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ASTRONOMY FOR YOUNG AUSTRALIANS ***




Produced by David E. Brown and the Online Distributed
Proofreading Team at http://www.pgdp.net (This file was
produced from scans of public domain works at The National
Library of Australia.)









ASTRONOMY

FOR

YOUNG AUSTRALIANS.

BY
JAMES BONWICK, F.R.G.S.,
AUTHOR OF “GEOGRAPHY OF AUSTRALIA,” ETC., ETC.

MELBOURNE:
PUBLISHED BY SAMUEL MULLEN,
COLLINS STREET EAST.
1866.




PREFACE

TO

SECOND EDITION.


A cheap edition of the “Astronomy for Young Australians” having
been desired, to meet the requirements of the Public Schools of the
colonies, Mr. Bonwick respectfully submits the present issue to the
favourable attention of Teachers.

ST. KILDA,
November 1, 1866.




ASTRONOMY

FOR

YOUNG AUSTRALIANS.


A Fine ship was buffeting the waves, like a strong swimmer in his
sport. The clouds dashed wildly to and fro, but left many wide spaces
of blue to be dotted with stars. The sea was in high spirits, throwing
up spray, as if to quench the bright lights that looked down so kindly
upon it.

The vessel quite enjoyed the fun, and her prow bobbed in the current,
and gathered up the foam to send it flying up the rigging, or leave it
hissing and fuming by her sides. How skittish she was this evening, as
the light clouds ran over her head, and the wind puffed her laughingly
along! She was young and strong. Her timbers were tight; and her yards
well braced. She had just left port, painted and clean, with a new suit
of sails, and her copper as bright as a fresh-coined penny.

And where was she going? Laden with the treasures of English cotton and
woollen mills, of iron-works, and other industries, she was away to the
far-off land of Australia, on the other side of the round globe.

And whom had she got on board? There were sailors to manage the
merry ship; there were passengers, going from the white cliffs of old
England, to dwell in the gum forests of the kangaroo home.

As the shore of Britain melted away in the distance, men and women
hung over the bulwarks, dreaming of sweet vales they left behind, and
sighing deeply as they thought of loved ones there. How long would it
be before they saw those vales again, or smiled with friends beside
that hearth of love!

The love of one’s country, as the land of our friends, the joy of
freedom, the defence of the right and true, is a duty as well as a
delight. And those born in the new land of Australia should cherish
so beautiful and healthful a home, and help to make it happy in the
virtues of its people.

One little intelligent fellow, about ten years old, hung closely
against his mother’s dress, and caught hold of his father’s hand. James
knew his parents must feel sad at leaving dear ones behind; and he
inwardly resolved to be a great comfort to them now by being a good,
loving son.

The last speck of land was gone, and every body turned round to the
ship, preparing to make that a home. Mr. and Mrs. Marple thought of
their only child, for others had been left to sleep in a church-yard
far away. He was their hope and joy. They determined to add to his
happiness, and secure him from the evil of idle ship-life, by improving
his time and his mind.

This very evening, therefore, the kind father took James beside him
in a quiet corner on deck. They spoke at first of aunts and uncles,
cousins and friends. Then a plunge of a porpoise turned their eyes to
the sea, or a scream of sea-fowl set them talking of natural history. A
sober chat followed, and a moral lesson came.

A sudden unrolling of clouds brought out such a dazzle of starry
splendour, that both gazed with delight upon the ever-wondrous heavens.
It was then that Mr. Marple determined, among other subjects, to make
the boy understand astronomy. This he was to do by leading the lad
himself to observe; and, under his direction, to find out the laws of
the universe himself.

A few stars were pointed out. There was the North Polar star, that
keeps such a constant place. There were the gentle Pleiades. There was
the beautiful Belt of Orion to the south. There was, too, though very
near the waves to the southward, the bright Dog-star, _Sirius_.




DAILY MOTION OF THE EARTH.


The following dialogue took place one evening on deck: -

“Well, my little fellow, what is there which so attracts your
attention? for you have scarcely moved for this last half-hour.

