Part 2

As I want you to remember it, take my pocket-book and mark off with the pencil a picture of the star and its four neighbours, putting a cross under the one that has just risen.

There it is,--the under one of this cluster I have sketched. I shall know him again.

Now, then, you must go below, and resume your observations to-morrow. I took the exact time--seven p.m.--when you saw the star rise. You must look for it at seven to-morrow evening.”

At four minutes before seven the next night, up rose the pretty object, and stole a glance at the boy across the ocean.

“Father, father! it’s up, it’s up!” sang out the happy lad.

His father looked at his watch, and said--

“Your star did not come to his time. It was four minutes to seven o’clock when he appeared, and that is four minutes too soon.

How is that, father? I must ask you, for it is of no use asking the star.

Perhaps you will still wonder when I tell you that to-morrow evening it will be eight minutes to seven before it gets up at this place.

Why, this is like the sun among the stars--always getting out of his place. But the sun gets right after a year.

Do you think our star will ever get right--so as to rise at seven some other evening?

I do not know, father.

If it rise four minutes earlier every day when will it be an hour earlier?

In half-a-month; and twenty-four hours earlier in twelve months. But that is the time when it will be visiting at night again.

So you have ascertained that the stars go an apparent journey round the earth in a year; like as the sun takes a year to go his apparent course among the stars.

Then both must have the same cause. As I found the daily motion of the stars was owing to the real daily motion of the earth, I suppose this yearly motion of the stars must be caused by some yearly motion of the earth.

Either the sun turns round the earth once a year, or the earth does round the sun.

Well, father, the sun appeared also to turn round the earth once a day. I do not think I could run round the deck in two minutes while I am two hours doing it. Then, as I am satisfied it did not have a daily motion, I am the more disposed not to trust to this apparent yearly motion. It is more likely, too, that the little world should run round the great sun, than that the great sun should revolve round the earth.

I perceive you are as settled upon the annual or yearly motion as you were upon the diurnal or daily revolution.”

THE SEASONS.

“James, my boy,” said Mr. Marple “do you know why there should be winter and summer?

No, father, I really cannot tell. I know that in England the sun was higher up in the day, and that he stopped up longer in summer than in winter. But then as I believed that the sun ran about as he liked, I was not surprised at his change of place. Now, however, I am very anxious to know more about this subject.

When were we in the circle of the Pleiades, that is, the place where at some time or other of the twenty-four hours they would be overhead?

O, that was long before we got to the equator.

Yes, it was about 1500 miles to the north of us. Now we are in the circle of the Virgin cluster.

But the Bull and the Virgin belong to the twelve constellations, of which we were talking, forming a broad circle, along which the sun appears to travel.

True. But they are not in an upright circle. Their line will cross our equator Line.

If that be it, the sun does not appear, then, to go round us over the equator.

No; for if it did, it would be seen overhead at that place every day at twelve o’clock; whereas it shifts its course continually. One day it is over one part at noon, and upon another day over another part.

Why, if the circle of the Zodiac be inclined to the circle of the equator, it will cross it in two places. I suppose that is the reason why, on the celestial globe, one constellation, as the Virgin, is over the equator circle; others, as the Bull, over some place to the north of it; and some others, as the Scorpion, to the south.

You guess rightly.

Then if that be it, the sun must appear to cross the equator twice in the year.

It does so on the 21st of March and 23rd September. These are the vernal or spring equinox, and the autumnal equinox. March is the vernal equinox of London, or equal day and night in spring; but it is the Australian autumnal equinox. In September the Australias have the spring equinox, and the English the autumnal. The sun is seen overhead to the furthest north on the 21st of June, and to the furthest south on the 21st of December.

Why, the first is the summer of England, and the last the winter. Then I suppose that is the reason why in summer the sun looks so high up at noon. Then in winter, as it is shining more over parts south of the equator, the north would neither see nor feel so much of him.

That is correct. You now see why the Australian summer is at the time of the English winter.

Yes, father; because they are on opposite sides of the equator. But you know that the sun does not really move about like this. I want to know what does move.

The earth, to be sure.

I understand that; but neither the rotundity nor motion seems to explain this mystery of the Zodiac. But let me see. When the world swings round the master light, is it obliged to keep bolt upright--north pole up and south pole down?

How would your equator then be, James?

Right opposite the sun. But that will not do, for it is not always opposite to the sun. Perhaps the world goes waddling along like a duck, first on one side a little bit, and then on the other. This would bring the sun opposite to either place.

It would be a very regular waddle. We will try your scheme. Pick up that round wooden ball there. Carry it round the capstan, and see whether by twisting it from one side to the other you can keep the south pole shone upon for six months, and the north pole shone upon for the other six months.”

