In the last chapter we said that all the stars in the sky, including our Sun, are fixed in their positions; by this we mean that if we were to look at the Big Dipper every night for a hundred years we could see no change in the positions of any of the stars forming this constellation.

But if we look at the sky along the line of the ecliptic—that is the path of the Sun—one night after another we are likely to see a bright point of light which looks exactly like a star and yet it is certain that this point of light really does move among the other stars. What kind of a heavenly object then is this?

The bright point of light which thus seems to us to be a star when we look at it with the naked eye is really another world, or planet as it is called, and very like our own Earth. To prove that this moving point of light is really a world, or planet, and not a distant star, all you need to do is to look at it through a pair of opera glasses, or a small telescope, when it will be seen to be a round body, whereas a star when viewed through the greatest telescope is never larger than a mere point of light. (See Fig. 50.)

The reason the planets, some of which are smaller and some larger than our Earth, can be seen to move is because they are quite near our Earth; that is, they are near when compared with the fixed stars.

Again, the reason the planets shine like the stars is not because they are hot and flaming bodies like our Sun and the other stars, but because the light from the Sun which strikes them is reflected to the Earth in exactly the same way that the sunlight falling on a mirror is reflected away in another direction.

Names and Sizes of the Planets.—The names of all the planets, and there are eight chief ones, should be learned as well as the order in which they are arranged around the Sun. The names of the planets are given below in the order of their size.

How to Know the Planets.—While it is not an easy thing to tell a planet from a star with the naked eye, still there are several ways of doing it.

First, always look for the planets along the path which the Sun and Moon travel. As all the planets are nearly in the same plane with the Sun and Moon they must all follow the same path across the sky.

Second, it is useful to remember that none of the planets, except Mercury, ever twinkle, unless they are very near the horizon.

Third, by watching a planet closely for a few hours it will be found to have moved a little. To note this change of position the planet and some fixed star near it must be closely watched and their distances compared from time to time.

Fourth, and last, the surest way of finding the different planets is by using an almanac which will tell you which planets can be seen at certain times of the year and in what part of the sky they are to be found.

Seeing Mercury.—Mercury is so near the Sun that it can only be seen with the naked eye at certain times. Mercury should be looked for just above the eastern horizon for about an hour before the Sun rises in the spring; and above the western horizon for about an hour after the Sun sets in the autumn. You will have no trouble in knowing Mercury if you can only see him, for he is very bright and will be near the horizon. His pale ash color will also help you to single him out from the stars about him. Mercury goes through phases like our Moon, but these cannot be seen with the naked eye.

Mercury is a curious planet in that his day and his year are of exactly the same length, just like our Moon; this means that he turns on his axis once in exactly the same length of time it takes him to travel round the Sun. This causes one side of Mercury to be always turned toward the Sun, and of course this side is hot and light, while the other side is always turned away from the Sun and, consequently, it is dark and cold. Three views of Mercury in different phases as seen through a telescope are shown in Fig. 52.

Mercury is 36 millions of miles from the Sun.

• His diameter is 3,000 miles.
• His day is 88 of our days long.
• His year is 88 of our days long.

Seeing Mars.—We hear more of Mars than of any other planet for two reasons: first, because great lines can be seen on his surface which are thought to be canals, and second, since Mars sometimes comes almost as close to the earth as Venus, there has been a great deal of talk since the invention of wireless telegraphy about our signaling to him.

That people could live on Mars is possible, for the Red Planet is like our Earth, in that it has land, water and air, weather and seasons, with a warm equator and ice-covered poles. Seen through a telescope he looks like Fig. 53, though much smaller.

But Mars cannot always be seen, for sometimes he disappears for a couple of years, but when he finally does return he is a good planet, for he stays a long time, and you cannot mistake him, for he will shine ruddy and bright with never a merry twinkle.

Mars is 141 millions of miles from the sun.

• His diameter is 4,200 miles.
• His day is 24½ of our hours long.
• His year is 687 of our days long.

Seeing Venus.—Venus is so much brighter than any of the other stars or planets that you will know her the instant you see her. Indeed, when Venus is the brightest and the Sun is far enough away from her, she can often be seen with the naked eye in the daytime if the sky is clear. Three views of the phases of Venus are shown in Fig. 54.

