I AM going to try to tell you in this book a number of things that you will like to know about the planets that are our neighbors in the solar system; about the sun which is the center of it; and about the stars that lie all about us in space. That is, they seem to lie round about us, but they are really very distant neighbors indeed. This will be a book easy to read, for I shall generally tell you what astronomers have found out, and not stop to say how they have done it.
It is not hard to understand that Copernicus discovered that all the planets—our earth being one of them—moved about the sun as a center, but it would be very hard indeed if I were to tell you what long calculations he, and others that came after him, had to make to find this out. What he discovered, more than three centuries ago, seems simple to us now.
It would be a mistake if we should fail to appreciate the genius of those old masters of science. We call them masters, and they truly were so. Many of their discoveries are simple to us nowadays, but you must not forget the thousands of long nights that they spent in watching the stars, nor their long days of toiling over calculations.
When you and I wish to know a fact we can look for it in a book. We can find it in a cyclopedia. In five minutes we can add that one fact to the other facts that we know, and use them all to help us to think. But it was different with Copernicus and Galileo. When one of them wanted a fact, such a one as is in every cyclopedia to-day, he had to find it out for himself.
One of Sir Isaac Newton's greatest discoveries was postponed for years just because the size of the earth was not known in his time—how many miles it was through the earth from outside to outside. Every school geography tells that to-day. It is 8,000 miles in diameter, or, more exactly, 7,918 miles.
We are too apt to forget how much we owe to the great men who lived before us. Every one of them has made some one thing plain to the whole of mankind—one thing at least, and perhaps many things.
It is as if the whole human race were trying to climb higher and higher on a great stairway. Some of them reach the uppermost step and can go no farther till some wise and strong man lays another step for them to mount on. Every single step has been laid in this way by some great man building on the steps laid by other great men before him. After the stairway is built it is not hard for the rest of us to climb. We are all of us doing it to-day. But just consider how much thought and labor and anxiety of mind has gone to the building! The foundations were laid ages and ages ago by the shepherds in Eastern countries, who were early awake watching the stars and waiting for the coming of the dawn. The early navigators, who crept round the coasts of the Mediterranean Sea in their slender crafts, sailed from headland to headland, never daring to venture out of sight of land, until it suddenly occurred to some wiser one among them that he might shape his course by the stars.
It was not long before he and his fellows were making voyages from Carthage to Britain to bring back tin from the mines of Cornwall, or down the Red Sea to the east coast of Africa to fetch back gold for the temple that King Solomon was building.
It is a great step from navigation such as theirs to the sureness with which our fleets of steamships traverse the world from end to end, arriving on their appointed days, and never losing their way in the trackless ocean. The first foundations for their triumphant voyages were laid by that wise sailor of long ago, who first thought of steering by the stars.
In this little book we shall talk of some of the results that the great astronomers have attained to, and not very much about the ways in which they went to work to find these results.
If you see a photograph of sun spots, made last year at the Lick Observatory on Mount Hamilton, you can hardly understand it fully without knowing that it was Sir Isaac Newton, in 1717 (nearly two hundred years ago), who first proposed to put telescopes on high mountains where there might be, he said, "a serene and quiet air "; that it has taken the labors of the best chemists for a hundred years to bring the making of photographs to its present perfection; and that Galileo, so long ago as 1611, first discovered that the sun had spots on his surface.
So that this photograph is only the upper step of a stairway, all of whose stones have been laid for us, one by one, by the great men of past times—Galileo, Newton, and the rest. It makes the picture far more interesting to remember all this history.
The very first thing for us is to see how the solar system is arranged, how it would look if we could get very far away from it, and look at it like a distant landscape. The sun is at the center of it, and is the source of all its light, of all its heat, and maintains all its life—for life could not exist for a moment without heat, you know, and all the heat comes from the sun.
