Page:The moon (1917).djvu/7

126 pole is similarly tipped. Therefore we see a little beyond first one pole and then the other each month. This slight variation we call the libration in latitude. Further, since the Moon's orbit is an ellipse its motion in its orbit will be variable, being slower when it is farthest from the Earth and faster when it is nearest; but its motion of rotation on its axis is perfectly uniform. This produces what we call the libration in longitude and permits us to "see alternately a few degrees around the eastern and western edge of the lunar globe." Finally, the Moon when it rises and when it sets is practically on a plane passing thru the center of the Earth while we are about 4,000 miles above that plane; therefore we look a little past the western limb of the Moon as it rises and a little past its eastern limb as it sets. The net result is that 41/100 of the Moon is always visible, 41/100 is never visible, and the remaining 18/100, along the limbs, is sometimes visible and sometimes not.

The Moon is so near the Earth that its distance can be measured with very great accuracy. One method of doing this is, in principle, precisely like that which a surveyor employs to determine the distance to an inaccessible object. The surveyor measures off a base line of suitable length from both ends of which the object is visible. At each end he then measures the angle included between the other end of the line and the object. This gives him a triangle in which he knows the size of three independent parts—one side and two angles—and from these he can readily compute the other parts. In the case of the Moon we measure its distance from the zenith at two stations having nearly the same longitude but widely separated in latitude, the observatories at Greenwich, England, and at the Cape of Good Hope, South Africa, for example. Knowing the latitudes of our stations we have for our base line the length of the line between them drawn thru the Earth's crust, and the measures of the Moon's zenith distance supply our angles. Then we calculate the distance from each observatory to the Moon and from these values the distance to the Moon from the Earth's center. The mean value has been found to be 238,862 miles; but it is easier to remember the value 240,000 miles, a round number that is sufficiently exact for any one except the specialist. Having the Moon's distance, our measures of its apparent angular diameter can be converted into