The Moon, from a photograph taken with the great telescope of the Lick Observatory.
The Earth as a Planet. —The children were looking at a map of the world one fine afternoon and studying the way the land and water are distributed, when Agnes said: "I never knew before how little land there was on the earth. Why, there is very much more water than land." "Oh, yes," said Tom, "there's very much more water on the surface; but it's all land at the bottom of the ocean. The sea is about three miles deep, you know, and then you come to the ocean bottom, and that is solid land again. The earth is nearly all rocks and soil; only a little of it is water, after all, but that little is on the surface, of course, and that is why it shows."
Agnes. So the earth is almost all land; if you dig down deep enough, you should come to rocks, even below the oceans?
Tom. Yes, and if you went up high enough, you would come to nothing. You would come to air first, and then by and by to no air, and then you would come to just nothing—to empty space.
Agnes. Well, it isn't quite empty, as you call it. There are other globes in space. There are other planets, and the sun and the moon, and there are simply thousands of stars. So space isn't empty; it is pretty full!
The Old World
Distance of the Moon and of the Sun from the Earth. —Here Tom's big brother Jack looked up from his book and said: "Well, that depends on what you call full. It is 240,000 miles from here to the moon, and the moon is the very nearest of all the heavenly bodies to us. There is a good deal of empty space between us and the moon, it seems to me."
Agnes. Two hundred and forty thousand miles! Oh, Jack, is that right?
Jack. Why, that isn't a beginning; how far off do you suppose the sun is? It is 93,000,000 miles—millions this time, not thousands; and some of the planets are much farther off yet, and every one of the stars is farther off still.
This picture shows the height of land on the earth compared to the depth of the sea. If you could cut the earth through and through with a knife and look at one part only, it would look something like the picture. All the shaded part is land. The curved line drawn all across the picture, near the top, is the curve of the surface of the oceans. Part of one of the oceans is shown by the white space below this curved line and above the floor of the ocean itself,—the shaded land. The curve of the ocean surface is continued across the picture underneath the mountains. If the surface of the earth were all water, the bounding line would be this curve. From side to side of the picture is about 350 miles. If the whole circle of the earth were drawn, it would be about eight feet in diameter. That is the scale of the drawing.
Agnes. Jack, tell us about it, will you? We don't know, and you do.
Jack. The very first thing you have to think about is the size of the earth. How far is it through and through the earth, Tom? If you pushed a stick through the earth from New York to China, how long would the stick be?
The Diameter of the Earth. —Tom. The geography says that the diameter of the earth is 8000 miles; so the stick would have to be 8000 miles long,—as long as from Cape Horn to Hudson Bay, my teacher says.
A Balloon—Balloons carrying men have gone up more than five miles, and small balloons carrying thermometers, etc., have been sent nearly ten miles high. The atmosphere of the earth extends upwards a hundred miles or so, but beyond this there is no air—nothing but empty space.
Jack. That's about right. Suppose there were a railway from Hudson Bay to Cape Horn, and express trains running on it at the rate of 40 miles an hour. Let us see how long they would take to go the 8000 miles. They would go 40 miles in one hour, and 80 miles in two hours, and 960 miles in a day—say 1000 miles a day. Well, they would take eight days to go the 8000 miles, then. Now, suppose we could build a railway to the moon. How long would an express train take to go the distance? Take your pencil, Tom, and cipher it out.
The Full Moon Rising in the East.
Tom. You said the distance from the earth to the moon was 240,000 miles. If the train goes 1000 miles a day, it would take 240 days. I don't need any pencil.
Jack. Sure enough; and 240 days is eight months (8 x 30 = 240). It would take the train eight months to go from the earth to the moon, then—eight whole months, traveling night and day at forty miles and more every hour.
Agnes. I should be nearly a year older when I got there than when I started, then.
Jack. Yes, and recollect that there are no stations on the railway to the moon. The moon is the heavenly body that is nearest to us, so that space is pretty nearly empty, after all.
A School Globe.
Distance of the Sun from the Earth. —Tom. How far did you say it was from the earth to the sun—93,000,000 miles?
Jack. That's right. You will need your pencil to figure out how long the express train would take to go from the earth to the sun, Tom.
Tom. Yes, it is like this, isn't it? The train goes 1000 miles in a day; then it will take 93,000 days to get to get to the sun.
30 * 93000 days
12 * 3100 months
258 1/2 years
It would take 3100 months, that is more than 258 years, to get to the sun. That's a long journey! You would have 258 birthdays on the road, Agnes.
Jack. Put it this way, Tom: 258 years ago takes you back to the year 1643 (1901 - 258 = 1643). The Pilgrims had been in New England only twenty-three years in 1643, for they came in 1620 (1643 - 1620 = 23). Suppose one of those Pilgrims to have stepped on to the train at Plymouth Rock; he would have been traveling all these years, and he would only have arrived at the sun a few years ago; that is, if he had lived to make the journey.
The Pilgrims landing on Plymouth Rock from their ship the "Mayflower," Dec. 20, 1620.
Tom. Two hundred and fifty-eight years!
The Planets Mercury and Venus. —Jack. Yes, and nearly all that space is empty too. There are only two planets between the earth and the sun—Mercury and Venus.
Agnes. Venus, the evening star?
Jack. Yes, Venus is the evening star sometimes. Venus and Mercury are the only planets that the Pilgrim would pass on the road from earth to the sun. Space is rather empty, isn't it?
Agnes. Aren't there any stars in between the earth and the sun, Jack?
Jack. Not one; the real stars are thousands and thousands of times farther off. We call Venus the "evening star," but Venus is not a star at all, but a planet. Let me tell you, so that you can make a sort of picture of it all in your minds. The sun is there in the middle of space and all the planets move around him, just as the earth does. Nearest to the sun is the planet Mercury, and then comes the planet Venus, and then the planet Earth.
Agnes. That sounds queerly—"the planet Earth"—though of course we know the Earth is a planet.
The Planets Mars, Jupiter, Saturn, Uranus, and Neptune. —Jack. Yes, exactly so. And then there are other planets farther away from the sun than the earth; Mars for one, and then Jupiter, and then Saturn, and then Uranus, and then Neptune. That is all we know of; there may be more of them. Neptune is thirty times as far from the sun as the earth is. Here is a little table that I will write down for you to keep. You need not memorize it, only recollect that Mercury and Venus are nearer to the sun than we are, and that all the others are farther away.
Distances of the Planets from the Sun
The planet Mercury is 36 million miles from the sun
The planet Venus is 67 million miles from the sun
The planet Earth is 93 million miles from the sun
The planet Mars is 141 million miles from the sun
The planet Jupiter is 483 million miles from the sun
The planet Saturn is 886 million miles from the sun
The planet Uranus is 1782 million miles from the sun
The planet Neptune is 2791 million miles from the sun
Jupiter is five times, and Neptune is thirty times, as far from the sun as the earth is.