Our family of planets forms what is called a system. The sun is the center and the chief of the system. The earth and all the other large planets—Mercury, Venus, (the Earth), Mars, Jupiter, Saturn, Uranus, and Neptune, with a great many very small planets that are called asteroids, a great many meteors, and a few comets—make up the system, the family of the sun.
You know how large the earth is—nearly 8,000 miles through from outside to outside, and about 25,000 miles around in the largest part—and you can imagine that the whole earth must be very heavy. Just New York alone would weigh a great deal, if we could only weigh it; and think what the Atlantic Ocean would weigh if we could put it in a huge box and then weigh it in enormous scales! The whole earth, then, would be very heavy indeed. Well, the sun weighs 330,000 times as much as the earth. Astronomers have found that out, for one thing.
The sun stands in the center of his family, and all the planets go round him. The earth goes round the sun in one year of 365 1/4 days, and turns round its own axis in twenty-four hours—a day.
This picture is intended to show in a general way that the earth is a globe, and has an axis (like the needle) round which it turns once every day. The next picture of a school globe is, perhaps, a better picture for us to use.
The earth turns on its axis once every day, and makes the changes from day to night. It also moves around the sun in a path somewhat like a circle (it is really more like an oval), and it takes just a year to go once round its path and to come back to the same point again.
Every one of the planets has two motions like these motions of the earth's. Every planet—Mercury, Venus, and the rest—turns on its own axis, and makes its own day and night, and also moves around the sun in its own path.
If I had a very large sheet of paper I could draw a kind of a map of the different paths of all the planets on it for you. But on this small page I shall have to make two pictures, and to put half of the planets in one map and half in the other.
The first map is made to show the paths (we call them orbits) of the planets that are nearer to the sun than the earth is; and the planets farther away from the sun than the earth are shown on the second map.
Let us look first at the next picture.
In the center of the drawing you see a little dot to stand for the sun, and four other dots to stand for the four planets that are nearest to the sun—Mercury, Venus, our own earth, and then Mars. First, there is the sun in the middle of the whole, and around him part of his family of planets.
The nearest planet of them all is Mercury; nearest to the sun, that is. The name is Mercury, and the sign for the name is ☿. He moves about the sun in the direction of the little arrow in the picture, and it takes him about eighty-eight days to go completely around his path (his orbit) once. If he is in a certain place in his orbit on the first of January he will be very nearly in the same place on March 29th, eighty-eight days later. And so every eighty-eight days he goes once round his orbit, and has been doing so for thousands and thousands of years.
We go around the sun once in three hundred and sixty-five days, which makes our year of spring, summer, autumn, and winter, and after one year is finished another begins. In the same way Mercury travels round and round with a short year of eighty-eight days—three of our months. As soon as one year is finished another begins.
You see that Mercury is the planet nearest to the sun and that he must get more light and more heat than any of the others; for, of course, the nearer you are to the fire (and the sun is a kind of a huge fire) the warmer you are, and the nearer you get to the light (the sun is a kind of an electric light, too) the brighter.
Venus is farther from the sun than Mercury, and moves (in the direction of the arrow in the picture) in the same direction as Mercury, but, of course, in a different path. In fact, all the planets move round their paths in the same direction. It takes Venus two hundred and twenty-five days to go completely around the sun once, so that two hundred and twenty-five days, which is about seven and one half of our months, is the year of Venus.
The third planet, outward from the sun, is our particular planet, the earth. It moves in its own orbit, always in the same direction as the other planets, and makes one complete circuit around the sun in three hundred and sixty-five days—or in three hundred and sixty-five and one quarter days, to be more exact. Three hundred and sixty-five days is our year, just as the years of Venus and Mercury are two hundred and twenty-five, and eighty-eight days.
Farther away from the sun than the earth there is a planet, Mars, which has a year of six hundred and eighty-seven days—a little less than two of our years, and very much more than one year—something like twenty-three of our months.