To see Venus you must look for her early in the morning in the east before the Sun is up, or in the evening in the west just after the Sun has gone down. This is the reason Venus is sometimes called an evening star and sometimes a morning star. Venus goes through phases like Mercury and our Moon, but these cannot be seen with the naked eye.

When Venus and the Sun get too close together she cannot be seen, for the light of the Sun is so powerful that her reflected light is dimmed by it. Venus is a bright straw color and she is a beautiful object in the sky when visible, but there are weeks at a time when she cannot be seen, by reason of the Sun outshining her when she is between us and the Sun or on the other side of the Sun. The day and the year of Venus are, like Mercury, probably equal, and hence one side of Venus is always turned toward the Sun and basks in its light and heat, while the other side is turned away from the Sun and is doomed forever to cold and darkness.

Venus is 67 millions of miles from the Sun.

• Her diameter is 7,700 miles.
• Her day is 225 of our days long.
• Her year is 225 of our days long.

Our Earth.—The Earth is the third planet in distance from our Sun, and although it is small compared with some of the others it is so important to us that it will be described in a separate chapter. Fig. 55 shows a view of the Earth as she would be seen from the Moon.

The Earth is 93 millions of miles from the Sun.

• Her diameter is 7,920 miles.
• Her day is 24 hours long.
• Her year is 365 days long.

Seeing Jupiter.—Jupiter, the largest planet which revolves round our Sun, is fifth in distance from him. He is not as bright as Venus, but he is brighter than any of the fixed stars, and by his brightness and silvery color he is quite easy to recognize.

This great planet seems not to have cooled down yet into a nice world like our own Earth or someone’s else Mars, but rather he is a ball surrounded by clouds of hot vapor. Still he is not hot enough to give out any light himself, but like the rest of the planets he shines by the reflected light of the Sun. Fig. 56 is a view of Jupiter as seen through a telescope; one of the moons, and its shadow on the planet, is shown.

Jupiter is crossed with several bands and he has nine moons to light up his great surface on a dark night, but neither his belts nor his moons can be seen without a glass.

Jupiter is 483 millions of miles from the Sun.

• His diameter is 87,000 miles.
• His day is about 10 of our hours long.
• His year is almost 12 of our years long.

Seeing Saturn.—It is not quite as easy to single out Saturn with the naked eye, for his light is just about as bright as Capella in the constellation of Auriga, or any of the other first magnitude stars.

This disadvantage is offset by the fact that when he rises at sunset he can be seen during the whole night, so that you will not only have plenty of time in which to find him, but to observe him as well. Again, Saturn may be seen any clear night during the winter months until the year 1920.

Like the Earth, Saturn is believed to have a more or less solid core, but hotter and with layers of gas around him. It is the sixth planet from the Sun, and with his beautiful rings, which are formed of millions of little pieces—each a moon in itself—and with his ten large moons, when seen through a telescope he is far and away the mightiest sight in the whole sky at night. He and his wonderful rings are shown in Fig. 57.

Saturn is 886 millions of miles from the Sun.

• His diameter is 73,000 miles.
• His day is 10 of our hours long.
• His year is 29 of our years long.

Seeing Uranus.—If you have sharp eyes and will look for Uranus in the spring and summer months, you should be able to see him. He has a pale green color, and on a clear moonless night is visible to a good eye.

Uranus is the planet star from the Sun and before Herschel discovered him with his homemade telescope astronomers frequently mistook him for a fixed star. As seen through a large telescope Uranus shows some indistinct markings.

Uranus is 1,780 millions of miles from the Sun.

• His diameter is 32,000 miles.
• His day is thought to be 11 hours long.
• His year is 84 of our years long.

Neptune.—Neptune is so far away from the Sun he cannot be seen with the naked eye, but he can be easily seen with a good opera glass. He is attended by one moon. He is simply a disk of light when seen through a big telescope, as shown in Fig. 59.

Neptune is 2,790 millions of miles from the Sun.

• His diameter is 36,000 miles.
• His day is unknown.
• His year is 165 of our years long.

