from the sun and in the course of four or five days increases to a semicircle. "The Moon her monthly course had now begun, And with increasing horns forsook the Sun;" --_Lucan's Pharsalia._