The moon is in more senses than one an illuminating object for both the earth and the skies. As a beginning for earth study it is an object lesson, illustrating what air and water do for our world and incidentally for us; while as the beginning of the study of astronomy, it is the largest and brightest object seen in the sky at night; and since it lies nearest us, it is the first natural step from our world to outer space.
The moon is a little dead world that circles around our earth with one face always towards us, just as a hat-pin thrust into an apple would keep the same side of its head always toward the apple no matter how rapidly the apple was twirled. As we study the face of the moon, thus always turned toward us, we see that it is dark in some places and shining in others, and some ignorant people have thought that the dark places are oceans and the light places, land. But the dark portions are simply areas of darker rocks, while the lighter portions are yellowish or whitish rocks. The dark portions are of such a form that people have imagined them to represent the eyes, nose and mouth of a man's face; but a far prettier picture is that of a woman's uplifted face in profile. The author has a personal feeling on this point, for as a child she saw the man's face always and thought it very ugly and, moreover, concluded that he chewed tobacco; but after she had been taught to find the face of the lady, the moon was always a beautiful object to her.
The moon is a member of our sun's family, his granddaughter we might call her if the earth be his daughter; and since the moon has no fires or light of its own, it shines by light reflected from the sun and, therefore, one-half of it is always in shadow. When we see the whole surface of the lighted half we say the moon is full; but when we see only half of the lighted side turned toward us, we say the moon is in its quarter, because all we can see is one-half of one-half which is one-quarter; and when the lighted side is almost entirely turned away from us we say it is a crescent moon; and when the lighted side is entirely turned away from us we say there is no moon, although it is always there just the same. Thus, we can understand that, although we can never see the other side of the moon, the sun shines on all sides of it. Our earth, like the moon, shines always by reflected light and is almost four times as wide as the moon. Think what a splendid moon our earth must seem to the lady in the moon! When we see the old moon in the new moon's arms, the dark outline of the moon within the bright crescent is visible because of the earthshine reflected from it. Sometimes pupils confuse this appearance of the moon with a partial eclipse; but the former is the new or old moon, which is one edge of the moon shining in the sunlight, the remainder faintly illumined by earth light, while an eclipse must always occur at the full of the moon when the earth passes between the sun and the moon, hiding the latter in its shadow.
It is approximately a month from one new moon to the next, since it takes twenty-nine and one-half days for the moon to complete its cycle around the earth and thus turn once around in the sunshine. Therefore, each moon day is fourteen and three-quarter days long and the night is the same length. The moon always rises in the east and sets in the west, following pretty nearly the sun's summer path. The full moon rises at sunset and sets at sunrise, but owing to the movement of the earth around the sun the moon rises about fifty minutes later each evening; however, this time varies with the different phases of the moon and at different times of the year. This difference in the time of rising is so shortened in August, that we have several nights when the full moon lengthens the day; and it is called the "harvest moon," because in England it adds to the hours devoted to harvesting the grain.
A Visit to the Moon
If we could be shot out from a Jules Verne cannon and make a visit to the moon, it would be a strange experience. First, we should find on this little world, which is only as thick through as the distance from Boston to Salt Lake City, mountains rising from its surface more than thirty thousand feet high, which is twice as high as Mt. Blanc and a thousand feet higher than the tallest peak of the Himalayas; and these moon mountains are so steep that no one could climb them. Besides ranges of these tremendous mountains, there are great craters or circular spaces enclosed with steep rock walls many thousand feet high. Sometimes at the center of the crater there is a peak lifting itself up thousands of feet, and sometimes the space within the crater circle is level. Thirty-three thousand of these craters have been discovered. And, too, on the moon, there are great plains and chasms; and all these features of the moon have been mapped, measured and photographed by people on our earth. For a boy studying geometry, the measuring of the height of the mountains of the moon is an interesting story.
But we could never in our present bodies visit the moon, because of one terrible fact—the moon has no air surrounding it. No air! What does that mean to a world? First of all, as we know life, no living thing—animal or plant—could exist there, for living beings must have air. Neither is there water on the moon; for if there were water there would have to be air. And without water no green thing can be grown, and the surface of the moon is simply naked, barren rock.
If we were on the moon, we could not turn our eyes toward the sun, for with no air to veil it, its fierce light would blind us; and the sky is as black at midday as at midnight, since there is no atmosphere to sift out the other rays of light, leaving the beautiful blue in the sky; nor is there a glow at sunset because there is no air prism to separate the rays of light and no clouds to reflect or refract them. The stars could be seen in the black skies of midday as well as in the black skies of night, and they would be simply points of light and could not twinkle, since there is no air to diffuse the sun's light and thus curtain the stars by day and cause them to twinkle at night. The shadows on the moon are, for the same reason, as black as midnight and as sharply defined; and if we should step into the shadow of a rock at midday we should be hidden as much as if we had stepped into a well of ink, or put on the invisible cloak of fairy lore. And because of no layers of air to make an aerial perspective, a mountain a hundred miles away would seem as close to us as one a mile away.