The Asteroids.—The Asteroids are a group of small planets moving around the Sun in orbits between the Earth and Mars and, hence, at times these little bodies come very close to us. The group occupies a place in the sky where we should expect to find a single large planet.

But when the planets were made, Jupiter, with his great bulk pulled the soft pieces apart of which a planet would have been formed, and instead of one planet of respectable size there are hundreds of little planets ranging anywhere from 10 miles to 500 miles in diameter.

One of these small planets is called Vesta, and although she is only 240 miles in diameter she may be seen on certain occasions with the naked eye.

Positions of the Planets Round the Sun.—It was mentioned in the beginning of this chapter that all of the planets lay not far from one plane, and that the Sun’s equator is nearly in this plane. Now let us see just what this means.

Suppose we lay eight marbles around a large marble placed on top of a table and in the center. This means that all the marbles lie in the same plane which is the top of the table, as shown in Fig. 60. The planets are all arranged round the Sun, as shown in Fig. 61, just the same as if they were all placed on a big, level board or table top. If we draw a picture of the plane view of the planets (Fig. 60) and the top view of the planets (Fig. 61) together, we shall have a picture of the solar system, as the Sun and all of his planets are called, as shown in Fig 62. From these pictures you will see that the planets are not arranged, in distance from the Sun, in order of size.

To remember the arrangement of these planets in their relation to the Sun commit to memory this simple sentence:

Men Very Early Made Jars Serve Useful Needs.

As the first letter of each of the above words is the same as the first letter of a planet, you will be able to instantly recall the proper place of any one of them.

How the Planets Are Held in Space.—If you will take an ordinary dinner plate and half an eggshell, and give the eggshell a slight spin on the rim of the plate—the rim should be slightly moistened—you will find that by tilting the plate a trifle the eggshell will revolve in two directions; first, it will spin round on its own axis, and second, it will travel round the rim of the plate, which we will call its orbit, as shown in Fig. 63.

This double motion of the eggshell is exactly like the double motion of a planet—each one turns on his own axis and each travels round the Sun in its own orbit; moreover, all the planets travel round the Sun in the same direction, the nearest planets taking the shortest time to complete the circle or orbit, while those farthest away take the longest time to go round their orbits just as we might expect them to do.

In the beginning of things the planets were a part of the Sun, as we shall see further on, and when they were thrown off by him they spun round their own axes, and they continued to spin round their axes just as a ball continues to do so after it has left the pitcher’s hand.

The tendency to fly off at a tangent which every particle of a rotating body feels is called centrifugal force. Now, no one knows what centrifugal force is, but how it acts is very well known, and you can find out for yourself by making the following experiment.

Take a stone and tie one end of a string to it; now swing the stone round in a circle; if the string should break, or you should let it go accidentally (on purpose) the stone will shoot off at a tangent, that is in a line away from the circle in which it was swinging, as shown in Fig. 64.

This is just what made the planets, one after another, separate from the Sun, but they could not get very far away, for another force which is not only in the Sun but in every particle of matter in the universe, pulled them back toward the Sun, just as it pulls a ball when thrown to the Earth, and this force is called gravitation.

So the centrifugal force keeps them spinning round on their axes and flying round in their orbits about the Sun. If it had not been for the attractive force of gravitation of the Sun and the planets, the planets would have kept right on going out into space and left the Sun to shift for itself forever after.

As it is they whirl round the Sun, never able to get any farther from him and yet never getting any nearer to him, for the centrifugal force tends to make them fly out and away and the force of gravitation tends to pull them back to the Sun. The result is that these two great opposing forces exactly offset or balance each other, and the planets are held in their orbits just as securely as the stone is held out by the force the boy exerts to move it, and in by the string he holds in his hand.

This balancing of opposed forces was nicely shown some years ago by Sir Robert Ball, the great English astronomer, at one of his lectures in the Royal Institution of London, and I reproduce it here.

To the ceiling over his lecture table he had fastened a thin wire, the lower end of which was secured to a hollow iron ball. When the ball was pulled aside, it would swing like a great pendulum, forth and back, in a plane, as shown in Fig. 65.

