CHAPTER X.. How the World Journeys.

HOW THE WORLD JOURNEYS.

You see now how it is that on the Earth we have day and night. The whole Earth spins round and round, and so each part of her in turn comes into sunlight.

This is not the only way in which our Earth moves. She also journeys round and round the Sun, revolving always on her axis every day as she goes.

A year on the Earth is about three hundred and sixty-five days long, or twelve months. Our “year” means just that time in which the Earth travels once round the Sun. And in that year, as she journeys, she turns right round upon her axis three hundred and sixty-five times.

So there are two separate movements of the Earth. A boy may, if he likes, stand still, and spin round like a top. Or he may walk round the table without spinning. Or he may do the two things together: he may walk round the table, and as he goes he may keep spinning like a top. That is how the Earth goes round the Sun.

As she moves she is always at much the same distance from the Sun--about 92 millions of miles off. In one part of her pathway she is a little farther, and in another part a little nearer; but there is never very much difference.

Sometimes she is on one side of the Sun, sometimes on another side. Always, day after day, and year after year, she keeps steadily journeying round and round the Sun.

This is how we get our seasons upon Earth. Spring is followed by summer, summer by autumn, autumn by winter, winter by spring again. It is on and on, the same thing, year after year.

In an earlier chapter you heard about the Equinoxes and the Solstices. There is an Equinox in the spring, a Solstice in the summer, an Equinox in the autumn, and a Solstice in the winter. At each of these times the Earth is in a different part of her pathway round the Sun. Also she is differently placed.

It is of course quite clear to you by this time that our Earth is a round solid globe in the sky. Also that the axis of the Earth is a straight line from her north to her south pole, right through the middle of her.

Now I want you to understand that, as the Earth goes round the Sun, her axis leans over a little in one direction, and always in the same direction.

Have you a good-sized soft ball to picture the Earth? Stick a big bonnet-pin right through the middle of it; that pin is the Earth’s axis. The pin’s head shows the north pole, and the pin’s point shows the south pole.

Properly, of course, if this little Earth is about four inches through in size, it ought to travel round and round a huge shining balloon, three-quarters of a mile off, to show how the Earth goes round the Sun.

But this, I am afraid, you will hardly be able to manage. So you must let distances alone, and just have a candle or a lamp on a table, and learn with that how we get our summer and winter.

First, now, hold the ball on one side of the candle. Let its north pole--the pin’s head--point in a sloping way over the candle-flame; not exactly towards the candle, and not up straight towards the ceiling, but in a slant.

This means Summer for the northern half of your little world, and Winter for the southern half. You must notice carefully how the north pole is towards the candle and the south pole is away from it. So, at the same time, we in the north have our Summer Solstice and people in the south have their Winter Solstice.

Next, carry round your ball to the other side of the candle, just opposite to where you have been; but do not turn it in your hand as you go. The slant or lean of the pin must be the same; and the pin’s head must point still just where it pointed before. You will see now that the south pole is towards the candle, and the north pole is away from it.

This means Summer for the southern half of your little world, and Winter for the northern half. So we in the north have our Winter Solstice while friends in the south have their Summer Solstice.

Between Summer and Winter lie the Spring and the Autumn Equinoxes.

For either of these you must carry your ball to one side of the candle, half-way between the summer place and the winter place. Your pin must still slant exactly as it did before, with no change in the direction of its head.

You will then find neither north pole nor south pole towards the candle. The pin lies sideways to it, and the candle-light falls on both poles alike. So here, as the Earth spins, days and nights are of just the same length; and this is one of the Equinoxes.

Over the greater part of the Earth days and nights are always altering in length between the Spring Equinox and the Autumn Equinox. Days are getting longer and nights shorter; or nights are getting longer and days shorter.

In the very far north, and in the very far south, near the poles, things are different. There, when the pole is turned towards the Sun, one full day lasts for months, with no sunset. And there, when the pole is turned away from the Sun, one full night lasts for months, with no sunrise.

Some chapters back we were thinking about our Earth as she floats in the sky, with stars all around her everywhere. You heard how the stars seem to travel every night across the sky, and how this seeming journey of theirs is brought about by our own Earth’s daily spinning on her axis.

The stars which we see in our sky are not exactly the same all the year round. Some are the same, but some are different. Fresh star-groups come into view in the evening at one time of the year, and vanish again at another time.

This is because we can only see those stars which lie in a direction away from the Sun. It is impossible for us to see those which lie beyond the Sun; for they are above the horizon when he is above it, and their faint glimmers are quite hidden by his radiance.

As we go round the Sun, we see him month by month in a fresh part of the sky, and behind him lie fresh star-groups. So our journey makes the Sun seem to move among the stars, and the Sun’s seeming pathway we call the Ecliptic.

As the Earth travels, her north pole always points exactly in one direction--always towards the Pole-star.

If a man were standing at the north pole and looking upwards, he would see the Pole-star always, at any hour of the night, in just the very same spot.

When we think of our Earth as a globe floating in the sky, we must try to remember that in the Sky there is no real “up” or “down.” This you have heard before.

Our “up” is always towards the sky, and away from the ground. But as the Earth turns round and round our “up” is every hour in a fresh direction.

For the blue heaven is all around us, and from every part of the Earth we look up into the depths of the sky.

We speak of some stars being in the “northern sky,” and of other stars being in the “southern sky.” For our own use we have given the name “northern sky” to one part of the heavens, and the name “southern sky” to another part.

Only “north” does not mean up, and south does not mean “down.” The only true “up” for us is from any part of the Earth where we may be towards the sky over our heads, and the only true “down” is towards the middle of our Earth, under our feet.

QUESTIONS.

1. What is a Year?

The time that our Earth takes to journey round the Sun.

2. How long is the Earth’s Year?

Twelve months, or about 365 days.

3. How many times does the Earth turn round on her axis in one Year?

Three hundred and sixty-five times.

4. What is meant by the Seasons?

Spring, Summer, Autumn and Winter.

5. How are the Seasons caused?

The Earth travels round the Sun with her axis slanting.

6. How does it slant?

Always in one direction, with her north pole pointing to the Pole-star.

7. What brings summer to us in the north?

When the Earth is on one side of the Sun her north pole is towards the Sun, and the northern half of the Earth gets most of his heat and light.

8. What brings winter to us?

When the Earth gets round to the other side of the Sun her north pole is turned away from him, and so we in the north have less heat and light.

9. Do they have summer and winter in the south of the world?

The southern half of the world has summer when we have winter in the north, and winter when we have summer in the north.

10. How is this?

When the north pole is towards the Sun the south pole is turned away, and when the north pole is away from the Sun the south pole is towards him.

11. When are the Equinoxes?

In Spring and Autumn, half-way between Summer and Winter.

12. In which part of the world is the Equinox?

All over the world at once.

13. Which pole is then turned towards the Sun?

Neither pole. The Earth’s axis is then sideways to the Sun, and his light falls on north and south pole alike.

14. What is the Ecliptic?

The path which the Sun seems to take in the sky through one year.

15. Where would a man at the north pole see the Pole-star?

Always exactly overhead.