MOON, in astronomy, the name given to the satellite of any planet, specifically to the satellite of the earth (a common Teu tonic word, cf. Ger. Mond, Dut. maan, Dan. maane, etc., and cognate with such Indo-Germanic forms as Gr. AO, Skt. mas, Irish ml, etc. ; Lat. uses luna, i.e., lucna, the shining one, lucere, to shine, for the moon, but preserves the word in mensis, month; the ultimate root for "moon" and "month" is usually taken to be me-, to measure, the moon being a measurer of time).
The moon being an opaque body, shining by reflected light, only that portion of the hemisphere which is illuminated and turned towards the earth can be seen. At new moon, M', the hemisphere turned towards the earth is wholly unilluminated by sunlight, while at full moon, the fully illuminated hemisphere is turned towards the earth. At intermediate phases the hemi sphere turned towards the earth is only partially illuminated. Near new moon (on both sides) the moon appears as a narrow crescent with the horns turned away from the sun ; near full moon the defective edge is that farthest from the sun's direction, i.e., before full moon the eastern side (left in northern hemisphere) and after full moon the western side. In the diagram the plane of the paper may be taken as that of the earth's motion round the sun. The moon's orbit does not lie exactly in this plane but is inclined to it at an angle of about 5°. If this were not so the moon would eclipse the sun at every new moon and would itself be eclipsed by the earth at every full moon. The line NN' may be taken as representing the line of intersection of the two planes. This line, known as the line of nodes, is in slow rotation, making about one revolution against the arrows in about 18.6 years. At two seasons each year the sun is near the direction of NN' as seen from the earth, and at these seasons eclipses occur. (See illustrations and article ECLIPSE.) To understand the positions in which to expect the moon at any time it is most convenient to consider its position relative to the sun. When the moon is new it is in the same direction as the sun. Consequently it rises and sets with the sun, crosses the meridian with the sun and cannot be seen from the earth. Relatively to the stars the sun moves eastward about I° per day, while the moon moves about 13° in the same direction. The moon, therefore, ap pears about 12° farther east of the sun day by day, and 13° farther east of the stars in its neighbourhood. When the moon is one day old it will lie about 12° east of the sun, it will be due south about 48 minutes after the sun and it will describe in the sky the same arc as the sun will do about 12 days later, if it is assumed, as is approximately true, that the orbital planes of the sun and the moon coincide. Similarly when the moon is at first quarter or 90° east of the sun it will be due south 6 hours after the sun and it will describe in the sky the same arc as the sun will describe in three months' time. Consider the case of a moon
a week after new on March 21. On that day the sun will rise six hours before noon and set six hours after. At sunset the moon is due south and in the position occupied by the sun at noon on 21. Consequently, for observers in the northern hemisphere, it will be more than i2 hours above the horizon, rising more than 6 hours before it souths. In the latitude of London it will rise about 81 hours before it souths, i.e., about 9.3o A.M., and it will not set till about 2.3o the next morning. As is well known, in fairly high north latitudes, the moon near the full rises at nearly the same hour for several nights in the autumn, giving rise to the phenomenon of the harvest moon. The explanation of this is very simple. The full moon in the autumn is in the same part of the sky as the sun is in the spring. Now, in the spring, the time the sun is above the horizon is steadily increasing from day to day. This is happening for the autumn full moon 13 times as fast. Consequently, although on the average the moon is 48 minutes later day by day in southing, its rising is not retarded by anything like this amount. In the spring the phenomenon is reversed, and the rising of the full moon is delayed by more than 48 minutes from day to day. Referring to the diagram, it will be seen that when the moon is near the position MI although the hemisphere turned towards the earth is not illuminated directly by sunlight it is illuminated indirectly by light from the illuminated hemisphere of the earth. This light must be about 12 times as bright as the full moon and consequently near the time of new moon, after twilight, the whole hemisphere of the moon can be seen faintly illuminated by "earthshine." This phenomenon can be seen very well in a small telescope.
The moon always presents nearly the same face to the earth, from which it follows that, when referred to a fixed direction in space, it revolves on its axis in the same time in which it performs its revolution. The rate of actual rotation is substantially uni form, while the arc through which the moon moves from day to day varies somewhat. Consequently, the face which the moon presents to the earth is subject to a corresponding variation, the globe, as seen from the earth, slightly oscillating in a period nearly equal to that of revolution. This apparent oscillation is called libration and its amount on each side of the mean is commonly between 6° and 7°. There is also a libration in latitude, arising from the fact that the axis of rotation of the moon is not pre cisely perpendicular to the plane of its orbit. This libration is more regular than that in longitude, its amount being about 6° 44' on each side of the mean. The other side of the moon is invisible from the earth, but in consequence of the libration about six-tenths of the lunar surface may be seen at one time or another, while the remaining four-tenths are forever hidden from view. It is found that the direction of the moon's equator remains nearly invariable with respect to the plane of the orbit, and therefore revolves with that plane in a nodal period of 18.6 years. This shows that the side of the moon presented to observation is held in position, as it were, by the earth, from which it also follows that the lunar globe is more or less elliptical, the longer axis being directed toward the earth. The amount of the ellipticity is, how ever, very small.