Tom. Isn't there a map of all these planets that we can see?
Jack. No, and there's a good reason why. Suppose you tried to make a map of them, and suppose you took the distance from the sun to the Earth on the map to be an inch. Don't you see that the distance from the Sun to Neptune would have to be thirty times one inch, and the page of your book thirty inches wide—nearly a yard wide?
Tom. Of course, no book has a page as big as that; but you might make little maps.
How to Make a Map that shows the Sun and Planets. —Jack. You and Agnes can make a map yourselves to-morrow morning, if you want to, when you go out for a walk, and I'll tell you how to do it.
Suppose you take the large globe in the library, that you were looking at just now, to stand for the Sun. It is two feet in diameter. Well, the diameter of the real sun is 870,000 miles, and your map has to be made all to one scale. Every step of yours is about two feet long, isn't it, Tom? Try it.
Tom. Yes, my steps are almost exactly two feet long.
Jack. Well, remember to-morrow that every step you take along the road to the village is really two feet long, but that it stands on the map for 870,000 miles.
Agnes. Are we going to make the map along the road?
The Road to the Village.
Jack. My dear, you have to do it that way; your map is going to be nearly a mile and a quarter long. You have to use the whole country round to make it.
Agnes. Well, that is a map!
Tom. How shall we make it, Jack?
Jack. You start, you know, with this globe in the house to stand for the Sun. The globe is two feet in diameter, and the real Sun is 870,000 miles in diameter.
Scale of the Map. —"So, recollect, every two feet on your map is 870,000 miles. Every one of your steps, Tom, stands for 870,000 miles.
"You must take with you
a very small grain of canary-bird seed to stand for the planet Mercury
a very small green pea to stand for the planet Venus
a common green pea to stand for the planet Earth
a rather large pin out of Agnes' work box, and let its round head stand for the planet Mars
and orange to stand for the planet Jupiter
a golf ball to stand for the planet Saturn
a common marble to stand for the planet Uranus
a rather large marble to stand for the planet Neptune
Sizes of the Planets compared to the Sun. —"If this globe, two feet in diameter, stands for the Sun (which is really 870,000 miles in diameter), then a common green pea is just the right size to stand for the Earth (which is really 8000 miles in diameter) and an orange is just the right size to stand for Jupiter, and so on. You are going to carry all the planets off in your pocket, and when you have put them down in the right places you have made your map."
The sizes of the planets of the Solar System (the Sun's Family) compared with each other.
Tom. How shall we know where to put them down?
Jack. I will give you the right number of steps to take between the Sun and every one of the planets. If one of Tom's steps is 870,000 miles, then
Mercury (the canary seed) is 41 steps from the Sun (the globe at the house)
Venus (the small pea) is 77 steps from the globe that stands for the Sun
Earth (the pea) is 107 steps from the globe that stands for the Sun
Mars (the pin's head) is 162 steps from the globe that stands for the Sun
Jupiter (the orange) is 555 steps from the globe that stands for the Sun
Saturn (the golf ball) is 1019 steps from the globe that stands for the Sun
Uranus (the small marble) is 2048 steps from the globe that stands for the Sun
Neptune (the large marble) is 3208 steps from the globe that stands for the Sun
Those are the right distances, and you can make your map tomorrow morning when you go for a walk. Recollect that the globe in the house stands for the Sun. You are to walk away from it along the road to the village until you've take 41 steps. Stop there and put down the canary seed to stand for the planet Mercury. Then go on 36 steps more and you will be 77 steps from the model of the Sun. This will be the place to put the small green pea that stands for the planet Venus; then go on 30 steps more and you will be 107 steps away from the Sun. This will be the place to put down the green pea that stands for the Earth, and so on. The last planet—Neptune—will be 3208 steps away from the house,—about one and a fifth miles away.
Agnes. I don't believe we can count such large numbers, Jack; we shall be sure to forget them and lose the count.
Jack. True enough, Agnes. Let me see if I can't make it simpler for you. I will write down on a card all that you have to remember, and we can make the numbers that you have to count smaller. We can do it this way; instead of counting the distances from the Sun to each planet, we will count the number of steps between each planet and the next one: this way. Here is the card that Jack wrote:
If one of Tom's steps is 870,000 miles, then:
The distance from the model of the Sun to the canary seed that stands for the planet Mercury is 41 steps; the distance from Mercury to Venus is 36 steps farther; the distance from Venus to the Earth is 30 steps farther; the distance from the Earth to Mars is 55 steps farther; the distance from Mars to Jupiter is 393 steps farther; the distance from Jupiter to Saturn is 464 steps farther; the distance from Saturn to Uranus is 1029 steps farther; the distance from Uranus to Neptune is 1160 steps farther.
NOTE—The numbers that are needed to make the map are obtained in this way: If one step is 870,000 miles, then
the distance from the Sun to Mercury = 36,000,000 miles = 41 steps
the distance from Venus = 67,000,000 miles = 77 steps (36 steps difference)
the distance from Earth = 92,000,000 miles = 107 steps (30 steps difference)
the distance from Mars = 141,000,000 miles = 162 steps (55 steps difference)
the distance from Jupiter = 483,000,000 miles = 555 steps (393 steps difference)
the distance from Saturn = 886,000,000 miles = 1019 steps (464 steps difference)
the distance from Uranus = 1,782,000,000 miles = 2048 steps (1029 steps difference)
the distance from Neptune = 2,791,000,000 miles = 3208 steps (1160 steps difference)
In the last column are the differences between the numbers just preceding; 77 less 41 is 36, 107 less 77 is 30, 162 less 107 is 55, and so on. If the model of the planet Mercury must be 41 steps from the model of the Sun, and if the model of the planet Venus must be 77 steps from the Sun, then the model of Venus must be 30 steps away from the model of Mercury, and so on for the others.
When the next day came, Tom and Agnes set out to make the map of the Sun and all the planets. The school globe in the house stood for the Sun, and they carried the models of the planets with them, as well as the card that showed how far apart the planets were to be on the scale of their map. Agnes kept the card in her hand and told Tom how many steps he was to take. At the house she said: "Tom, you must take 41 steps, and then stop." So Tom walked off, counting his steps till he had made 41, then he put down the little canary seed that stood for the planet Mercury. The globe in the library stood for the Sun; this tiny seed stood for the planet Mercury; the distance from the globe to the seed stood for the real distance of the real planet Mercury from the real Sun. Thirty-six steps farther they put down the small green pea that stood for the planet Venus; and 30 steps farther still they put down the green pea that was to stand for the Earth.
