Page:The moon (1917).djvu/5

124 crescent, the time of rising grows ever later until we see it rise for the last time in the month just before sunrise.

Of course this retardation in the time of its rising is due to the fact that the Moon is really moving about the Earth from west to east. Watch it for a few hours in any clear moonlight evening and you will find that in an hour's time it moves eastward among the stars about the distance represented by its own apparent diameter. Continue your observations and in due time you will learn that it requires approximately 27⅓ days to return to its original position among the stars so far as its eastward motion is concerned; but now it may be a little farther north or a little farther south than it was a month earlier. This is a little more than two days less than the time it requires to pass from new Moon back again to new Moon, and the reason is obvious when we recall the fact that because of the Earth's motion in its orbit the Sun also seems to move eastward among the stars. In a month's time it travels over nearly of its orbit, and the Moon must catch up with it before it can again reach the new Moon phase. It is also clear that the phases must in some way be related to the change in the Moon's position with respect to the Sun, for full Moon always comes when the Sun and Moon are on opposite sides of the Earth, new Moon when they are nearly in line on the same side.

Note the Moon's size and you will find that it is always about the same but that it does vary slightly. At one time in the month it is a little larger than the average, at another a little smaller. But in making this observation be careful to watch the Moon when it is about the same distance from the horizon, for it always looks larger when near the horizon than when it is higher in the sky. This is an illusion, for the Moon is then really farther away and actually its disk is a little smaller.

After careful and long continued observations of this kind the ancients were able to conclude in the first place that the Moon's orbit about the Earth—its apparent path among the stars—makes an angle of about 5° with the ecliptic. This explains why the Moon sometimes rises north of the east point, and sometimes south of it, for the ecliptic itself makes an angle of 23½° with the plane of the Earth's equator, and the Sun is south of the equator from the autumnal equinox, about September 21, to the vernal equinox, about March 21, and then north of it thru the next six