But when a powerful magnet was placed on the table and the ball was set swinging in a plane as before, just as it came close to the end of the magnet the latter pulled the ball with its mighty force toward it, and so changed its course, but the ball, instead of being attracted directly to it, swung in a graceful curve around it, as shown in Fig. 66.

This is precisely the case of the planets swinging round the Sun, and shows very nicely the balanced forces of the Sun and the planets and why the planets stick to their orbits.

Why the Planets Do Not Stop Spinning.—Now you may ask why the planets do not stop turning on their axes and revolving in their orbits round the Sun. And the answer is because there is nothing to stop them.

If you spin a top on a plate it may keep going for a long time, but it will finally die down and stop. This is because there are other forces which oppose its centrifugal force. The forces a spinning top has to overcome are friction between the point of its spindle and the plate, and the resistance between the surface of the spinning top and the air pressing on its sides, and the friction and the air resistance together soon use up the energy stored in the top and which makes it spin.

But if you could spin and throw a top far enough away from the Earth so that it would not meet with friction or resistance of any kind, it would go on spinning forever just like the planets.

How to Plot the Position of a Planet.—One of the tests which a Boy Scout must pass in order to obtain a merit badge for starcraft is to “plot on at least two nights per month for six months the positions of all naked eye planets between sundown and one hour thereafter. The plot of each planet shall contain at least three fixed stars with their names and designations, colors of planets and stars to be recorded by him.”

Now, by looking at your almanac under the head of Morning and Evening Stars you will find all the planets which are listed as Evening Stars, together with the dates when they can best be seen. From your almanac for 1915 you will learn that

The first thing to do is to find what constellation is on your meridian at 9 P. M. for the month that the planet you wish to plot is to be seen. This you can do by looking at the map of the stars shown in Fig. 67. You can see any of the stars or constellations on your meridian by looking for them at 9 o’clock P. M. on the months marked above them.

This done, consult your almanac and find out what time the Sun sets on the day that the planet you are looking for can be seen.

As an example let’s take Mercury, which the almanac says is an evening star about February 5, and which can be seen in the west just after sunset. The almanac will also tell you that the Sun sets on February 5 at 5 o’clock.

The star map (Fig. 67) will show you that the constellation of Gemini, the Twins, is one which can be seen in February on your meridian at 9 o’clock at night, and you know that Mercury can be seen close to the western horizon from 3 to 4 hours earlier.

Since Gemini, the Twins, is on the meridian at 9 o’clock, Taurus, the Bull, which is the next constellation to it, will be on the meridian at 7 o’clock, and Aries, the Ram, will be on the meridian at 5 o’clock.

Now the next constellation to the west of Aries, the Ram, is Pisces, the Fishes, and the next one to Pisces, the Fishes, is Aquarius, the Water Bearer, and as this constellation sets about an hour after the Sun, it is in this constellation that you will find Mercury in the early February evenings.

The diagram (Fig. 68) shows how it is that when Aries is on the meridian at 5 o’clock in the evenings of February, Gemini will be rising in the east, and Aquarius, with Mercury in it, will be getting ready to set in the west.

You can find all the other planets in the same way, but it is much easier to find them in the sky than it is to try and imagine their positions after reading how it is done. You should, though, by all means, read the chapter on The Stars of the Zodiac before you try to plot a planet, for there is much useful information in it which you ought to know.

To plot the position of a planet, take a sheet of stiff paper or cardboard about 8 inches square and divide it into a square 6 inches on the side, as shown in Fig. 69. This will leave a margin of 1 inch on both sides, ½ inch on top and 1¼ inches at the bottom of your paper.

Fasten the paper to your starboard, described in Chapter II, and go forth with it and your acetylene or flash lamp and find your planet. Having found it, mark with a pencil in the squares on your ruled paper the positions of three of the nearest fixed stars which are shown in the star chart of Fig. 67. Now mark the position of the planet as you see it on your chart by making a little circle and your outdoor observation is ended.

Once inside, mark down the chart number, the date and the time you made the observation; also the names and colors of the stars and the planet, and finish the record by signing your name. Should any of the other planets be visible at the time of your observation you will, of course, have to plot two charts, unless the planets are very near each other.