Here they stopped for a minute to think about it all. This little bit of a green pea was the huge Earth, very, very much smaller than the globe that stood for the Sun. They could not even see the small green pea that stood for Venus, nor the little seed that stood for Mercury, though they knew about where they were, of course. There were no other planets in the real space between the real Earth and the real Sun exceptjust those two, Mercury and Venus, and space was almost empty, after all, as Jack had said, except for few, very few, planets that were exceedingly far apart. "Why, we can't even see the models of Mercury and Venus from here," said Agnes. "No," said Tom, "but if they were shining things, as the planets are, we could see them. They ought to be painted white so that the sunlight would make them glisten."
A plan of the orbits of Mercury, Venus, the Earth, and Mars.
So the children went on putting the models down in the road at the right distances apart. Agnes read the right numbers from the card, and Tom walked away counting his steps up to the thousands. He got rather tired of it, but they kept on until finally all the models were put down at the right distances apart, and their map was made. By this time they were nearly a mile and a quarter away from home, and they had spent the whole morning in the work. But the work was not wasted. They really understood what they had been doing, and realized, as very few people—even grown people—do, how immensely large space is, and how few—very few—planets there are to fill it.
A plan of the orbits of Mars, Jupiter, Saturn, Uranus, and Neptune. (The scale of this drawing is much smaller than that of the preceding one.)
When the children came home that day there was a great deal of talk about the map—the model—that they had made. All the older people and some of the neighbors were interested in it. They found their work had not been wasted and that they had really learned something.
The Solar System; the Sun and Planets. —Jack told them some interesting things about the sun and the planets. They knew already, of course, that all the planets moved round the sun in paths that were called orbits. The earth, for instance, goes once round the sun every year,—every 365 1/4 days. Every one of the planets goes round the sun, too, in its own particular orbit, in its own year. For instance,
Mercury goes round the Sun in 88 days = about 3 months
Venus goes round the Sun in 225 days = about 7 months
Earth goes round the Sun in 365 days = about 12 months
Mars goes round the Sun in 687 days = about 23 months
Jupiter goes round the Sun in 12 years
Saturn goes round the Sun in 29 years
Uranus goes round the Sun in 84 years
Neptune goes round the Sun in 165 years
Tom's father told them about one of the kings of Spain who, long ago, used to play chess on a huge chessboard with real living persons for chessmen. These men moved from square to square on the chessboard as the game went on; and Tom's father said that the map of the solar system with its eight planets ought to have had eight little boys who would walk in circles round the model of the sun, carrying the models of the planets in their hands. One boy would carry the canary seed that stood for Mercury, and he would have to walk once round his circle in three months; another boy would carry the small green pea that stood for Venus, and he would have to walk around a larger circle once in seven months; still another would carry the green pea that stood for the Earth, and he would have to walk around the circle of the Earth's orbit once in each year; and so on for all the other planets. The boy that carried the marble that stood for Neptune would not get all the way around his circle for 165 years. "He would be quite grown up by the time he got round, wouldn't he?" said Agnes. "Well," said Jack, "Papa is right; that is the real way to make the model. The sun is in the middle. All the planets move round him in circles; each one of the planets takes a different time to go once around its orbit. All of these planets together make up the solar system,—the family of the sun."
In this picture the large circle stands for the sun. Each of the small dots stands for the earth. The size of the dots and of the circle are in the right proportion. It would take 109 earths in a row stretched across the disk of the sun to reach from edge to edge. Count them.
Three drawings of Jupiter as seen in a telescope. The lower drawing shows Jupiter with his four bright satellites. It is on a smaller scale than the others.
Tom. Why do they call it the solar system, Jack?
Jack. Just because it is the sun's system; sol, in Latin, means "the sun," and solar means "belonging to the sun." All the planets go round the sun, and round nothing else. That's why. The sun is so much larger than any of the planets, or than all of them put together for that matter, that it is the sun's system.
Relative Sizes of the Planets. —"You see," said Jack, "that the sun is very large indeed. He is as much larger than the earth as the library globe is larger than a green pea. If all the solar system were to shrink and shrink until the earth—the huge earth—had shrunk to the size of one green pea, the sun would still be as big as the globe in the library—it would be two feet in diameter."
The real diameters of the sun and planets are:
The Sun is 866,400 miles in diameter
The smaller planets
Mercury is 3,030 miles in diameter
Venus is 7,700 miles in diameter
The Earth is 7,918 miles in diameter
The Moon is 2,162 miles in diameter
Mars is 4,230 miles in diameter
The giant planets
Jupiter is 86,500 miles in diameter
Saturn is 73,000 miles in diameter
Uranus is 31,900 miles in diameter
Neptune is 34,800 miles in diameter
"Oh!" said Agnes, "we left the Moon out of our model."
"So we did," said Tom; "let us go this afternoon and stick a pin in the ground to stand for the Moon, alongside of the green pea that stands for the Earth."
Drawings of the planet Saturn as seen in a telescope at different times. In the upper figure we are looking at Saturn's rings edgewise, and they appear as a thin line. In the next drawing we are looking down on the rings. In the third drawing we are also looking down on the rings.
The Moons of the Planets. —"Well," said Jack, "that's all right. Only you must choose a pin with a very small head. And, while you are about it, you had better put in some more pins, for several of the other planets have moons—satellites, they are called—and they go around their planets just as the Moon goes around the Earth. Mercury has no satellite that we know of; Venus has no satellite that we know of; the Earth has the Moon for satellite; Mars has two very small satellites; Jupiter has four large satellites about the size of our Moon, and five extremely small ones; Saturn has nine satellites, one larger than our Moon; Uranus has four satellites; Neptune has one satellite almost the same size as our Moon."
The Starlit Sky.
The Great Comet of 1858.
The Minor Planets; the Asteroids. —"Yes, and at the same time you might as well sprinkle about 500 grains of sand in the space between Mars and Jupiter to stand for the 500 minor planets that they call asteroids. There are about 500 of them known now, and, I've no doubt, hundreds more not yet discovered. When you read in the newspaper that a new planet was discovered last night by some astronomer, that means that another one of these minor planets has been found. They find them by photography with a large telescope."
Comets. —"And, by the way, put in two or three thin wisps of cotton wool somewhere to stand for comets. Comets are mostly made out of shining gas—they aren't solid. But they look a little like wisps of cotton wool, anyway."
Tom. Is that all? Shall we put in anything else?
Jack. That is all for the solar system, except clouds of very little stones, almost like dust, that make the shooting stars or meteors.
The Stars. —"What about the stars?" said Agnes.
Jack. Oh, the stars are not part of the solar system, Agnes; they are millions and millions of miles outside of it; the very nearest star is thousands and thousands of times farther from us than even the planet Neptune.
Tom. How far off are they, Jack, anyway? Could we get the nearest of the stars on our model? Where would it be? In the next country?