The drawing shows the four planets that are nearest to the sun, moving round him in four orbits, as they might look to a very distant eye up among the stars. Such an eye would see only the five little dots in the drawing, one central dot—very bright it would be—for the sun, and four fainter dots for the planets. If the eye would watch patiently for eighty-eight days it would see that Mercury had in that time gone completely round its orbit and had come back to the place it started from.
Venus would have gone quite a long way forward in its particular path, but, of course, not all the way around—only about half way. And the earth would have moved about one quarter way round its circle, and Mars would have moved, too. If the eye would wait patiently for six hundred and eighty-seven days, about twenty-three months, Mars would have completed its circuit also.
Now these planets have been watched by men on the earth for thousands of years, and they always have gone round and round in the same paths. Thousands of years before the first man looked at them they were moving as they now move; and long after our time they will still be traveling quietly in these paths that we call their orbits.
Not only have these four planets been moving about the sun for all these years, but also the four other great planets—Jupiter, Saturn, Uranus, and Neptune. We could not draw a picture on this page to show all of them at once, so that we have had to make two pictures. The second one shows a part of the orbits of the other four planets, and the orbit of Mars has been put down also.
In the first picture the orbit of Mars was quite large, you remember. (Will you please to turn back and look at it? You see that the path of Mars is about three inches across in the first drawing.)
Now in the second picture we have had to draw the whole orbit of Mars a dozen times smaller. It is shown in the little circle nearest to the dot that stands for the sun. (Look at the bottom of the picture, please.)
The lowest dot stands for the sun. Quite close to it is another dot to stand for Mars Then there are some letters, "The Minor Planets," to show where there is a swarm of very small planets—the asteroids—that we will speak of by and by. They are very small for planets, and much smaller than our moon, though they are a few hundred miles through the center from outside to outside. We need not stop to speak about them now, but we will look at the orbit marked Jupiter.
The little dot stands for the planet Jupiter, and it moves about the sun in a path, of which a part only, is drawn. You must imagine the rest of it. Of course when completed it is an entire circle; and of course Jupiter goes around its path and comes back to the same point, and goes around again, and so on forever. It takes Jupiter about twelve of our years to go around its path once.
The earth takes a year to go completely around its path, you know. The earth goes around its orbit twelve times, that is, while Jupiter is going around its orbit once only. The year of Jupiter is then twelve times as long as the year of the earth. Its spring is twelve times as long as our spring, its summer is twelve times as long as our summer, and so with its autumn and winter. You must not think that its seasons—spring, summer, etc.—are like ours; but whatever they are, they are longer than ours—twelve times longer, each one of them.
Outside of Jupiter—farther away from the sun—there is a large planet called Saturn, and the picture shows a part of Saturn's orbit, too. To go completely around the sun, and to come back to the same point again, Saturn takes twenty-nine of our years. The year of Saturn is twenty-nine times as long as ours, and its spring is twenty-nine times as long, and so forth.
Outside of Saturn is a planet—Uranus—with a year as long as eighty-four of our years. It takes Uranus eighty-four years to go once around the sun in its very large orbit. And last of all comes Neptune, that has a year as long as one hundred and sixty-five of our years on the earth. Its orbit is so large that it takes one hundred and sixty-five of our years to move around it once.
An eye outside of the solar system, looking at it patiently, would see all the eight planets moving around the sun, which is the head of the family, in their eight different paths. In three of our months (eighty-eight of our days) Mercury would go around the sun once; and every three months Mercury would make another turn around the sun. Every year the earth would go once around its orbit, too. Jupiter would take twelve whole years for its course; and so with the other planets. Neptune moves so slowly that one hundred and sixty-five of our years are needed for this planet to move once completely around the sun.
The farther away a planet is from the sun the slower it moves, you see. The nearer to the sun, the quicker it moves. But they all move regularly, all the time, "without haste, without rest." Before there were any men living on the earth they were moving in these very paths, and long after our time, as I said, they will be so moving.
