Robert Grant Aitken

“ And our crowded hurrying life with its insistent and ever growing demands upon our time affords ever less leisure for quiet observation and thought, and city lights too often hide from us the lights in the sky. That is why I am devoting the first part of this lecture to a simple account of the Moon as we see it in the sky. ”

“ We see the Moon rise above the eastern horizon, circle the sky towards the west, and set below the western horizon. The points of rising and setting are not always the same nor does the Moon cross the meridian always at the same altitude, and the times of rising change from day to day. The observer will quickly learn to associate the times of rising and setting with the Moon's age and its phases. For a day or two at new Moon time he will not see it rise or set at all. Then, if he is sharp-eyed and the air is very clear, he will see it rise shortly after sunrise, a slender crescent. ”

“ Taking the phases into account, it is found that in 13 seconds we receive as much light and heat from the Sun as we do from the Moon in a whole year! Evidently, then the Moon's heat is quite unimportant to us; a light cloud passing in front of the Sun deprives us of more heat than the Moon ever sends us. In the second place, storm centers travel across the Earth, generally from west to east in our latitudes, and can often be traced clear across a continent, or even half way round the globe in the course of a week or, two. ”

“ The question of the origin of the various lunar surface features is one on which astronomers are still in doubt. It is not difficult to conceive of the formation of the mountain ranges, lofty as some of them are, and of the valleys or canyons and of the smaller craters, at least, by forces similar to those which have produced corresponding features upon our Earth, especially when we consider the fact that, because of the Moon's smaller mass, a given force acting against gravity there would be about six times as effective as here. ”

“ The waters of the ocean are freer to move, that is all. Now it can be shown that when a body rotates upon its axis in the same direction as its motion in its orbit, and the rotation time is shorter than the revolution period such a tidal force acts as a brake to slow up the rotational motion until the two periods are equal. It is thought by most astronomers that the Moon originally rotated much faster than it does now and that the cumulative effect of the Earth's tidal action upon it thru the ages is responsible for the fact that now its rotation time equals its revolution period. ”

“ To account for the observed motion under the law of gravitation, taking all the disturbing factors into consideration, is a problem that has exercised the highest powers of great mathematicians from Newton's time to the present day. ”

“ It requires no observatory equipment—not even the smallest telescope—to gain a knowledge of the apparent motions of the Moon in the sky. It is only necessary to watch it with seeing eyes, as the ancients did thousands of years before the telescope was invented. Any intelligent boy or girl can repeat these observations and verify what I am going to say, and I hope that many of you who hear me tonight will do so. When it comes to the real motion of the Moon the story is very different. ”

“ If the Moon raises tides upon the Earth, then the Earth must likewise exercise a tidal strain upon the Moon and because the Earth's mass is so much the greater of the two, this strain must be about 20 times that exerted by the Moon upon the Earth. ”

“ 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 231⁄2° 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 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. ”

“ In a month's time it travels over nearly 1/12 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. ”

“ If the Moon moved simply under the mutual attraction between it and the Earth, the problem would be the comparatively simple one known as the two-body problem. But the Sun's attraction is a powerful disturbing—or, in technical terms, perturbing—force ”

“ Not one of them has a basis of fact, but they cling tenaciously to men's minds and still influence the actions of some. In a certain region of the San Joaquin Valley, for instance, no farmer, even today, plants his cabbages without first consulting an almanac! ”