James now makes a trial. But after all sorts of turns he failed in producing the English winter and summer in the proper time and for the proper length of time.

“I must give it up, father.

Just slant your ball a little out of the perpendicular, and carry it round your sun without moving it from its position.

Capital! It explains the whole. When it was in front of the capstan, the south pole was toward it; that was the Australian summer. When I went to the right side, neither end was inclined toward it; this was the sun over the equator. When I came to the back of it, the north pole was toward it; this was the English summer. When I reached the left side, my equator was opposite to it; this was the sun’s second crossing of the Line. When I got to the front again, there was my south pole pointing to the capstan again.

Then you really understand the seasons. Of course the spring in England is the autumn of Australia. But I have something else to recall to your recollection. Have you forgotten the old church on the hill at home?

No, father, and never shall. How pretty it looked when the sun rose over it upon a fine spring morning, with the birds all alive with their song, and the bees down upon the flowers!

Did the sun rise over the church at hay harvest?

No; I remember then it came up from behind the squire’s house to the left of it.

Where was the sun first seen upon a cold winter’s morning?

Aye, I fancy I see his red face looking lazily over the parsonage to the right of the church, and that when we had done breakfast, too.

How came the sun to jump about in that manner?

Why, do you see, he did not hop from one side to the other. He would get a little further from the church each morning till he rose over the squire’s house: then, as if ashamed, he seemed to creep back to his old place. Afterwards he appeared to take it into his head gradually to get on to the parsonage, and then he would once more turn to the church.

Which was on the north-side of the church, James?

The squire’s house. Now, I think I can explain all without your questions this time, my dear father.

Then go on by all means.

The sun rose over the squire’s house in summer, for that was the north side; the north pole was then inclined to the sun. At spring it was half-way between the squire’s and the parsonage; this was the sun over the equator. In winter the sun rose at the furthest part to the south; this was where the north pole was turned away, and the south side turned to the sun.

Well, then, you have satisfactorily explained the seasons.”

PRECESSION OF THE EQUINOXES.

It was with no little impatience that James had waited for the first appearance of the Southern Cross, the emblem of Australia. He first noticed it when a long way north of the equator, and each evening afterwards bestowed a considerable attention upon it. As he made southing on his voyage the glorious constellation rose higher and higher in the heavens. Hour after hour would he sit, marking its progress, wondering much to see it more or less upright according to the time of night, and more or less declining from the perpendicular at the same returning hour of evening upon different days.

James could understand the Cross seeming to perform a circle round the south pole of the heavens, as he had seen the Bear round the north pole, from the rotundity of the earth and its daily motion.

It was quite natural, then, that the lad should be so enthusiastic when he spoke of the constellation to his father in these terms:

“O that pretty Southern Cross! I like it more and more as I get down to the south. I did feel sad to see the old Bear drop gradually, night after night, till it seemed to sink for ever in the northern Atlantic; but I have a new friend here, that keeps rising higher and higher each night, as if to welcome us to our southern home. How it would please many boys in Europe to see the Cross!

But it was seen in Europe formerly.

Another puzzle, dear father. Did it run away from the south pole to have a look at the north one?

It was not a run, but a gradual slide out of its place, and as gradual a slide back into it again.

What a queer trick! Is it on its rambles now, I wonder?

It is always on the move.

Yes; I know it describes a circle in the sky once in twenty-four hours, but that is owing to the world rolling over to get its daily light.

Yet it has another motion; not really its own any more than the other. And this leads it further away from the central point over our south pole, and then brings it to its place again.

Well, I am glad it is here now for me. Will there be time for my London cousins to catch a glance at it when it wanders northward again?

O no, no; it does not move quite so fast as that. But if Abraham had been sojourning in southern Europe, instead of Asia, he could have seen it, though it was then making its way back to the south.

How could the Cross slip away from the other southern stars to go on such migrations?

Not so. All the stars keep their places relatively to each other, as you see them do in the nightly progress from east to west. In the daily motion none get before the other, nor did the Cross get before its neighbours.

Well, I am fairly done. That is a riddle.

Look at the question, boy. What makes the apparent daily motion of the stars?

The real daily motion of the earth.

And if, then, you observe any other peculiarity of movement among your bright friends up there, to what may you reasonably ascribe it?

I should imagine some peculiar twist, roll, or slipping of this world of ours.

True. And if there be seen among the polar stars, north as well as south, a slight but regular movement, a sort of a swing round, so that some stars get farther off the spot we call the south pole of the heavens, while others approached it nearer, and yet so swung round that at last all find their old places again, how could you get a motion of the earth to make up for all that?