I have been looking at the stars, father.

What is so wonderful in them this evening?

There is something wonderful in them which I never noticed before.

What is that?

I will tell you. When I first came upon deck after tea, I saw a bright
star rising like as if it came out of the water. While looking at its
pretty light, it seemed to get higher and higher up the sky.

Did it run away from the other stars, and get a head of them?

No. But I thought it did at first, until I had watched, and then I
found that the stars around about kept at the same distance from my
bright star, and appeared to keep company with it while climbing the
sky.

And what else did you see?

Afterwards I thought I would watch the beautiful Sirius. This seemed to
be moving on, too. Then I looked at others. But they all appeared to be
rolling along after one another.

What do you make of all this?

I can make nothing of it.

What puzzles you, my boy?

Several things. I cannot understand where all the stars are rolling to,
why they all keep together so cleverly in their motion, or why they are
moving at all.

Did you see any stars setting as well as rising?

Yes, father.

Well, as I want you to find out this subject yourself, I shall allow
you to stay up later this evening, to give you time to make a few more
observations. Now, follow your bright star a little further. Look at
that blue one overhead, and trace his journey. Have your eyes upon
Sirius, and the band of Orion.

I will, father, and thank you.”

* * * * *

The boy was left at his star-gazing, and intently was his mind fixed
upon his work. It was no vacant stare he gave at the heavens. He had an
object before him.

The conversation was afterwards renewed by the father: -

“I should think you were sleepy, James.

No, father: the stars kept my eyes open.

How, now, does your star get on?

Look up there. He has got as far as that.

Where is the blue-coloured one?

O, that has gone down to the western edge.

What of my old friend Sirius, and his neighbour Orion?

They have been travelling the same road.

Well, you must stop star-gazing to-night.

But will you not explain the reason of this curious motion, my dear
father, before I go down?

No, my lad, you must have another look at the stars to-morrow evening
first.”

The little fellow retired slowly and thoughtfully to his berth; and
dreamed of stars and ships in mingled confusion. How he longed for the
sun to leave off his shining! He never watched that orb so as he did
that day! He saw it rise, ascend, descend, and set. When the short
twilight was over, the little twinkling bodies came out one by one, as
a few western clouds changed from gorgeous red to colours of a darker
hue. He first distinguished Sirius, and then the band of Orion. The
blue star took some time to come forth; but when it did, there it was
right overhead, as it had been early the last evening. The bright
star was again on the tip of the distant waves. He stared again. He
remembered how he had left them all the night before, and now the stars
seemed in their old places again. His kind father came up to him.

“O, father,” cried the boy, “all my stars have gone back into their
places again this evening.

Did you see them travel back as you saw them travel forward?

No. But there they are, though I do not know how they got there.

I think I saw you looking pretty often at the sun, to-day. Did you
notice anything peculiar about his movements?

Yes. I noticed that he seemed to go the same road as my stars did the
evening before.

But you saw him ascend in the east, rise nearly overhead, and then set
in the west. You did not observe your stars do that.

No, father; but I have it. It took a long while for the sun to go all
that distance; and I fancy that if I had watched my bright star as
long, I should have seen that set in the west also.

Certainly you would.

Well, but how did they get back into their places again?

How will the sun manage to get on the east side, to-morrow morning?

Ah! I see. What will explain the movement of the stars will explain the
movement of the sun.

I think you will find it so.

There is only one way by which I can account for this - they must all
turn round the earth. Is that it, father?

Everybody used to think so.

But what surprises me is this; as the stars are turning round us, none
get before or behind the others. They keep the same distance apart.

Do you not know that all the stars are stuck in a huge mass of blue
stuff, called the sky, and that when the sky turns all of them must
turn with it, and, unless they tumble out, they must keep in their
places?

O, father, you are joking.

Well, then, if they are not joined together, why do they move so
uniformly?

That I cannot tell.

No two of the stars are of equal distance from us. They are all
scattered through space, like a lot of marbles in a scramble.