Distances of the Stars. —"Let me see," said Jack, "the nearest star of all is 20,000,000,000,000 miles from the sun— twenty millions of millions of miles! If you were to put it on your map, it would have to be about 9000 miles from where we are now—it would have to be somewhere in China."
Agnes. Is that the nearest star, Jack?
Jack. Yes, the very nearest. If you should put another school globe in the Chinese emperor's palace at Peking, that would stand for the nearest star to our sun, which our school globe in the library stands for. The sun is a star, and stars are about of the same size. So a school globe may stand for any one of them.
Tom. Well, space is empty if planets and stars aren't any closer than that. What is the difference between a planet and a star, anyway?
What is a Planet? —"The greatest difference," said Jack, "is this: the stars shine by their own light, just as an electric street lamp shines; and the planets shine by light reflected from the sun, just as a football would shine if you held it up in the sunlight."
Tom. Do you mean that Venus and Jupiter do not shine by their own light?
Jack. I mean just that. Venus and Jupiter are two great globes something like the earth, made out of rocks and soil, with clouds all around them—clouds something like our clouds. The sun shines on them, and they shine, and we see them. If the sun were to stop shining on them, they'd go out like a candle.
Agnes. But, Jack, Venus shines at night, in the dark sky, when the sun has stopped shining.
Jack. The sun has stopped shining on you and me at night because the earth has turned round and we are in the earth's shadow; you know that. But all the while the sun is shining just the same. It is shining on the other side of the earth, where it is daytime, and it is sending out sunbeams above the earth and below it, everywhere all the time. Some of these sunbeams fall on Jupiter and Venus and make them bright, and we see them. What we really see is the sun's brightness reflected back to us, just as you might see an electric light at night shining on a mirror. You might be in the dark yourself; the electric light might be round the corner of the street, but the mirror would be bright.
Tom. So planets are bright because the sun shines on them. Why are the stars bright then?
Jack. Stars are bright just as the sun is bright. The sun makes its own light as an electric lamp makes its own light. The stars are like the sun. They shine by their own light. Planets shine by borrowed light. They borrow their light from the sun. If you were to go off and sit on the nearest star and look at the solar system, you might see the sun in the middle of it shining away all the time—all day and all night, too. And if you could see our little group of eight planets wheeling around it, they would be bright on the side nearest the sun—on the side shined upon; and be dark on the side away from the sun. The sunlight cannot go through them. The sun can shine only on that part of a planet that is turned towards it.
Phases of the Moon (New Moon, Full Moon, etc.). —"Don't you know the moon is often only half bright, and sometimes three-quarters bright, and so on? Venus looks that way in a telescope sometimes; in a telescope you can see Venus like a crescent moon—like a sickle. You do not see it like that with your eye, because Venus is so bright that your eyes are dazzled. You see the glare, and it looks like any other dazzling glare; you do not see its true shape."
Tom. You can't see the true shape of a sheet of tin that the sun shines on; it looks just like a dazzle of light.
The New Moon Setting in the West.
The Moon in the First Quarter.
Fred Watching the Full Moon Rise in the East.
Jack. That is the way with the planets when you do not use a telescope. Now the moon looks so large, and the light from any part of it is so faint, that you can see its shape. It does not dazzle your eyes. They call those different shapes of the bright part of the moon its phases. Venus has phases, too. The moon is a globe, you know, about 2000 miles in diameter. One half of it is always turned towards the sun, and that half of it is always bright, day and night. If we were on the sun, we should always see the whole circle of the moon bright. But we are on the earth, and the bright part of the moon is not always turned towards us. We see only so much of the bright part as is turned towards us—so much and no more.
A schoolroom experiment to show how the sun lights up half of every one of the planets, and only half. The room should be darkened; the lamp should have a ground-glass shade; the orange that stands for the earth or planet should be fastened by a knitting needle to a pincushion. The pupils should see that half, and only half, of a globe (a planet, the earth, the moon) is illuminated. They should also see that by going to different parts of the room different portions (phases) of the illuminated part are visible. The phases of the moon can be explained by this experiment. Half of the moon is lighted by the sun; all of the illuminated half that is turned towards the earth is seen bright; the moon moves round the earth and turns different parts to it at different times.
Agnes. Sometimes we see the whole circle of the moon bright—at full moon.
Jack. Yes, we see it so when the sun is setting in the west and the moon rising in the east. The sun is shining full on the moon, and the bright half of the moon is turned full towards us.
Tom. When the moon is a sickle it is often in the west, not far from the sun about sunset.
Jack. That is the phase we call new moon.
Tom. The moon goes round the earth, doesn't it?
This picture shows why the moon's disk has different shapes at different times. The sun is supposed to be far away in the direction of the top of the page. It shines on the earth and lights half of it. It is night on the unlighted half of the earth. The moon goes around the earth in its orbit in the direction of the arrow. Wherever the moon is, one half of it is lighted—the half turned towards the sun. A person on the earth sees one half of the moon—the half turned towards him. The little circles outside the orbit in the picture show the shape that the bright part of the moon will have at new moon, full moon, etc.
Jack. It goes round the earth once in every month. The moon's month begins when the moon is a new moon. Every night the bright part gets larger, and in about a week, a quarter of a month, we see a quarter of the moon bright; that is the first quarter. Two weeks after the new moon the full moon comes; and a week later comes a moon that is only partly bright again; that is the third quarter. By and by, in four weeks, comes another new moon, and so on forever.
Agnes. One of my storybooks says the old moons are cut up to make stars out of. They wouldn't be bright enough, would they?
Jack. Not exactly. Stars are the brightest things there are except the sun, which is the very brightest thing we know.
Agnes. There are faint stars, though—some that you can scarcely see.
Tom. They are faint only because they are far off. If you were near them, they would be bright like the sun.
Jack. That's right. The stars are suns, and our sun is a star. All of them are really very much alike, though the stars do not look at all as the sun does. The sun looks large, and it is hot, because it is close to us. The stars look small because they are so far off, and we get no heat at all from them, though we get light. You know you can see the light of a lamp much farther than you can feel its heat.
Number of the Stars. —Agnes. There are thousands and thousands of stars, Jack; do you know how many there are?
Jack. There are about 6000 stars that you can see with the naked eye, not more; and you cannot see all those at once. Probably you never see more than a couple of thousands at any one time.
Agnes. Why, there seem to be many more than 2000.
Jack. Well, my dear, the only way to know is to count them. And the astronomers have counted them, and made maps that show every one of them by a little dot. That is the way they know how many there are. But if you take an opera glass, you can see very many more; and if you take a telescope, you can see thousands and thousands. The largest telescopes that we have will show perhaps a hundred million stars. The brightest stars are nearest to us, and the faint ones are very far away indeed—inconceivably far, in fact.
Tom. You said the nearest star was as far away from the sun on our map as New York is from Peking. Are all the stars as far apart as that? Aren't some of them close together?