Ah! I must contrive that the world should do three things. It must roll over once a day, roll round the sun once a year, and yet wriggle about in another way at the same time.--Stop a bit--I have it. My top turns round itself, and pretty quickly, too; it shifts about on the floor almost in a circle as it turns round; and I see it, especially when it is quietly spinning, have a slow roll of the head, like a sailor when walking ashore, as if its head were a little giddy.

You have hit it exactly, my good fellow. The three motions of your top are much like the three motions of the world. That top-heavy slow swinging of your top while it is spinning in full force is like a sort of head-rolling of the earth. The poles seem to have a roll of their own, independent of the regular roll.

Yes; and that would make the Polar stars seem as though they were swinging. But how long are they before they are in their regular places again?

Astronomers calculate 25,850 years.

But will not this changing make the star-charts of the ancients all wrong?

Indeed it does. Aries the Ram, for instance, as a _sign_, is the first thirty degrees on the celestial globe; but as a _constellation_ it has shifted to between the thirtieth and sixtieth degree on the ecliptic. This is called the precession of the Equinoxes.

How is that? I know when the equinoxes are--March 21st and September 23rd--when it is equal night all over the world.

The European vernal equinox took place at the first point of Aries. But by this precession, or more properly recession, or going backward of the stars, the Equinox takes place when the sun is in Taurus the Bull, two thousand years after it was in the Ram.

I see. The stars not only get earlier four minutes a-day to accommodate the earth in its annual motion, but make a change to accommodate the swinging of its pole.

Can you tell me, James, how much the stars slip back in a year?

Let me calculate it. There are 360 degrees in a circle, and sixty times as many minutes; that is, 21,600. As it is 25,000 years in the circle, the stars would not shift one minute of distance, and the sun’s apparent size in the heavens is thirty minutes, or half-a-degree.

By this Precession a star comes to the same spot about twenty minutes later every year. It is, as it were, sliding back. In Europe, James, one effect has been to bring what we call the Polar star within one degree of the north pole of the heavens; whereas, a few thousand years ago it was twelve degrees off it. The bright star of the Lyre Constellation will some day be the North Polar star.

Ah, but in 25,000 years the present Polar star will be in its old place. The end of the little Bear’s tail seems now fastened on the north pole; but it must be a sort of greasy pole for the stars to slip off it as soon as they get near it.”

FIXED STARS NOT FIXED.

It had been a sultry day. The sun rose with a fiery glance, and the sea blushed with a deep red as his hot beams glided over the heaving surface. The clouds hung heavily and gloomy in one part of the horizon, sending back to the sun a glowing, angry look in exchange for his fierce gaze. It was not long before the massive clouds came rolling up, as they were pressed forward by the western breeze, which was rushing forward as hastily as the rest. The sun tried to frighten the advancing vapours, but could not. They came onward till they covered the sky, and drove the bright rays from the gilding of the waters.

The wind, rejoicing in its power to drive the clouds, now flew upon the ocean and buffeted the waves. Affrighted at its blows, they fled hither and thither in their distress, and mingled their roar of terror with the shrill shrieks and cries of their invisible foe.

It was a storm. The captain, who had been attentively watching the signs, and losing his breakfast, saw what was coming, and prepared for it. The sails were taken in, and the ship made snug and safe. The storm came with a wild burst all at once, and reeled the vessel almost upon its side. But the officer was there, the helmsman was awake to his duty, and the lounging barque kept on her course in spite of her erratic movements.

James was not insensible to fear. He was but a boy. And when, in addition to the noise of wind and billows, the thunders came pealing from the heavens, he instinctively clapped his fingers to his ears, as if to drown the sound and his fears. The vivid stream of lightning, as it darted from cloud to cloud, played round the ship, or plunged madly into the sea, added to the interest and the terror.

But the echoes died away to whispers. The clouds put on their snowy robes. The ripples gently laughed on the bosom of the ocean. The sun, no longer angry, smiled kindly upon the scene. In the evening, a few lines of vapour were motionless in the sky; and the long, uncertain heavings of the waters told that a calm was softly ruling all.

The boy was found by his father looking at the placid face of the heavens.

“What, boy! at your old post, staring at Orion again.

I’ll tell you what I was thinking of, father. You see the world is like this ship--always on the rock, never really still--and I was thinking how pleasant rest is, and what a lot of it they must get up there in the fixed stars.

Not so; we are now learning that they are, like some boys, not so steady as they look.

You surely don’t mean to say that such respectable old folks as Sirius and his bright friends ever go dancing about in the heavens like giddy Venus and Mercury? I know they appear to tumble about over our masts in a storm at night, but that is because the ship is rolled over by the winds and waves.

It is really true that they do not keep to their places, though the change of position is so very slight as not easily to be discovered. Your friend Sirius, for instance, has been closely watched, Dog as he is, for two thousand years. We know what his place was that time ago, and what it is now; and it has shifted to the southward half-a-degree.

