Seeing an Eclipse.—The word eclipse is taken from the Greek and means to fail to appear.

In starcraft, when the Earth passes across the Sun, and the Moon is hidden in its shadow, we say that it is an eclipse of the Moon; or when the Moon passes across the Sun and covers it we say that there has been an eclipse of the Sun.

How eclipses of the Sun and of the Moon are caused can be shown by a very simple experiment and you ought to make it.

An Eclipse of the Moon.—First let us suppose it is the Moon which is eclipsed by the Earth. All that is needed to show how this is done is a lighted lamp on a table and an apple with a knitting needle through its center. Let the flame of the lamp represent the Sun, your head the Earth and the apple the Moon.

Hold the apple by the knitting needle in a line between your eyes and the flame and turn round just as you did in the experiment showing the phases of the Moon. When you have turned halfway round the apple will be in the shadow of your head, when the apple is eclipsed, as shown in Fig. 119.

This is just what happens when the Earth gets between the Sun and the Moon and all these bodies are in a straight line, as shown in Fig. 120; the Moon will then be in the shadow of the Earth, and thus it is that the Moon is eclipsed.

If the Sun, Earth and Moon were always in the same plane with each other, every time the Earth passed between the Sun and the Moon the latter would be plunged into the Earth’s shadow and the Moon would be eclipsed. But the Moon does not revolve round the Earth so that it and the Earth make a straight line with the Sun every revolution for the reason that the Moon’s orbit is tilted a little and hence it is only once in a while that all of these bodies get in a straight line with each other, and when these times do happen then eclipses are produced.

Again when the Moon is eclipsed it is not always entirely hidden from view by the great shadow of the Earth, for the air on the Earth sends the sunbeams round it so that the shadow is never very deep, and for this reason the Moon can still be seen, as shown in Fig. 121. So, then, some of these bent sunbeams fall on the Moon and are reflected from its surface back to us and it is then that we see the Moon in an entirely new light.

Sometimes during an eclipse of the Moon the man can still be seen, but instead of having a bright silvery face he will have changed it to the copper color of a red Indian. The whole eclipse of the Moon often lasts longer than six hours and sometimes the moon is in the deep shadow of the Earth as long as two hours.

A total eclipse is one in which the Moon passes completely into the shadow of the Earth, while a partial eclipse is one where only part of the Moon is in the shadow of the Earth and part of it is in the sunlight. Eclipses of the Moon occur quite often and you can look up the time of the next one in your almanac.

Eclipse of the Sun.—When the Sun is eclipsed it is caused by the Moon passing between the Earth and the Sun and so the light is cut off from the former.

To show how this is done all that is needed is to continue the experiment with the lighted lamp and the apple which you used for the eclipse of the Moon. You will remember in that experiment you were turned with your back to the lamp and with the apple held in front of you.

Now to show how the Sun is eclipsed by the Moon keep on turning round until you face the lighted lamp with the apple in between and in a straight line with your eyes and the flame, as shown in Fig. 122.

Again you will find that the side of the apple nearest you will be in a deep shadow, though usually it can still be seen, and though you can see the light all round the apple yet you cannot see the flame.

This is exactly what takes place when the Moon gets between the Earth and the Sun and all of these bodies are in a straight line, as shown in Fig. 123; the Moon will then cover up the Sun and the shadow of the Moon will fall upon the Earth in a circle about 100 miles in diameter, as shown in Fig. 124. It is thus that the Sun is eclipsed.

As the Moon is traveling round the Earth and the Earth is turning round on its own axis, the Moon’s shadow moves across a path or trail that is only about 100 miles wide, and it moves very fast, too, for it usually takes less than five minutes for the Moon to sweep over the face of the Sun.

There are three kinds of eclipses of the Sun. The first is called a total eclipse, and this takes place when the Moon covers the entire face of the Sun, as in Fig. 125.

The second is called an annular eclipse and this takes place when the Moon does not completely cover the Sun but leaves a bright ring exposed, as shown in Fig. 126. The reason the Moon covers the Sun completely during a total eclipse and does not cover all of it during an annular eclipse is because the orbit of the Moon around the Earth is not a perfect circle, and so sometimes the Moon is nearer the Earth than at other times and this makes the Moon seem larger or smaller, as the case may be.

The third kind is called a partial eclipse. If we are not in the direct path of the shadow of the Moon we may see the Moon pass over only a part of the Sun, as shown in Fig. 127.