Clusters of Stars. —Jack. Well, there are some groups of stars fairly close together; but generally one star is about as far from the star nearest to it as our sun is from the nearest star. If you were making a map of the whole universe, you would begin by making a model of the solar system just as you did yesterday. The library globe would stand for the sun, which is one of the stars, you know. The nearest star to it would be shown on the map by a globe set down at Peking, 8000 miles away from us, and 8000 miles from Peking there would be another globe, and 8000 miles farther another one, and so on. Every 8000 miles on your map there would be a globe to stand for a star, and there would be at least a hundred million globes on your map of the universe, because, you know, the telescopes show us at least a hundred million stars. Of course these stars are scattered all around us; they aren't in a straight line one after another, but they are scattered all over the surface of the night sky.
The Group of Stars Called the Pleiades (The six brightest stars can be seen with the naked eye. To see the others a small telescope must be used. The Pleiades may be seen high up in the sky and to the south of the point overhead about 10 P.M. December 21, about 9 P.M. January 5, about 8 P.M. January 20, every year. Or you may see them rising to the north of the east point of your horizon about 10 P.M. August 23, about 9 P.M. September 8, about 8 P.M. September 23.)
The stars in space are arranged somewhat as in the picture. On the whole, no one of them is nearer to any other one than the sun is to the nearest star,—20,000,000,000,000 miles. The Sun is just one out of a countless number of stars—one out of millions. No one of the planets of the solar system can be seen from the nearest of the stars.
Agnes. The planets move around the sun; do the stars move around the sun, too?
Jack. No, they are so far off from us that the sun has nothing to do with them, nor they with the sun. The sun has its own family of planets, and it is possible that the stars—which are suns—have their own planets, too; but we do not know whether they have or not.
Agnes. Why don't you know, Jack?
Jack. Because the stars are so far away. We can see the stars like bright shining points in the sky. They shine by their own light and are bright. Now suppose any one of the stars really had a family of planets around it. Those planets would shine by the light from that star, and they would be faint, much too faint for us to see, even if the planets were really there; and the only way to know about stars and planets is to see them; you cannot touch them or hear them. If you cannot see a planet it does not exist, so far as you know.
A photograph of a part of the Milky Way. Each little dot in the picture is a star, and there are thousands of them even on one photographic plate. You can see the Milky Way like a bright belt in the sky—a belt made of stars—overhead early in the evenings of August and September or of November, December, and January, or parallel to the northern horizon early in the evenings of April and May.
Tom. Couldn't a man on the nearest star, looking at our sun, see the planets of our system,—Venus and Jupiter?
Jack. No, indeed; he would see our sun, but the light of our planets would be too faint. He could not possibly see them.
Do the Stars have Planets as the Sundoes? —Tom. You say you don't know whether the stars have planets round them. What do you think about it? Haven't you any idea?
Jack. There is a great deal of difference between knowing and thinking. I certainly do not know that the stars have planets, for I have never seen them. But I do think that it is very likely that they have families of planets, just as the sun has. I think it is likely—very likely; but I don't know.
The Starlit Sky.
Tom. And do you think those planets, if there are any, have people on them? Are they inhabited as the earth is?
Jack. That is a hard question. In the first place, it is not certain that there are any planets around the stars, and then it is a mere guess whether there could be inhabitants on them. That is one of the questions we shall have to give up. It is too difficult.
Agnes. I am going to believe that every star has planets round it, just as the sun has.
Jack. Well, that is reasonable enough. Very likely you are right. Who knows?
Agnes. And I am going to believe that some of these planets round the stars have men on them.
The Shower of Shooting Stars seen on Nov. 13, 1866 The round dots stand for stars; the arrows for the tracks of meteors that were seen. Notice that nearly all the meteors radiated from a spot near the center of the picture.
Jack. I can't say you're wrong; I can't prove that you are wrong. Who knows? You can believe what you like about it. Wait till we know more.
Shooting Stars; Meteors; Fireballs. —On the night of August 10 the children stayed up late to watch the shooting stars that are regularly seen every year on that particular night. On almost any night that is clear any one who will watch for an hour will see a dozen or more; and the easiest way to understand what they are like is to watch for them. In the country, where the sky is dark and where there are no electric lights, it is not hard to see them. In the city it is not so simple; the sky is too bright and the street lamps interfere too much. Any one can see the stars. If one of the stars should suddenly get brighter and move quickly away from its place and then suddenly disappear, as if it had been blown out like a candle, it would look just as the shooting stars do. The real stars stay in the same place night after night, year after year, century after century. They are called fixed stars because they are fixed in their places. The shooting stars are small pieces of stone or iron that are moving about in space, as the planets move. One of these pieces comes near to the earth and falls to the ground just as a stone falls. It moves rapidly through the air and gets hot, as your hand will get hot if you move it very rapidly to and fro on your desk. The shooting star moves very fast and gets very hot indeed—hot enough to burn. Usually the meteors (shooting stars) get so hot in their flight through the air that they are quite burned up before they reach the ground. Sometimes a piece of iron falls and is picked up. The picture shows a piece of the sort. Fig. 32 shows how such a meteor (a very large one—much large than a shooting star) looks as it is falling.
The Great meteor that fell in California in 1894.
A meteoric stone that fell in Iowa in 1875.
The Zodiacal Light—The best time to see it in the United States is in February, March, and April in the early evening, above the western horizon.
The Zodiacal Light. —Space is full of such meteors, most of them small, like dust. The sun shines on them, and you can often see a triangle of faint light or glow, which is called the zodiacal light. If you live in the country, where the sky is dark, be on the lookout for it. The street lamps of the city make the sky entirely too bright for you to see it in towns.
Nebulae. —Nebula, in Latin, means cloud; and nebulæ is the plural. There are several spots in the sky that, even with the naked eye, on a clear night look as if the stars in those spots were covered with a thin veil of cloud. When these spots are looked at with a telescope you see bright forms like those in the pictures Figures 35 and 53, and they are, in fact, bright clouds of gas and small particles of dust. They shine by their own light.
The Great Nebula in Andromeda, from a Photograph made with a Telescope
Rising and Setting of the Sun. —Tom. We know that the sun rises in the east every day—
Agnes. And goes across the sky and sets in the west.
Jack. Why does it? Does the sun really move?
Agnes. No; the earth turns round and the sun stands still; but the sun seems to move.
Jack. The sun seems to move across the sky from rising to setting every day; the moon does the same thing; each one of the thousands of stars rises and then sets every night. There are just two ways to explain these things. Either the earth stands still and all these different heavenly bodies really move around it—every one of them—in twenty-four hours, or the heavenly bodies stand still and the earth turns round on its axis every day. The last explanation is the true one, as you know very well, and so we have to say the sun appears to move from rising to setting (for the sun really does not move at all); and we have to say the stars appear to move from rising to setting (for the stars do not really move at all). It is the earth that turns, and as it turns everything in the sky appears to move from east to west.
