CHAPTER XVII.. Long-Tailed Comets.

LONG-TAILED COMETS.

A good deal has been said about empty gaps in the sky between and around the pathways of the worlds. But those gaps are at least not always quite empty.

Comets, with long bright tails, flash through the darkness by hundreds, perhaps thousands. Meteors travel in vast swarms, by millions of millions. Each comet gives forth a radiant shining, and each little meteor is bright in the sunlight. I am going to tell you about Comets first, and then about Meteors.

The word “comet” means “a hairy star.”

But comets are not stars really, though they have often been mistaken for stars, especially when first seen without any tail.

There may be any number of comets as far away as the stars, millions of them in each direction. But those we cannot possibly see. We only see such comets as belong to our Sun and travel about in his kingdom, or else those which come to pay him a visit from far away.

No comet that is outside the Solar System can be visible to us on the Earth. The distance becomes too great. For the light of a comet is not like the light of a star, and it cannot reach through billions of miles, as the light of a star can.

Once in a while a splendid comet makes its appearance, with a tail reaching half across our sky. But this is not at all common. Most of those seen are small and faint, and the greater number can only be seen at all in telescopes.

Almost every year some fresh ones are found in the sky, and hardly a day passes in which at least one may not be noticed, in some part of the heavens, with a good telescope.

Each comet, like each world, has its own pathway in the sky round the Sun. But a comet-pathway is much more oval in shape than a planet-pathway. Sometimes it is a very long and very narrow oval indeed, with the Sun almost close to one end of the long oval.

To get round such a pathway as this takes a good while. At one part the comet gets quite near to the Sun, and then rushes at a tremendous speed. After which he wanders far away from the Sun, and creeps along more and more slowly.

There are comets belonging to the Solar System which draw closer to the Sun than Mercury and go farther away than Neptune.

Comet-pathways do not keep to the level of the chief planet-pathways. They slope about in all manner of ways, like the paths of the little Planetoids.

Very many comets belong to the Sun’s kingdom. They journey round and round the Sun, and appear again and again from time to time. Some take only a few years for their journey, while others come back only once in the life of a man; and others again may be hundreds of years absent.

And some comets never return. They do not belong to our Sun, but only pay him a single visit. These are strangers to our kingdom of worlds, travelling from the kingdom of some other far-off sun, perhaps one of the twinkling stars in our sky.

A stranger comet comes, like other comets, slowly from the distance, quickening his speed day by day as he gets nearer to the Sun. Then he rushes at a mad rate round the Sun and flies off in a new direction, to quite another part of the heavens.

What wonderful stories these bright visitors might tell us, if they could speak, of the skies from which they arrive!

A comet is made of three parts: the nucleus, or the thickest portion of the head; the coma, or the bright fog round the nucleus; and the train or tail.

Sometimes there is no nucleus, and sometimes there is no tail; but there is always a coma--a soft hazy cloud of light, perhaps small enough to look like a dim star at first.

But a comet watched from the Earth can never be so far off as the stars. Even the very largest comets are seldom to be seen farther away than Jupiter.

There are comets of all sizes, from the huge to the tiny. Perhaps one would find as much difference between comets in the sky as between a whale and a minnow in water.

Under a certain size they are hidden from us; but tinier comets than we can see may float in the sky by myriads.

Of those which we can see, the thickest and heaviest part of the whole--the “nucleus”--may be only about fifty or a hundred miles through, or it may be some thousands of miles. The coma, or bright fog surrounding this thickest part of the comet, is generally as much as ten thousand miles across; and sometimes it is a hundred thousand miles. As for the bright train, it is, when fully formed, seldom less than ten millions of miles long, and sometimes it is a hundred millions of miles. Such a tail as this would reach the whole way from our Earth to the Sun.

Yet a comet is not heavy. Its make is most wonderfully light; far more so than the very lightest world in the Sun’s kingdom. Saturn is not so solid or so heavy as water; but a comet really almost seems to be less solid and heavy than a mist.