Since there is no atmosphere on the moon to act as a buffer between the cold of outer space, which is estimated to be 250° below zero, and the heat of the sun, which is 500° above zero, the temperature of the moon would vary 750° between day and night, or between sunshine and shadow, because there is no air to carry the heat over into the shadow or to blanket the world at night. But this great change of temperature between sunlight and darkness is the only force on the moon to change the shape of its rocks, for the expansion under heat and contraction under cold must break and crumble even the firmest rock more or less. Our rocks are broken by the freezing of water that creeps into every crevice, but there is no water to act on the moon's mountains in this fashion or to wear them away by dashing over their surface. However, the rocks and mountains of the moon may be changed in shape by the battering of meteorites, which pelt into the moon by the million, since the moon has no air to set them afire and make them into harmless shooting stars, burning up before they strike. But though a meteorite weighing thousands of tons should crash into a moon mountain and shatter it to atoms there would be no sound, since sound is carried only by the atmosphere.
Imagine this barren, dead world, chained to our earth by links forged from unbreakable gravity, with never a breath of air, a drop of rain or flake of snow, with no streams, nor seas, nor graced by any green thing—not even a blade of grass—a tree, nor by the presence of any living creature! Out there in space it whirls its dreary round, with its stupendous mountains cutting the black skies with their jagged peaks above, and casting their inky shadows below; heated by the sun's rays until hotter than the flame of a blast furnace, then suddenly immersed into cold that would freeze our air into solid ice, its only companion the terrific rain of meteoric stones driven against it with a force far beyond that of cannon balls, and yet with never a sound as loud as a whisper to break the terrible stillness which envelops it.
Leading thought—The moon always has the same side turned toward us so we do not know what is on the other side. The moon shines by reflected light from the sun, and is always half in light and half in shadow. The moon has neither air nor water on its surface and what we call the moon phases depend on how much of the lighted surface we see.
Method—Have the pupils observe the moon as often as possible for a month, beginning with the full moon. After the suggested experiment, the questions which follow may be given a few at a time.
Experiment for recess—Darken the room as much as possible; use a lighted lamp or gas jet or electric light for the sun, which is, of course, stationary. Take a large apple to represent the earth and a small one to represent the moon. Thrust a hat pin through the big apple to represent the axis of the earth and also the axis about which the moon revolves. Tie a string about a foot long to the stem of the moon apple and make fast the other end to the hat pin just above the earth apple. Hold the hat pin in one hand and revolve the apple representing the moon slowly with the other hand letting the children see that if they were living on the earth apple the following things would be true:
1. Moving from right to left when the moon is between the earth and the sun it reflects no light.
2. Moving a little to the left a crescent appears.
3. Moving a quarter around shows the first quarter.
4. When just opposite the lamp, it shows its whole face lighted turned toward the earth.
5. Another quarter around shows a half disc, which is the third quarter.
6. When almost between the sun and the earth the crescent of the old moon appears.
7. Note that the moon always keeps one face toward the earth.
8. Note that the new moon crescent is the lighted edge of one side of the moon, while the old moon crescent is the lighted edge of the opposite side.
9. Make an eclipse of the moon by letting the shadow of the earth fall upon it, and an eclipse of the sun by revolving the moon apple between the sun and the earth. The earth's orbit and the moon's orbit are such that this relative position of the two bodies occurs but seldom.
1. Describe how the moon looks when it is full.
2. What do you think you see in the moon?
3. Describe the difference in appearance between the new moon and the full moon, and explain this difference.
4. Where does the new moon rise and where does it set?
5. When does it rise and when does it set?
6. Where and when does the full moon rise and where and when does it set?
7. How does the old moon look?
8. Could the crescent moon which is seen in early evening be the old moon instead of the new; and, if not, why not?
9. When and where do we ordinarily see the old moon when it is crescent shaped?
10. Does the moon rise earlier or later on succeeding nights? What is approximately the difference in time of moonrise on two successive nights?
11. Do you think we always look at the same side of the moon? If so, why?
12. Is more than one side of the moon luminous? Why?
13. How many days from one new moon until the next?
14. How long is the day on the moon and how long the night?
15. How many times does the moon go around the earth in a year?
16. What is the difference between the disappearance of the old moon and an eclipse of the moon? In both cases the moon is hidden from us.
The Physical Geography of the Moon
Questions for the pupils to think about and answer if they can—
17. Since it has been proved that there is no air or water on the moon, could there be any life there?
18. Supposing you could do without air or water and should be able to visit the moon, what would you find to be the color of the sky there?
19. Would there be a red glow before sunrise or beautiful colors at sunset?
20. Would the sun appear to have rays? Could you look at the sun without being blinded?
21. Would the stars appear to twinkle? Could you see the stars in the daytime?
22. How would the shadows look? If you could step into the shadow of a rock at midday, could you be seen?
23. Could you tell by looking at it whether a mountain was far or near?
24. It is estimated that the temperature of outer space is 250 degrees below zero, and the sun's direct heat is 500 degrees above zero. If this be correct, how hot would it be in the sunshine on the moon? How cold would it be at midnight?
25. Why is it so much hotter and colder on the moon than upon the earth?
26. If you could shout on the moon, how would it sound? If one hundred cannons should be fired at once on the moon, how would it sound?
27. Is there any rain or snow on the moon? Are there any clouds there? If there are no air and water on the moon, would the intense heat and the powerful cold affect the soils or rocks, as freezing and thawing affect our rocks?
28. Professor Newton estimated that the earth meets seven million meteorites (shooting stars) every twenty-four hours. Why do we not see more of these? What happens when a meteorite strikes the moon?
29. The moon is so small that the force of gravity on its surface is one-sixth that on the earth's surface. If a man can carry seventy-five pounds on his back here, how much could he carry on the moon? If a boy can throw a ball one hundred yards here, how many yards could he throw on the moon? If a boy can kick a football one hundred and thirty-five feet in the air here, how far could he kick it on the moon?