The Setting Sun.
The Way the Sun seems to move from Rising to Setting—The man in the picture is looking towards the south, and his arms are stretched out to the east and to the west. If he stood there all day, he would see the sun rise above the horizon in the east, gradually rise higher and higher and be highest at noon, just to the south, and then decline towards the west and set in the west at the end of the day. The dotted line shows the apparent motion of the sun. The picture was drawn at about three o'clock in the afternoon. Why? Because the sun in the picture is where the real sun will be every day about three o'clock.
The Celestial Sphere. —"Think of it this way. You are on a globe—the earth—that turns around every twenty-four hours. Above you is the sky. It looks exactly as if it were a hollow globe, and as if you were inside of it. In the night-time the stars look like little shining marks fastened to the hollow globe all around you. In the daytime the sun (and sometimes the moon) seems to be fastened to the inside of the hollow globe of the sky. We call the hollow globe of the sky the celestial sphere. You are in the middle of it, and you see all the stars at night slowly moving from rising towards setting. The celestial sphere is the surface of the sky to which the sun, moon, and stars appear to be fastened. They look as if they were fastened there, anyway. They all seem to be at the same distance."
Tom. They can't all be fastened to any one sphere, because they are at very different distances from us. The sun is very much further away from us than the moon, and the stars are much further off than the sun.
The Celestial Sphere (The Hollow Globe on Whose Inner Surface all Stars seem to Lie) The earth is supposed to be at O, and some stars at p, q, r, s, t, t, t, u, v. You see the stars as if they were all projected on the celestial sphere at P, Q, R, S, T, U, V. You think of them as if they were all at the same distance from you.
Jack. True enough. If you will look at this picture I am drawing, you will see how it is. You are supposed to be in the middle of the celestial sphere at O. The earth is at O (Fig. 38), and you are on it. All around you are stars, p, q, r, s, etc. You see the star q along the line Oq—along the line that joins your eye and the star. The line seems to pierce the celestial sphere at Q, and you think the star q is really at Q. In the same way you think the star r is at R, the star s at S, and so forth. If there were really three stars, t, t, t, all in one line, Ot, you would see only one star at T. All the stars seem to be lying on the surface of some sphere, and all of them seem equally far away.
This picture shows the northern sky as it appears in the early hours of the evening every August to people who live in the United States. If you face north, you see the horizon (Pronounced ho-RYE-zon.): the surface of the ground. Above that comes the sky with many stars in it. Towards the west and pretty high up is the dipper: The Great Bear (Ursa Major). Two of its stars: the pointers: point at the north star: Polaris, (Pronounced po-LAY-ris.) it is called. High in the east is Cassiopeia, (Pronounced kas-ee-oh-PEE-ya.) a group that is sometimes called The Lady in the Chair. Every child that owns this book should try to find these stars. They are always there, in the north. If he looks in August they will be just as in the picture. If he looks in other months the book must be turned a little. By taking a little pains the book can be held so that the picture will look as the stars do.
Tom. That is true, I know. When I look at the stars at night they certainly do seem to be all at one distance—just like shining tacks driven into a darkish globe above my head and all around me.
Agnes. And in the daytime the sun and, sometimes, the moon seem to be the same way—shining circles fastened on to a shining globe.
Jack. Of course there isn't any real globe there. It is only an appearance. But it looks real, and we have a name for the appearance because it is convenient to have names for things we always see, or even for things that we always think that we see.
Tom. You would have a model of the celestial sphere by making a huge hollow globe as big as a barn and getting inside of it.
Agnes. Yes, and by lining it with black velvet and driving bright-headed tacks into the lining for stars; only you would have to drive them in the right places.
Jack. A model like that would be worth making, but it would be expensive. We shall have to do with pictures and flat maps. They will explain what we really see in the sky.
The next night Jack took the children out of doors. He made them face towards the north; the east was on their right hand, the west on their left. First of all he showed them the Dipper—the Great Bear (Ursa Major in Latin)—and the pointers.
The Dipper is made up of seven bright stars and is always easy to find. Three of its stars make the handle, four make the bowl, and two stars of the bowl are the pointers. After you have found the pointers it is easy to find the polestar. Now if you imagine a line drawn from the polestar to the center of the earth (under your feet), that line will be the axis of the earth. The earth turns round that line every day. Every part of the axis itself stands still, and every point not in the axis moves. The center of the earth stands still while the earth turns; and Polaris stands still. All the parts of the earth not on the axis appear to move, and all the stars except Polaris appear to move—they move from rising to setting and back to rising again. The stars in the east move upwards, then over the pole towards the west, and then downwards (in the direction of the little arrows in Figs. 39 and 40).
The Dipper—the Great Bear—as it appears at Different Times—Sometimes it is above the pole, sometimes below it; but if you lay ruler on the picture, you will see that the pointers always point to the north star—Polaris.
Jack kept the children out of doors till long after their bedtime to let them see the stars rise higher and higher, but finally they had to go to bed. They could not watch any longer.
On the next night Jack showed the children how the southern stars appeared to move from rising to setting. He took them out into a large open field and made them face towards the south. The east was on their left hand, the west on their right hand, and the stars appeared to move from east to west,—from rising towards setting—just as the sun does. The apparent motion of all the stars—of the south stars as well as of the north stars—is caused by one thing and one thing only. The earth turns round on its axis underneath the starry sky.
A photograph of a part of the northern sky near the pole. A camera was pointed at the pole early in the evening and the plate was exposed all night and only shut off at daybreak. Each star moved about half of its course round the pole, and as it moved it left a trail on the plate. All the trails in the picture are half circles. The star Polaris is not exactly at the north pole of the heavens (though it happens to be pretty near it). Its trail is the brightest one on the plate. The other stars left their trails, too.
A photograph of a part of the southern sky, showing the trails of southern stars as they moved across the plate from rising towards setting. This photograph, and the one like it for the northern stars, prove that the stars really move with respect to the photographic plate. But it is not the stars that move. The plate moves with the earth as the earth turns round its axis. The stars stand still.