Think of this whole family that belongs to the sun. They are moving millions and millions and millions of miles away from him, so that you might think they were going off into space to join some other sun (for each of the stars is a sun like our sun). But the planets come punctually back to the same place, time after time, and they regularly move, year after year, in the same paths. We know exactly how fast each of them is moving, and we can calculate precisely where each one of them will be a year from now or a hundred years from now; or where every one of them was a year ago or a hundred years ago.
If we read in some very ancient book that Jupiter was in a certain place a thousand years ago, we can calculate just the spot in the sky where it was at that time. If our calculation does not agree with the saying in the book we know that the book is wrong; not that our calculation is wrong, for we are certain about these things.
Usually, of course, we find that such books are exactly right. The Chinese emperors used to have astronomers, centuries ago, to note the places of the planets and to write the places down in books. Usually we find that the books are quite right. Sometimes there is a mistake in the writing or the printing, and we can now, four thousand years afterward, correct a slip of the pen of one of these astronomers of long ago.
I am telling you this only to make you see that we are certain about such things, not to show that we are wiser than the wise men of past times. Every single thing that we know now is built on foundations that were laid long ago.
Now I hope that you have in your mind a kind of a picture of what the family of the sun would look like if you could only see it from very far away—if you could go up to some bright star and look back at the solar system.
You would see only one picture, you know, with the bright and very large sun in the center of it, and with eight planets—Mercury, Venus, our earth, Mars, Jupiter, Saturn, Uranus, and Neptune—moving about it, each in its own orbit.
Shut your eyes now, please, and see if you can imagine how they would look—eight little faint specks moving round a brighter speck, much brighter and much larger than they are.
This brighter speck is our sun. It is very large, but if you were to go very far away from it, it would not seem any larger or brighter than one of the stars. In fact, it is a star like the other stars that you see, only much nearer to us, while they are very far away.
The planets that move around the sun are not all of the same size. Some of them are much larger than the earth, while some of them are far smaller.
Here is a picture that will be of some use to us. Look at the little circle marked "Earth" on the left hand of the illustration below. If you look at the earth from your house it looks simply immense. If a bird looks at it from the sky the earth is still enormously large, of course. The bird is still too close to see the whole earth as a globe.
When you look at the moon in the sky the moon seems not so very, very, large. It is, in fact, an enormous globe, but it does not look enormous because you are very far away from it—thousands and thousands of miles away.
Suppose that you could get very, very, far away from the earth, thousands and thousands of miles away. The farther away you went the smaller the earth would appear. At one time it would look no larger than the moon looks to you now, and as you went farther and farther away, it would seem to grow smaller and smaller. If you went far enough it would by and by seem to be as small as the little white disk in the picture—the little disk that is marked "Earth."
Now if you could stand still at such a distance and look at all the planets of the sun's family one by one, what would you see? Nearest to the sun you would find a tiny little planet that would seem to be of the size of the dot marked "Mercury" in the drawing. Of course, the planet Mercury is really very large, but from your very distant point of view it would appear to be quite small.
A little farther from the sun you might see another planet, Venus, a little larger than Mercury and a little smaller than the earth. (Look at the third dot in the picture.) Then you might see the earth. The planet next farther from the sun would be Mars. It is farther from the sun than the earth is, and it is much smaller than the earth. It is shown by the second dot in the picture.
Next, beyond Mars, is the largest planet of all—Jupiter. When the earth looks only as large as its little white disk in the picture, Jupiter appears to be very much bigger—as large as the drawing marked "Jupiter."
The planet Saturn is the next beyond Jupiter as you go away from the sun, and it is the next in size to Jupiter, too. Uranus and Neptune are the next two planets, and they are equal to each other in size.
