Let us now turn our thoughts beyond the earth and its atmosphere to the phenomena which may properly be described as astronomical. We see a procession of objects moving ceaselessly across the sky—the sun by day, the moon and stars by night. These all appear to cross the sky from east to west, because the rotation of the earth, from which we view the spectacle, causes us to move continually from west to east.

The most conspicuous phenomenon is of course the daily motion of the sun across the heavens, producing the alternations of light and darkness, heat and cold, which we describe as day and night. The rising and setting of the moon and its passage across the sky are only one degree less conspicuous, and must have been not only noticed, but also familiar, since the days when human beings first appeared on the earth.

The sun shews no changes either of shape or brightness, except when our own atmosphere dims its light, but the moon continually varies in both respects. Every month it goes through the complete cycle of changes, which we call its “phases”. It begins as a thin crescent of light, which we describe as the new moon. This increases in size until after about a week we have the semicircle of light we call half moon, and then a week later the complete circle we call full moon.

We know that the moon always looks about the same size in the sky and from this we can conclude that it is always at about the same distance from the earth. And we can measure the distance in the same way as we measure the distance of an inaccessible mountain peak, or the height of an aeroplane.

When an aeroplane is up in the air, people who are standing at different points must look in different directions to see it. If it is directly overhead for one man, it will not be directly overhead for another man a mile away, and its height can be calculated simply by noticing how far its position appears to be out of the vertical for the second man. Using this method, astronomers find that the distance of the moon varies between the limits of 221,462 miles and 252,710 miles, the average distance being 238,857 miles. Thus, in round numbers, we may think of the moon as being a quarter of a million miles away.

At such a distance, we can hardly expect to see much detail with our unaided eyes. Indeed, as we watch the moon sailing through the night sky, we can detect nothing on its surface beyond a variety of light and dark patches, which, with a bit of imagination, we can make into the man in the moon with his bundle of sticks, or an old woman reading a book, or—as the Chinese prefer to think—a jumping hare.

Every year for more than a century, the Royal Institution has invited, some man of science to deliver a course of lectures at Christmastide in a style “adapted to a juvenile auditory”. In practice, this rather quaint phrase means that the lecturer will be confronted with an eager and critical audience, ranging in respect of age from under eight to over eighty, and in respect of scientific knowledge from the aforesaid child under eight to staid professors of science and venerable Fellows of the Royal Society, each of whom will expect the lecturer to say something that will interest him.

The present book contains the substance of what I said when I was honoured with an invitation of this kind for the Christmas season 1933—4, fortified in places with what I have said on other slightly more serious occasions, both at the Royal Institution and elsewhere.

It is a pleasure to acknowledge many courtesies and return thanks for much valuable help. I am indebted to Sir T. L. Heath for permission to borrow largely from his Greek Astronomy and other books; to many Institutions, Publishers and private individuals for the loan of negatives, prints, blocks, etc., and permission to reproduce these in my book—detailed acknowledgment is made in the List of Illustrations.

Let us leave the earth, in which we have burrowed for long enough, and turn our thoughts, and our eyes, upwards.

We all know what we may expect to see—the sun, the blue sky, and possibly some clouds, by day; stars, with perhaps the moon and one or more planets, by night. We see these objects by light which has travelled to us through the earth's atmosphere, and if we see them clearly, it is because the atmosphere is transparent—it presents no barrier to the passage of rays of light.

Perhaps we are so accustomed to this fact that we merely take it for granted. Or perhaps we think of the atmosphere as something too flimsy and ethereal ever to stop the passage of rays of light. Yet we know exactly how much atmosphere there is, for the ordinary domestic barometer is weighing it for us all the time. When the barometer needle points to 30, there is as much substance in the atmosphere over our heads as there is in a layer of mercury 30 inches thick. This again is the same amount as there would be in a layer of lead about 36 inches thick, for mercury is heavier than an equal volume of lead in the ratio of about six to five. To visualise the weight of the atmosphere above us, we may think of ourselves as covered up with 144 blankets of lead, each a quarter of an inch in thickness.

These are restless days in which everyone travels who can. The more fortunate of us may have travelled outside Europe to other continents—perhaps even round the world—and seen strange sights and scenery on our travels. And now we are starting out to take the longest journey in the whole universe. We shall travelor pretend to travel—so far through space that our earth will look like less than the tiniest of motes in a sunbeam, and so far through time that the whole of human history will shrink to a tick of the clock, and a man's whole life to something less than the twinkling of an eye.

As we travel through space, we shall try to draw a picture of the universe as it now is—vast spaces of unthinkable extent and terrifying desolation, redeemed from utter emptiness only at rare intervals by small particles of cold lifeless matter, and at still rarer intervals by those vivid balls of flaming gas we call stars. Most of these stars are solitary wanderers through space, although here and there we may perhaps find a star giving warmth and light to a family of encircling planets. Yet few of these are at all likely to resemble our own earth; the majority will be so different that we shall hardly be able to describe their scenery, or imagine their physical condition.