That is as much as the apparent diameter of the sun. But has he any company in his rambles?

Yes; the _Bull’s_ Eye, Aldebaran, has kept up pace with him.

Quite right to have an Eye upon the Dog. But has any exact difference of position been observed since we have had good telescopes?

The star sixty-nine of the _Swan_ has been well watched for fifty years, and found to have gone on four minutes--the eighth part of the moon’s face. A star in the _Indian_ has moved seven seconds in the year. They are not fixed, like the ancients thought they were, in a crystalline sphere; or riveted, as Aristotle taught.

Pray, is our sun no more fixed than the rest of them?

He, too, is on the move.

But where are they all going to?

That has been a matter of dispute. Some thought the stars were dancing round Sirius. But Mädler, the German astronomer, would have us believe that they have a greater fancy for some spot near the _Pleiades_. Others think of Hercules. But all would take many millions of years.”

DISTANCES AND SIZES OF THE STARS.

James had a long talk with the captain one day about the telescope, and got a capital lesson about the magnifying power being according to the character and size of the glasses used. The sailors called the instrument the “Bring ’em near;” and it does make distant objects look as if near. It was explained to the boy that the stars were of different magnitudes or sizes, first, second, third, &c., according to their relative distances. The following conversation followed between the father and son:--

“Now, my boy, I must try and give you some idea of the penetrating power of light, that you may get an idea of the enormous distances of the stars. You are aware that the more distant they are the less their light. A star of the first or nearest magnitude will have one hundred times the light of one of the sixth magnitude. A telescope, therefore, gathering one hundred times the ordinary light, will make the sixth look as near as the first.

And will it be one hundred times further off?

No; light increases or diminishes according to the square of the distance.

I know. If the light be one hundred times less, the star will be ten times further off, for the square of ten is one hundred. I can understand now that a thing is only seen by the help of light. I do not see many stars, because their light is too little for my eyes to take in. The telescope has bigger eyes to take in the light of the distant stars and nebulæ.

The pupil of the eye is but one-eighth of an inch in diameter. An object glass of twelve inches diameter is, therefore, ninety-six, or say one-hundred times as long. As the light seen is according to the square of the diameter, the telescope of twelve inches will receive one hundred times one hundred, or ten thousand times more light.

But is there a way of measuring the quantity of light?

There is. We find that the sun has twenty-two thousand millions of times more light than the nearest of the fixed stars.

Then, the square root of this ought to tell how much further off it is. Let me see. It will be about 150,000.

Yes. If the sun were put back 150,000 times further than it is, it ought to look as brightly as that star. If it does not, it is because it is really smaller than the star.

What! 150,000 times 95 millions!

But that is nothing; for it is only to the first rank. What of the twentieth magnitude?

Yes. But you say the nebulæ are further off than that.

I may tell you that if the sun moved three times as fast as the world does, in its six hundred millions of miles a year, it would take two hundred and fifty millions of years to get to as far as Lord Rosse’s telescope could see.

That takes my breath away.

Hear a little more. Light comes from the sun to us in eight minutes. It will take sixty thousand years to come from one of those stars Lord de Rosse saw. In fact his telescope has enlarged our universe one hundred and twenty-five million times.

Then I think religious people ought to thank astronomers for showing them more of the greatness of God. Those who only thought of him as the Creator of the three thousand stars, to be seen by the naked eye, could not have such a notion of his vast power as those who know of millions upon millions of suns.”

* * * * *

After this, James was left to digest such wonderful lessons. When his first astonishment had passed away, his curiosity was excited to know more about the distances of the stars, so that he might form a simpler idea of the thing. He took, therefore, another occasion of bringing up the subject in these words:--

“Father, do you really believe the stars are so far off?

I am obliged to believe many things I do not understand, upon the testimony of trustworthy witnesses; but in this case I can form a good guess of the truth. Do you remember what I once told you of the parallax, or angle of observation of the sun or moon?

Yes. That of the moon was 57 minutes, and the sun 8-3/4 seconds. A degree is 60 minutes, and a minute 60 seconds.

Very well. Then 57 minutes, or 3420 seconds, will be four hundred times as much as the other. If the moon be 240,000 miles off, the sun will be four hundred times further or 96,000,000.

But how do you get this parallax?

Distances are calculated by the angle made in looking at an object from two places. The two lines of sight cross one another. A great base is needed to view a distant object, or else no angle can be observed. Astronomers take the diameter of the earth’s orbit.

That is twice ninety-five millions of miles.

With that base--that is, looking at a star from both sides of our orbit, or at six months’ interval--we could get the two lines crossing one another, and so making an angle. The further the object, the more minute the angle. Only a few of the fixed stars could be observed in this way, as they generally are too far off to give an angle.