Page:The moon (1917).djvu/6

Rh months. Let us note just here that since the Moon at full is always opposite to the Sun, the full Moon must be north of the equator during our winter months, when the Sun is south of it, and south of the equator during the summer months. The full Moon therefore "rides high" in the sky and gives us the most light in the winter when we have the least sunlight, and rides low in the sky in the summer. In our latitudes this is not a matter of great consequence, but if we were at the North or at the South Pole, it would be pleasant, at least, to have the Moon above the horizon continuously for the 14 days from the first quarter thru full Moon to the last quarter every month during the long polar night.

Next, the ancients learned that the Moon's distance from the Earth varies by a slight amount corresponding to the slight variation in its apparent diameter and that this variation progresses in a regular manner, completing the cycle of its changes in the period of a month. This we now know is due to the fact that its orbit is not an exact circle but is flattened a little into the form of an ellipse. Of course they also learned that the Moon does not shine by its own light but only by reflected sunlight. This led to an understanding of the phases of the Moon.

A careful study of some of the prominent markings on the Moon's surface will soon convince anyone that they always remain in approximately the same position with respect to the limb; that is, that the Moon always turns the same face toward the Earth. This means that the Moon must turn once on its axis—make one complete rotation—each month. That is a puzzling statement to many people when it is heard for the first time but it is easy to show that it is true, and that in no other way could the Moon keep the same face turned toward us. Try walking around a table placed near the center of a room always facing the table as you walk and see what happens! You will find, that in making the round, you have faced each wall of the room in succession; that is, you have yourself turned once completely round during your walk.

I said just now that the Moon always keeps the same face turned toward the Earth. This is true in a general way but the statement is not quite exact. The Moon's equator is inclined 6½° to the plane of its orbit, consequently at one time in each month its north pole is tipped 6½° toward us, and two weeks later its south