Then if they had to move round the world, I am sure they never would
keep in such order, especially as some would have to go so much faster
than others; for they have a much greater distance to go. They cannot
turn round the earth in that way. But yet they seem to do so.

Yes. But just look forward. Do you see how the head of the vessel is
dipping into the Bay of Biscay?

I do, father.

But do you notice that as the bow of the ship sinks and rises, all the
stars to the southward seem to dance up and down?

Now I understand. Like as these stars appear to dance about according
to the motion of the ship, so the apparent motion of the stars round
the earth in twenty-four hours must be owing to the motion of the
earth. The earth must turn over once a day. It is easier to imagine
this, than to imagine such extraordinary motions to take place among
the stars.”




ROTUNDITY OF THE EARTH.


It was fine fun for the healthy, hearty English boy to be tossed about
in the Great Atlantic. What a noble, towering wave was that which
raised up the ship, but only to let it down in the deep trough of the
sea, while its crest hung over it, as if to drive it to the bottom!

As the day passed off, and the sombre clouds thronged the western
horizon, James got toned down, and became thoughtful and still. Then
was his face turned from the waves to the stars, being never tired of
watching their motions and admiring their beauty.

He had long been regarding the heavens one evening, when he turned, as
he heard his father’s step, and exclaimed -

“The more I study the stars, father, the more their motions bother me.

How is that, my son?

When we first left England I saw the Polar star and Great Bear a good
height up in the heavens, toward the north, and Sirius low down toward
the south.

Have they been playing any tricks with you?

I do not know that. But as we have been sailing towards the equator,
I have observed the Great Bear going gradually down, and Sirius as
gradually coming up. I am afraid I shall lose the Polar star altogether
soon. And now I have another trouble. I do not know what to make of a
number of new stars coming up from the southward.

Would it be owing to the shape of the earth, think you?

Aye, I never thought of that.

If we were sailing on a flat surface all this long way, would you see
these ups and downs of the stars?

No, I think not. The truth is, it seems to me that we have been going
up a hill, if the stars do not move.

That will account for your Great Bear going down; but what should cause
these new stars to appear in the south?

Why, that looks as though we were getting up to the top of the hill,
and should have to go down on the other side.

What shape, then, do you suppose the world ought to be, to account for
these curious apparent motions of the stars?

I fancy it ought to be round, and that is what I have always been told
it is, father.

I want to ask you whether you have noticed anything peculiar about the
apparent daily motion of the stars. Do they all rise in the east and
pass over to the west?

This is just the subject I wanted to speak about. I thought at first
that they did so, and ought to do so. But I have since found out that
this is not the case. They do not all describe the same sized circle.

What have you observed?

Some make a regular sweep across - others only rise to pop down again.
Yet there is a more wonderful thing. The stars near the North Polar
star perform their circle without going under at all. Then the greatest
puzzle is, that the Polar star always keeps its place without any
turning.

You do not think that the stars describe circles each day of different
sizes according to their distances from the Polar star?

No. I feel sure that it is all a cheat of the eye, though I don’t see
where the cheat is.

I thought you were quite settled in your mind that the apparent motion
of the stars was owing to the revolution of the earth.

True, father; yet I see no connection between the movement of the earth
and the stationary place of the Polar star.

What appearance would the heavenly bodies present, were the earth like
a big drum rolling over?

Why, I should fancy that then they would all rise in the east and pass
over, in an orderly, proper manner, to the west. Though I have been
told that the earth is round I do not understand how that should affect
the motion of all the stars, as we see them.

Come, let us see. If the world be round, then some parts would describe
a less circle than others in the daily revolution.

Of course, father.

Would there be any portion that would not turn at all?

Yes; the poles, or extremities.

Where would be the most motion?

At the equator.

Then, if the stars be still, and the earth move and be round, would
not -

O, I know all about it now. The stars over the pole would appear
still, because the pole is still. The stars over England would appear
to describe a larger circle than those of the Great Bear over Sweden.
Where we are now, the stars overhead describe a very large circle.

And at the equator the greatest circle of all?