The only total eclipses of the Sun which can be seen in the United States in the next 30 years are the following:

Finding a Comet.—To the naked eye a great comet looks like a bright star with a long, glowing tail. In the long ago a comet was called a hairy star, for the early Greeks pictured the tail of a comet as being made of long hair and so from their language we get the word comet, which means hair.

A comet is really made up of three parts, which are, (1) a bright head or core, called a nucleus, and this is covered with (2) a layer of hazy light, called a coma, to which there is attached (3) a luminous tail, all of which is shown in Fig. 128. The nucleus of a comet is formed of a bunch of stones and pieces of iron, all widely separated and which are held together by attraction as they speed through space. As a comet nears the Sun it begins to get hot and to throw off burning gases, which make up its coma, and these bright gases streaming along form its tail.

There are several ways in which a comet can be told from the planets. In the first place, new comets, that is, comets which have never been discovered before, appear suddenly and in any part of the sky, though they may be very dim at first, and after a while they fade away, sometimes never to return again.

Second, they shine chiefly with their own light like the stars; third, they do not travel in small circles around the Sun like the planets; but the paths they take are either long ovals, called ellipses, or great curves whose ends never meet, called parabolas and hyperbolas, as shown in Fig. 129; fourth, they do not travel through space in a line with the planets and Sun, but shoot in and out of our solar system from and to every direction; and fifth and last, they travel at enormously high speeds.

Now, a comet, however great it may grow to be, never bursts into view, big, bright and beautiful, but when one is found it is usually seen as a little, dim patch of light, and it would quite likely be mistaken for a nebula, if it did not move along so swiftly.

A comet can be told by its movement and its movement can be plotted in the same way I have described for plotting the position of a planet in Chapter IV; and so, if you see a dim, little ball of light in the sky and find that it changes its position when compared with the fixed stars near it you may guess that you have discovered a comet.

The next thing you should make sure of is, that you have not found one of our old familiar friends, the planets. If it is really a great comet coming toward the Sun, it will grow larger and brighter every night, and you will soon be able to see its tail, which is the real earmark of a really truly comet.

When a comet is nearest us its head may shine far brighter than any of the planets, and its great tail may be millions of miles in length, and spread out in a glowing arc and light up the whole sky.

When this time comes a comet is by far a greater sight to the naked eye than it is when seen through the largest telescope. You should observe the comet often and carefully, for another one may not appear for a long time.

The nearer a comet gets to the Sun the faster it travels; some comets have been known to move a thousand times as fast as a rifle-ball, and this is the more surprising when we consider that the great, flaming tail goes along with it at the same terrific rate of speed.

The tails of comets do some very strange things; for example we should rather expect the tail of a comet to always follow its head like a skyrocket, but while it does so when the comet is headed toward the Sun, when the comet is passing round and shooting into space the tail turns away from the Sun, and this causes it to move ahead of the comet, as shown in Fig. 130.

This shows that while the head of a comet obeys the laws of gravitation, the tail does not do so, but acts as if it was electrified like the Sun, when of course it would be repelled by it. Fig. 131 is a picture of Halley’s comet of 1910.

Although a great astronomer once said that there are more comets in the sky than fishes in the sea, there have only been 1,000 comets recorded since the beginning of written history.

Meteors and Meteorites.—If you will look at almost any part of the sky on a clear night when there is no Moon you will no doubt see a bright flash like a rocket. But if you will scan the sky during the dark nights of August and November you will be very apt to see dozens of these shooting stars or meteors.

Now, meteors, fireballs, shooting stars and meteorites are all one and the same thing to start with and they have their beginnings when some comet goes to pieces.

After a comet breaks up, the pieces of stone and iron which form it still travel round in the same orbit. Some of these pieces may get within range of the Earth’s force of gravitation when they are drawn to it, and on striking the air they are intensely heated by the friction, and if they are small they burn up before reaching the ground.

The smaller meteors burn up almost instantly and the shining tails they leave last only a few seconds. Fireballs, which are simply large meteors, leave burning trails which can sometimes be seen for several minutes. Shooting stars are merely meteors which are not very bright, while meteorites are meteors which have fallen to the Earth before they have had time to burn up.

Many meteorites have been found, but ninety-five out of every hundred are of the stony kind, the others being of the iron kind. The way to tell a meteorite from a common stone is by examining its surface. A true meteorite is covered with a black, shiny, burnt crust, caused by the intense heat to which it was subjected as it fell through the air. The test for an iron meteorite is to grind and polish a part of its surface and then cover it with a dilute solution of nitric acid, when markings like those shown in Fig. 132 will be etched upon it.