Time and Timekeeping. —We use the apparent motion of the sun from rising to setting to give us the time. Watches and clocks all over the world are now regulated by the sun. Long ago the ancients used to tell their time by the stars. They would say: "You must begin your journey when Pleiades are rising"; just as we might say: "I must take the train at 9 P.M." Groups of stars, like the Pleiades, were the moving clock hands; the dial was the celestial sphere. The stars moved steadily across the dial, and their motion told the hour. The sun moves regularly and steadily from rising to setting. When it is highest up in the heavens and exactly south of any place (a city, a town, any place), then it is noon at that particular place. Twelve hours later it is midnight; and twelve hours later than midnight it is noon again—noon of the next day of the week. A watch is a little machine arranged to drive a steel hand round a dial in twelve hours. The hand is set so as to mark XII o'clock at noon, and the machine is regulated so that when the next noon comes the hand shall be at XII again. To set our watches exactly, we must have a north and south line. Astronomers have a particular kind of telescope set exactly in the north and south line (the meridian), so that they can observe the exact instant of noon. Their watches are corrected so as to mark XII o'clock just at that moment; and made to run so that when the next noon comes they will mark XII o'clock again. They have other kinds of telescopes also, especially made to examine distant planets and to discover what is to be seen on their surfaces.
Telescopes. —The children were playing with a reading glass that belonged to their father. Tom used it to light a match with, and then to look at the wings of a fly, and noticed how it magnified everything—how it made it look much larger.
Then he said: "Jack, what is the difference between this magnifying glass and a telescope? Both of them magnify."
Jack. Well, the telescope magnifies very much more, for one thing; and a telescope is made up of more than one lens. The burning glass has only one.
A Meridian Circle—The eye end of the telescope is at M. The telescope is fastened to a horizontal axis which lies in an east and west line, and the telescope always remains, therefore, in the meridian. LL is a level by which the axis is made horizontal. The axis has two circles (H and K) fastened to it. These circles are divided into 360 degrees, and by them we can measure the altitude (height), of any star.
Jack took the burning glass and showed the children how to use it to make an image (a picture) of the window on the wall, as in Fig. 45.
Jack. You see that this glass makes an image of the window on the wall. Suppose that we should cut a hole in the wall just where the image is now. The image would be there just the same, for if you put a piece of white paper over the hole the image would show on the paper as it now does on the wall. Now suppose that you were in the other room beyond the wall and held another burning glass in just the right place to magnify the image in the hole. The second burning glass magnifies everything it looks at; well, you could use it to magnify the image formed by the first burning glass. If you did this, you would have a telescope. Two lenses combined so as to form a magnified image of any object make a telescope. One lens alone is not a telescope; it is a magnifying glass.
Agnes. Then a telescope must have two glasses?
Jack. Yes, two at least; the first glass forms an image of the thing you are looking at—a picture of the window, for instance. The second glass magnifies the image so that you can see it better and see it larger. All opera glasses and spyglasses have at least two lenses, usually more than two.
A Reading Glass, A Magnifying Lens, or a Burning Glass.
Tom. Here is a drawing of the great telescope of the Lick Observatory (Fig. 46). Where are the two glasses there?
Jack. One of them is the upper end of the long steel tube; they call it the object glass, because it is nearest the object you are lookng at. The other glass is at the other end of the tube; they call it the eyepiece, because it is next your eye. In the drawing you see a man looking through the eyepiece.
Agnes. But the telescope is inside a house, Jack. How can the astronomer see anything?
If you hold a burning glass in a room, you can make it form an image (a picture) of the window on the opposite wall. The image will be clear and distinct, but it will be upside down, as you can prove by trying. Most lenses will need to be held nearer the wall than that in the figure.
Tom. Why, you know, Agnes, that there is a long window in the dome that is opened when they want to look out to see anything. The telescope looks out through the open window.
Agnes. What is the long tube for?
Jack. It is principally to keep the object glass and the eyepiece at exactly the right distance apart and to hold them steadily where you want them.
Tom. The tube is on an iron stand, and you can go to the top of the stand by a winding stairway. What are those big circles at the top, Jack?
Jack. The circles are fastened to the telescope, Tom, not to the iron stand, you see; and they are arranged to show the latitude and the longitude of the particular star that the telescope is pointed at.
Agnes. Do they know the latitudes and the longitudes of stars?
Jack. Yes, that is the way they point at them. If I tell you to find on the map a town that has a latitude of 41 degrees and a longitude of 80 degrees you can find it, can't you?
Agnes. Here is the map, and the town is Pittsburg.
Jack. Well, the astronomers have maps of the stars, and they find the star they want by knowing its latitude and longitude, and by pointing the telescope there.
Tom. But the star would be moving from rising to setting. How do they manage to follow it?
Jack. If you will look at the drawing of the telescope (Fig. 46), you'll see a piece of machinery in the top part of the stand. It is really a powerful clock. That clock is arranged so as to move the telescope towards the west exactly as fast as the star moves toward the west. When you once have the star in the telescope the clock keeps it there.
Agnes. How large is the object glass of the Lick telescope?
The Great Telescope of the Lick Observatory.Its object glass is three feet in diameter, and it is nearly sixty feet long.
Jack. The object glass is three feet in diameter, and the tube is nearly sixty feet long, and the eyepiece is quite small—just the size to be convenient for your eye to see through, Agnes.
Tom. How much can you magnify with a telescope like that?
The Moon. —Jack. Well, you can arrange so as to magnify more or less as you please. For instance, you can magnify the moon about a thousand times—you can see the moon as if it were a thousand times nearer than it really is. How far off did I tell you the moon is?
Tom. Two hundred and forty thousand miles.
Jack. Then if the telescope will make it seem a thousand times nearer, how far off will it seem to be?
Agnes. Two hundred and forty miles.
Jack. That's right, my dear. The Lick telescope will show you the moon just as you would see it if you got within two hundred and forty miles of it—just as if the moon were at Pittsburg and you at Philadelphia.
Agnes. That does not seem very near.
Jack. Well, it isn't near; but it is wonderful to do even so well as that.
Mountains on the moon as seen in a telescope.
Tom. Then the planets, that are so much farther away than the moon, cannot be seen anything like so well?
Jack. No; Mars, for instance, is 50,000,000 miles away from us when we see it best, and so we never can make it seem nearer to us than 50,000 miles. That is better than nothing; but it isn't very close, after all. It is really wonderful that men have found out so much as they have about the planets when you consider what the difficulties are. The smallest spot that can be seen with distinctness on the moon would contain several acres; and when you come to looking at a distant planet like Mars a spot would have to be fifty or sixty miles square to be visible at all.
Tom. Then you might see a city on the moon? A city covers many acres.
Jack. You could see a city on the moon if it were there; or even a very large building like the Capitol at Washington; but there are no such cities or buildings on the moon. Astronomers have looked for them thousands of times without ever finding the slightest sign of any living thing.
Life on the Planets. —Agnes. Is there any sign of life on the planets?
Mountains on the moon as seen in a telescope.
Jack. Not one; life of some sort may be there—plants, trees, animals, or possibly men—but the telescope shows no sign of life at all.
Tom. Not even on Mars?
Jack. Not even on Mars—nowhere. Some people have talked about land and water on Mars, calling parts of Mars that are reddish, land, and parts that are bluish, water; but no one has any proof at all that the red parts are really land, or the blue parts water.