Yes, yes, - and I can now see why the Great Bear seems to go away from
us, as we get up the hill toward the equator. That constellation is
opposite to a part of the earth a good way to the north of us. As
we rise higher from that place, we must appear to rise higher than
the stars over it. How I do long to stand under that bright Belt of
Orion, and under Sirius! And how I should like to see the pretty Cross
stretched over the mast-head!

Wait a bit, my lad, and you may see all these wonders in time.

How delighted I am to make out that all those funny motions among the
stars are simply owing to the world being round.”




SIZE OF THE WORLD


It was while half tired of ship life already, and longing for the
excitements of shore in a new country, that the boy turned round to his
father, and said, with a sort of yawn -

“What a long while we shall be getting to Australia, father.

It is a great distance to go, James.

Well, I think the earth must be a good big ball to roll about among the
stars.

You ought to know its size.

I have been told that it is 25,000 miles round; but who has measured it.

Why as to that, you and I might measure it some evening by the stars.
You know that the great space between the Belt and Polar Star is
one-quarter of the great circle of the heavens.

Of course.

Every circle is reckoned to be divided into spaces called degrees, of
which 360 go to form the circumference; so that one-fourth will be
ninety degrees. You can imagine that space divided into ninety of these
portions, called degrees.

What rare compasses that would take!

Those two bright stars overhead, which are about twice the same
distance apart as the apparent diameter of the Sun, would be nearly
equal to one of these ninety portions. Now, you know that if you were
at the pole, you would have the Polar Star above you; and, if at the
equator, Orion’s Belt would be over your head.

I understand all that, father.

You know, then, that you might be in a place where one of those two
bright stars would be overhead, and you might journey on to another
place further south, where the other one would be at the zenith or
overhead.

This is all clear to me.

Then if you measured the ground you had gone over, it would be equal to
one-ninetieth part of the space between the pole and the equator, or
about seventy miles. Can you tell how many miles it would be from the
pole to the equator?

That would be seventy times ninety - which is 6300.

But that is only one-quarter of the way round - is it not?

Yes; the whole distance round would be four times 6300, or about 25,000
miles.

Well done - we have measured the circumference of the world.”




THE EARTH NOT QUITE ROUND.


“Father,” exclaimed James one day, “the captain has been giving me
a riddle which I cannot guess. He asks where a pound of shot is not a
pound. Is he joking with me?

Not at all, lad. A bag of shot weighing a pound in London, or even in
Melbourne, will not weigh a pound on the equator.

What queer things you do tell me! How can that be?

You know that the world attracts you.

Yes; I know when I jump up I am soon brought down, and sometimes
unpleasantly so, too.

As the world is round, and everywhere the world pulls things down to
its surface, it is as if it were drawing all to the centre. This is
called the _Attraction of Gravitation_.

And I suppose that like as fire is hotter the nearer you get to it, so
does the attraction get greater the nearer you get to the centre of the
earth.

True; but can you guess how this gravitation can be weight?

I know when I pull a thing, it is as if I pushed it, or put a weight on
it. So the pulling of the earth will make the thing attracted feel as
if a weight were on it.

And can you not see why a pound should somewhere not be a pound?

No; unless I made a big hole towards the centre, and then the pound
weight would be heavier. But you told me the world was round, and it
must be as far from the centre at London and Melbourne, as at the
equator.

But if from trial it be found that the pound really is less at the
Line, what would it prove?

It proves what I can’t see, that the earth at the equator is farther
off the centre than what it is at London or Melbourne, and that would
make the earth not a true Globe.

Neither is it my son: though the difference is but little. It is about
a dozen miles thicker down to the centre at the Line than at the poles.

Then, as London and Melbourne are each nearly half-way between the line
and the pole they will be nearer by six miles to the centre than the
equator is.

You have solved the problem, and understand that the world is not a
sphere, but what is called an Oblate Spheroid, being flattened at the
poles.

As I cannot easily find out by weighing that a pound is not a pound:
can you tell me any other way of proving that the earth is not quite
round, dear father?

Yes, I can. Do you see the reason why the pendulum returns down when it
is moved up?