The Milky Way.—You have, no doubt, often seen on a clear, dark night, a wide, ragged band of light stretching from the northern sky across the celestial equator and beyond. This band of light is the famous Milky Way.

If, as you were looking at the Milky Way, the Earth could be moved from under you and you were left standing in space alone so that you could see in every direction, you would find that the band formed a complete ring round the sky.

To the naked eye the Milky Way seems to be made up of mists of matter as thin as the stuff of which comets’ tails are made. There are some patches, though, that are very bright, but look at them as long as you will with the naked eye nothing more can be seen than just a milky patch of hazy white, as shown in Fig. 133.

With a very small telescope, however, this band of luminous matter will be instantly changed into thousands of stars, all separate and distinct, and some of these stars will look about as large as the stars of the Pleiades when these are seen with the naked eye, while others will shine as brightly as Venus or Jupiter when they are nearest to the Earth.

These bright patches, then, which form the Milky Way, are really groups of stars or star-clusters, and when some of these clusters are photographed through a telescope as many as 15 or 20 thousand stars can be counted, while the real number of stars that lie beyond and which cannot be seen is past all calculation, and, just think of it, every one of these stars is a Sun as large or larger than our Sun!

Those who have made a deep study of starcraft tell us that our Sun is one of the stars of the Milky Way, and since the other fixed star that is nearest to us, which is Alpha Centauri, is 25 trillion miles away, and Sirius, the Dog Star, which is the brightest star in the whole sky, is three times as far away as Alpha Centauri, we may gain some slight idea of the enormous distances of the fainter stars that make up the Milky Way.

The Nebulæ.—Unlike the bright, cloud-like patches in the Milky Way, and which the telescope shows to be formed of separate and distinct stars, are the faint misty spots in the sky called nebulæ (or nebule).

Now the nebulæ give us a clew as to how the stars were made, and how other stars are now being made, for it is believed that the nebulæ are the raw material which, when set in motion, produced heat and took on form and became Suns and planets like our own solar system.

The nebulæ are formed of luminous gas. Some of them are great irregular clouds of gas, many are more or less spiral in form, others are condensed so they look like fuzzy stars.

There are a few of these nebulæ which can be seen with the naked eye; one of these is the Great Nebula of Orion, and if you will look at Orion some night and draw an imaginary line a little below his belt and toward the east you will be able to find it. Another nebula that is bright enough to be seen with the naked eye is the Great Nebula in Andromeda. Both of these nebulæ are good tests for eyesight. Fig. 134 shows some of the different forms of nebulæ.

The Making of the Stars.—To explain how our Sun and the planets were made, as well as all the other stars in the universe, two ideas have been worked out, and although these are quite unlike in many ways, yet both start out with the nebulæ. The first of these is called the nebular hypothesis and the other and later one is called the planetesimal hypothesis.

The Nebular Hypothesis.—We have found out what nebula is believed to be and we should next understand exactly what the word hypothesis means. An hypothesis is an idea worked out so that there is a fairly good chance of its being true.

The nebular hypothesis, then, is an idea that has been worked out from what nebulæ are believed to be and what is known of the mighty forces of nature, and these when taken together seem to show that all the things in the sky—stars, planets, moons, comets and meteors—are made of nebular stuff.

The nebular hypothesis says that when the nebular matter, or star stuff of which the planets were made, was separated from the parent nebula by centrifugal force, they were all whirled away in the same plane, turning on their own axes and traveling round the Sun in the same direction.

The Planetesimal Hypothesis.—A later idea, is called the planetesimal hypothesis. Planetesimal means little planet, and as we know already what hypothesis means, by coupling the two words together we may easily guess that it is an idea worked out which accounts for the making of solar systems out of little planets.

The planetesimal hypothesis assumes that the sun, planets, and moons comprising the Solar System were formed from a star which was torn to pieces by another star which passed close by. The star was left in the form of a spiral nebula rather lumpy and with little particles of matter scattered all around.

The meteors, or little planets, making up the nebula, attract each other, like all other bodies, and when they get close enough they are drawn together and dense masses of matter, or cores, are built up. The largest core is seen in the center of a spiral nebula, and as it becomes more compact it grows hotter and a Sun is made, while the other and smaller cores turn round it and attract little planets to them. And so the cores grow in size, and with more and more weight bearing toward their centers the gases are forced out and these make the air and water.

It is in this manner that the planetesimal hypothesis explains how the Sun, planets and Moons of the solar system are made.