Very faint stars can be seen shining through thousands of miles of comet-thickness; while it does not take much of an earth-mist to hide the light of even the brightest star.

Not long ago people were much frightened at comets. If a big one appeared in the sky it was thought to be a sign of something dreadful about to happen. Nobody then had any idea what immense numbers of comets are always in the sky.

It was feared that, if a comet should run against our Earth, the whole world would be destroyed. Nobody knew how very light and delicate in its make a comet is.

If such a thing ever did happen, which is most unlikely, one cannot say that no harm would be done; but certainly our Earth would not be destroyed.

These comets seem to shine partly in the sunlight, and partly by their own brightness.

You must not think that a comet always has a tail. More often, when one is first seen in the distance, it is only as a little hazy patch, or like a dim star, with no train of light at all.

But as it comes hastening out of cold and darkness into the warmth and glare of the Sun great changes take place in its shape.

The nucleus very often gets a little smaller; and why this should be I cannot tell you. But the coma gets larger, and takes to throwing out bright jets. Then the tail begins to grow; and day by day it becomes larger and larger.

A comet drawing nearer to the Sun travels head-foremost, with the tail following after the head. This is only what one would expect.

But as the comet swings round the Sun with a mighty rush its tail is sent round also in a great outward sweep, pointing all the time away from the Sun.

Lastly, as the comet on the other side of its pathway goes away from the Sun, its tail travels first, end-foremost, and the head follows after the tail.

So the head of a comet always points towards the Sun, and the tail of a comet always points away from the Sun.

We know little as to the true nature of comets. They are, however, believed to be made partly of shining gases, and partly perhaps of small masses or lumps of more solid substance--in short, of little meteors.

Biela’s Comet was once a comet belonging to the Sun’s kingdom; but its story is rather curious. In 1846 it broke into two separate comets. These two kept company for a while, and then parted. One went ahead, and one dropped behind. After this both vanished, and in their stead our Earth in her journeying came across a shower of meteors. So perhaps the meteors are the remains of those two comets--the broken up bits, if one may so speak.

The Great Comet of 1882 was often to be seen in full daylight. When passing away, after its rapid whirl round the Sun, it could be perceived in telescopes at a distance greater than that of Jupiter--a very unusual thing.

In the picture of a Sun-Eclipse you will notice a tiny comet quite near the Sun. This little comet had been hidden by the Sun’s glare and nobody knew it to be there at all. But when the moon glided between, hiding the Sun’s great brightness, and a photograph was taken--then the tiny comet had its picture taken also, side by side with the dark body of the Moon and the light edge of the Sun, with the fiery sea and sharp mountains.

QUESTIONS.

1. What does the word “comet” mean?

It means “a hairy star.”

2. What is a comet like?

A star-like body, with a hazy kind of fog round it, and a long tail.

3. Do comets always have tails?

No; the tail generally appears when the comet comes near to the Sun.

4. Tell me the three parts of a comet.

The Nucleus, or thickest part; the Coma, or hazy part round the Nucleus; and the long Tail or Train.

5. Which of these is always found in a comet?

Only the coma. The nucleus and tail may be wanting.

6. What shape is a comet’s pathway?

A long oval: sometimes very long and narrow indeed, with the Sun close to one end of it.

7. How long is a comet’s year?

All lengths, from three or four of our years up to hundreds of our years.

8. Do all comets belong to the Solar System?

No; only a certain number seem to do so.

9. Where do others come from?

They seem to come from far-distant stars, paying one visit to our Sun, and then going off, never to return.

10. How does a comet carry its tail?

Always pointing away from the Sun.

11. Which goes first, head or tail?

When a comet is coming towards the Sun its head journeys first. But when a comet is going away from the Sun, its tail journeys first.

12. What is a comet made of?

It is believed to be made partly of gases, and perhaps partly of meteors.

13. Is a comet heavy, or light?

Very light indeed, compared with its great size.