You should look carefully at this picture and see exactly what it means. It means that if you could, by any chance, get far enough away from the earth to have it seem no larger than the little white disk marked "Earth," then all the other planets, which are really very large, would seem to you to be of the sizes of the other circles. Uranus and Neptune would seem to be of about the same size, as you see them marked with their names in the picture. Saturn would seem larger than they are. Jupiter would seem to be the largest of all. All the rest—Mars, the earth, Venus, and Mercury—would seem to be quite small.
It seems as if the planets had been created in two groups of two sizes. All the small ones—Mercury, Venus, the earth, and Mars—are near to the sun. The four giant planets are the four that are farthest away—Jupiter, Saturn, Uranus, and Neptune.
The picture shows the sizes that the different planets would seem to have when the earth looked like the little circle. How large do you think the sun would look then? Well, if it were properly drawn in the picture, it would have to be about twelve inches across—more than four times the width of the black ground of the picture, about ten times larger than the circle marked "Jupiter"!
You see now why the artist who made the picture did not put in a disk to show the size of the sun. The page of this book is not large enough to show it. The picture of the sun would have to be about twelve inches across—about a third of a yard.
Perhaps the illustration below will help you to realize how large the sun is. You see the large white circle? Well, and do you see the black line across the middle of it? It is made up of little black dots. Now if you possibly could get so far away from the earth that it would look no larger to you than a single one of these tiny black dots, the sun would then look as large as the white circle.
Do you understand how this is? The sun is really one hundred and nine times larger than the earth, and there are one hundred and nine dots side by side in the picture. One hundred and nine earths side by side would just reach once across the sun.
You see that all these pictures are drawn, as the artists say, to different scales. When we draw a picture of a house on a sheet of paper we make the line for the roof of the house only a few inches away from the line for the basement. Then we have chosen the scale, as they say. The windows, and the chimneys, and so forth, have to be drawn so that they will look just right, so that they will have the right proportions.
If we draw the same house on another sheet of paper we can make our drawing twice as large, if we like. But if we do so, we have chosen our scale—our proportions—and we are obliged to make the windows and the chimneys twice as large as they were in the first picture. It is just so in these pictures of the planets.
In the last one we chose to make the earth the size of a little black dot. If that is the scale, the sun must be one hundred and nine times larger. If we had made the earth in the picture as big as an inch, then the sun would have had to be one hundred and nine inches across; that is, more than nine feet across, about three yards.
Whatever the size of the earth in the picture, whatever scale we have chosen to begin with, the circle for the sun must be one hundred and nine times as large as the circle for the earth. We can choose any scale we please to begin with, but when we have once chosen it we must keep to it.
Turn back, please, to the picture before the last one—to the picture that shows the comparative sizes of the eight planets. In this drawing we have chosen the scale so that the earth, which is eight thousand miles through the center from outside to outside, is shown as a little white disk—the one marked "Earth." If eight thousand miles is properly shown by this little disk, then the other disks are of the right sizes to show the planets marked with their names.
It is just the same with the two pictures showing the paths of the planets. In the first we have chosen the scale so that the path of the earth, which really is one hundred and eighty millions of miles across from side to side, is drawn about two inches across.
If the path of the earth is drawn of this size, then the path of Mercury must be of the size given in the picture for Mercury's path, the path of Venus must be of the size drawn, and the path of Mars must be of the size that is given in the picture; that is, about three inches across it from side to side. Now in the second picture of orbits we have chosen a different scale of proportion, you see. Will you please turn to it?
Here, because the page is small, we have chosen to make the orbit of Mars only about a quarter of an inch across; and if the path of Mars is of this particular size, then the paths of Jupiter, of Saturn, of Uranus, and of Neptune must needs be of just the sizes we have given in the picture.
You understand this? Each picture is correct according to its own scale, but the different pictures do not fit together. We have to make them in this way because the page of this book is itself small—too small to hold a very large drawing. In the pictures of the orbits no attention is paid to the sizes of the planets themselves. The artist has simply put in little dots to show where they are. You have to get their sizes from other drawings, such as the next to the last one in this chapter, which has been given for this special purpose.