The gravitation, to be sure.

If a thing weighs heavier in one place than another through a
difference of gravitation will there be a difference in the two places
with the pendulum?

I should fancy that where the earth pulled the harder the pendulum
would come down the quicker.

If it came down quicker it would rise quicker; that is, the beat would
be quicker. What effect would this have upon the clock?

The faster it beat, the faster the wheels would be moved, and the
faster the hands would turn on the face.

Just so: the clock would be faster in time when the pendulum beat
quicker, and slower in time when the pendulum beat slower.

But how could I find this out, father?

This way. It is ascertained that in London a pendulum must be 39-1/7
inches from the top to the middle of the swinging-bob to beat a true
second at a time.

Then I am sure the London pendulum will not be pulled down so hard at
the equator, and so it will be longer coming and going.

What must I do, then, to make my London pendulum beat seconds on the
equator?

Why, shorten it a little bit.

Yes; or, what is the same thing, move up the weight at the end of the
pendulum a little. I must shorten it one-tenth of an inch, or else it
will lose sixteen hours in the year.

Then all the clocks carried to Melbourne from London will be wrong.

Will they go too fast or too slowly in Victoria?

They will be a little too slow, as Melbourne is nearer the equator than
London is.

If a Melbourne clock-maker want his second pendulum to beat seconds in
London what directions must he give?

He must tell the folks in London to lengthen the pendulum a bit.

You will see that by the world being flattened at the poles the degrees
of latitude will differ. One close to the equator measures 68-3/4
miles, in Victoria about 69 miles, in England 69-1/8, while in Lapland
it would be 69-1/4.

What would be the mean length of a degree?

About 69-1/20 miles.

Let me multiply that by 360, to find the mean circumference of the
world; I have it - 24,858 miles.”




ANNUAL MOTION.


The ship was much delayed in the Region of the Calms, as it is called;
that is, near the equator.

After a while the astronomical dialogue was resumed.

“Well, James, look now at your twinkling friends, and tell where you
are.

I suppose by the North Polar star resting upon the waters to the north,
that we must be now nearly on the top of the hill, by the equator.

We are so, my lad. But what time did the sun rise this morning?

At six, and it set at exactly six this evening.

Do you know what day of the month it is?

The twenty-third of September.

Have you ever observed the sun to rise and set at six o’clock before,
on the voyage?

No, father.

Well, then, you will recollect hereafter that you were upon the equator
on the twenty-third of September, when there was equal day and night.
But did you not know of equal day and night at home?

Yes; about half-way between winter and summer.

Just so; that is, at the end of September.

Why should it be so there in England and here too?

We will try and find out. I think I have pointed out to you the
Pleiades.

I know them well. They are in the Bull cluster.

There happens to be a band of twelve constellations forming a broad
circle of the heavens.

Is that what is called the Signs of the Zodiac?

The same. Can you repeat them?

Yes, father. Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer
the Crab, Leo the Lion, Virgo the Virgin, Libra the Scales, Scorpio the
Scorpion, Sagittarius the Archer, Capricornus the Goat, Aquarius the
Water-bearer, Pisces the Fishes.

Over there to the eastward is one of these that rose as the sun
was setting. The sun is seen opposite one or other of the twelve
constellations in each month of the year. It is observed, for instance,
to rise each morning for a month in the constellation of the Ram, and
for the next month in the Bull.

How can it be among those stars when you say they are so much farther
off?

By saying it is in such a constellation, I mean that it is in a line
between us and that cluster of stars.

Why, then, the sun appears to go a journey between the stars and us for
a whole year. Then it will rise in the Ram twelve months after it had
been seen opposite that constellation. I wonder how this can be. Did
you not say, father, that neither the stars nor the sun really moved?

I did.

Well, then, it must be either the stars turning about the sun, or the
sun taking an annual trip about them, and yet the stars do not move,
and the sun does not.

Let us leave the sun and his trip for a little, while we have another
talk about the stars.

Notice that red star just rising there on the larboard side.

I have it, father.

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


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