[AS. mona.] The globe or satellite which moves round the earth and reflects the light of the sun upon it. In form it is an almost perfect sphere of 2,163 miles in diameter, and revolves at a mean distance from the earth's centre of 238,840 miles. The moon presents as large a surface to the eye as the sun, but it is really many million times smaller, and looks as large only because it is so much nearer. It moves round'the earth in a nearly circular orbit in a little less than a month. The time occupied by the moon in passing from one star to the same star again is called a sidereal month. For every revolution in the moon's orbit it; rotates once on its axis, so that one side of the moon is always invisible to the earth. At times, however, owing to its vibratory motion, we see a little more than the side which usually faces the earth. When the moon is between the sun and the earth, it is invisible, and on becoming visible is called "the new moon," and when the earth is between the moon and the sun, the whole surface becomes visible, and in this state we have what is called "fullmoon." The interval from new moon to new moon again - that is, the time occupied by the moon in passing from the sun round to the sun again - is called the synodic month. It is longer than the sidereal month, its mean value being 29.53 days nearly, and this is the length of the ordinary lunar month. From "new" to " full " the moon increases in apparent size, and then begins to decrease in size, until it returns to the condition of the new moon. In the phases before and after new moon a faint illumination of the part not directly lighted up by the sun is visible. This is called the " earth shine," and is due to the reflection of light received from the earth. An eclipse of the moon takes place when it gets into the shadow of the earth, and an eclipse of the sun when the moon conies between the sun and the earth. To the naked eye the surface of the moon presents a mottled appearance, some parts being light and others dark. Viewed through a telescope the surface appears to be covered with mountains, valleys, and plains, like the surface of the earth ; only in the moon everything is barren and desolate, like the country around volcanoes, and there are no seas, lakes, or rivers. The harvest moon is the full moon that occurs nearest the autumnal equinox (Sept. 23). (See Eclipse.)