Tom. I have read about canals in Mars.
Jack. Well, whatever they are, they are not canals. The telescope shows narrow, straight, dark lines on the planet's surface (see Figure 49), and they were called canals because they crossed the red parts of Mars that were called continents. But the Lick telescope shows that the canals go across the oceans, just as they go across the continents; so that it is pretty clear that the canals are not canals at all, and that we do not know whether Mars has any water on its surface at all.
Tom. How is it about Jupiter?
Jack. Jupiter looks as if it were a very hot planet; like a huge red-hot ball covered with clouds of steam. All of Saturn that we can see seems to be clouds; and the same is true for Uranus and Neptune, and for Venus, too, for that matter. Mercury and Mars have no clouds and probably little or no atmosphere at all. All the others have atmospheres, but no one knows whether their air is the right kind of air to breathe. It is very doubtful whether any planet beside the earth is fit for men to live on.
Tom. Is there air on the moon?
Jack. There is no air on the moon at all, nor any water either; and it is so cold on the moon, and on Mars too, that no man could possibly live there for an instant.
Tom. Then there isn't any place in the whole universe where we are really sure that men can live except just the earth?
Jack. No. Men cannot live in the sun; the sun is too hot. Jupiter is too hot, also. Mercury and Mars have little or no air. Venus, Saturn, and Uranus, and Neptune are covered with clouds, and we do not know what is underneath the clouds. Men couldn't live in the stars; they are like the sun—too hot. And we do not know whether the stars have planets round them or not; very likely they have. If they have, some of their planets may be fit for men to live on. Agnes says she is going to believe it.
Agnes. Yes, I am. It makes the universe more interesting to believe that there are people like ourselves everywhere, or at least in many places.
Jack. Well, believe it, my dear. I half believe it myself; but there is no way to prove it, or to disprove it, for that matter.
Drawings showing two hemispheres of the planet Mars. The narrow lines are what have been called canals. The dark parts of the drawing should be colored blue and most of the white parts reddish in order to make it look as Mars does.
THE EARTH The earth is a globe flattened at the poles. Its shortest diameter (from pole to pole) is 7900 miles. Its longest diameter is 7927 miles. It turns on its axis once daily. It moves in its orbit round the sun once in a year of 365 days 5 hours 48 minutes 45 1/2 seconds. Its month (from new moon to new moon) is 29 1/2 days. The earth is 5 1/2 times as heavy as a globe of water of the same size. The sun weighs 333,000 times more than the earth. The distance from the earth to the sun is 93,000,000 miles.
THE MOON The moon is 2163 miles in diameter. The moon weighs about 3 1/2 times as much as a globe of water of the same size. The earth weighs 81 times as much as the moon. The distance from the moon to the earth is 240,000 miles.
ECLIPSES OF THE SUN AND MOON They are explained in Book II (Physics).
THE PLANET MERCURY Mercury is 3030 miles in diameter. It weighs about 3 1/8 times as much as a globe of water of the same size. It goes once round the sun every 88 days. It is 36,000,000 miles distant from the sun—less than 4/10 of the earth's distance, therefore.
THE PLANET VENUS Venus is 7700 miles in diameter (about the size of the earth, therefore). It weighs 4 8/10 times as much as a globe of water of the same size. It goes round the sun once every 225 days. It is 67,200,000 miles distant from the sun—about 7/10 of the earth's distance, therefore.
THE PLANET MARS Mars is 4230 miles in diameter. It weighs 4 times as much as a globe of water of the same size. It turns once on its axis in 24 hours 37 minutes 22 67/100 seconds. It goes round the sun once every 687 days. It is 141,500,000 miles from the sun—about 1 1/2 times the earth's distance, therefore. It has two very small moons.
A rough drawing of the full moon.
JUPITER Jupiter is 86,500 miles in diameter. It weighs only 1 3/10 times as much as a globe of water of the same size. It turns once on its axis in 9 hours 55 minutes. It goes round the sun once every 11 9/10 years. It is 483,300,000 miles from the sun—about 5 times the earth's distance, therefore. It has nine moons. Five of them are very small; the others much larger—about the size of our own moon, or of the planet Mars.
THE PLANET SATURN Saturn is made up of a globe with rings around it. The diameter of its globe is 73,000 miles. It weighs only 7/10 as much as a globe of water of the same size. The globe turns on its axis every 10 hours 14 minutes 24 seconds. It goes round the sun once every 29 1/2 years. It is 886,000,000 miles distant from the sun—about 9 1/2 times the earth's distance, therefore.
The rings of Saturn are made up of a swarm of countless little moons. The rings are about 28,000 miles wide and 168,000 miles in diameter, and only about 100 miles thick. Saturn has eight moons—one as large as Mars, one about the size of our moon, and the rest smaller.
THE PLANET URANUS Uranus is 31,900 miles in diameter. It weighs only 1 2/10 as much as a globe of water of the same size. It goes round the sun once in 84 years. It is 1,781,900,000 miles distant from the sun—about 19 times as far as the earth, therefore. It has four rather small moons. It turns on its axis every 10 hours 50 minutes.
THE PLANET NEPTUNE Neptune is 34,800 miles in diameter. It weighs only 1 1/10 times as much as a globe of water of the same size. It goes round the sun once in 165 years. It is 2,791,600,000 miles distant from the sun—about 30 times the earth's distance, therefore. It has one moon about the size of our own moon.
COMETS A few comets belong to the family of the sun and move around him as do the planets.
THE FIXED STARS Stars are suns, immensely distant from our sun and from each other except when they are grouped in clusters. Light, which travels nearly 200,000 miles in a second, takes 4 years to come to us from the nearest star. The light from Polaris (the polestar) takes 47 years to reach the earth. Many stars are millions of times as large as the earth and may give off 1000 times as much light as our sun.
The Total Solar Eclipse of 1871 in India—The black circle is the disk of the moon; behind it the sun's disk is hidden. The pale white streamers are the sun's corona, or crown. The corona always surrounds the sun, but is not visible every day because the streamers are so faint. Notice close to the edge of the moon's disk a few brighter spots. These are flames of hydrogen—a gas that is glowing as if it were white hot—in the sun's atmosphere.
NEBULAE Nebulae are masses of gas at about the same distance from the sun as the stars are. They are of all shapes and sizes. Many of them are spiral in shape—corkscrew shaped. If, as sometimes happens, a star burns up it may turn into a nebula; or, as sometimes happens, a nebula may solidify and become a star. Perhaps our sun and all the planets were once a huge nebula that cooled and solidified into separate globes.
A Cluster of Stars in the constellation of the Centaur—Each white dot represents a star.
Drawing of a large nebula (in Andromeda) as seen in a telescope. The white dots are stars; the shining white cloud is the nebula. (See also Fig. 35.)