We all know now that our sun is a very ordinary star, but it took men a long time to discover this. Perhaps this is not surprising, for certainly it does not look much like an ordinary star to us. The reason is, of course, that it is enormously nearer than any of the other stars.

We have seen how the ancients imagined the earth to be the fixed centre of the universe, round which everything else moved. The stars merely formed a background of light, against which they could map out the motions of the sun, moon and planets. They thought of the stars as attached to the inside of a hollow sphere, which turned round over the earth much as a telescope dome turns round over the floor of a telescope, or “as one might turn a cap round on one's head”. And although a few of the more philosophical of the Greeks gave reasons for thinking that the earth moved round the sun, they had no means of making their opinions or arguments known to a wide circle of people, so that these were forgotten as the world gradually became submerged in the intellectual darkness of the Middle Ages. Then, in 1543, a Polish monk, Copernicus, advanced views and arguments which were very similar to those which Aristarchus of Samos had propounded 1800 years earlier, although the extent to which he was indebted to his Greek predecessors is not clear.

There are nine planets circling round the sun, of which of course the earth is one. Of the other eight, five have been known from pre-historic times, while the remaining three—the three farthest from the sun—are comparatively recent discoveries.

The row of models exhibited in fig. 60 shew how greatly these nine planets differ in size. Those which are nearest to, and farthest away from, the sun are the smallest, while the middle members, Jupiter and Saturn, are the largest. Jupiter, the central member, is largest of all, with a diameter of nearly 90,000 miles, and a volume 1300 times that of the earth. Jupiter stands in the same proportion to the earth as a football to a marble, while on the same scale Mars would be hardly larger than a pea.

If we wish to complete our model by placing the objects shewn in fig. 60 at their proper distances, the nearest planet, Mercury, must describe an orbit which is not quite circular, but is such that, even at its nearest approach to the sun, the planet would be 20 feet away. The earth must keep at a distance of 50 feet from the sun, while Pluto, the farthest planet of all, must describe an orbit nearly half a mile in radius.

We see that the solar system consists mainly of empty space, and yet the emptiness of the solar system is as nothing compared to the emptiness of space itself.

The moon and planets look very conspicuous objects in the sky, but we know that these are very near neighbours which only look bright and big because they are near. For the rest our unaided eyes can see nothing of the universe except stars.

A small telescope or field-glass will shew us more stars in abundance, but it will shew us something else as well. A new class of object comes within our ken, the fuzzy indefinite patches of faint light which we describe as “nebulae”.

The word “nebula” is of course the Latin word for a mist or cloud. In the early days of astronomy it was used indiscriminately to describe any object of misty or fuzzy appearance—any object, indeed, which did not exhibit a clear outline. Since then it has been found that the nebulae fall into three distinct classes.

The first consists of objects known as planetary nebulae, which lie entirely within our system of stars. It is now known that these are themselves stars which, for reasons not altogether understood, have become surrounded by very extensive atmospheres. Examples are shewn in fig. 98 (facing p. 204). We described the red giant stars as large, but when their atmospheres are counted in, these stars are beyond all comparison larger. Our rocket, travelling at 5000 miles an hour, would take 9 years to travel through the biggest of red giants, but about 90,000 years to travel through one of these planetary nebulae.

So far we have been concerned only with the smaller of the objects in space. Smallest of all were the pellets of matter which we describe as shooting-stars when they fall into the earth's atmosphere; these are so small that we could hold thousands of them in each hand. The largest object we have discussed so far has been the giant planet Jupiter, with about eleven times the diameter of the earth. A box big enough to hold Jupiter would hold II × II × II or 1331 earths—eleven each way. Yet even Jupiter is quite small in comparison with the sun, which we shall examine in the present chapter, and the sun is smaller still in comparison with the larger stars and other objects that we shall examine subsequently. Broadly speaking, the sun is as much bigger than Jupiter as Jupiter is bigger than the earth—Jupiter could contain more than a thousand earths, but the sun could contain more than a thousand Jupiters. To carry on the sequence, each of the blue stars we shall consider later could contain more than a thousand suns, while each of the “giant red” stars could contain more than a thousand blue stars. And each of certain nebulae which we shall discuss in our last chapter of all not only could contain, but actually does contain, thousands of millions of stars.

In giving a course of recent wireless talks, I assumed that my listeners had no previous scientific knowledge of any kind, and tried to introduce them to the fascination of modern astronomy and to the wonder of the universe we see through the giant telescopes of to-day.

The present book contains these talks expanded to double their original length, still in the informal conversational style and simple non-technical language of wireless talks. It is totally unambitious, aiming only at providing an easy, readable and not over-serious introduction to the most poetical of the sciences.

We have seen how the stars shew as great a range of candle-power as there is between a glow-worm and a searchlight; while their range of size is as that between a speck of dust and a motor-car. The range in their weights is much smaller, but still it is about equal to that between a feather and a football. And in every respect the sun is somewhere about average. It could hardly be expected to strike the exact happy mean in every way, but it never misses it badly. To put the same thing in another and less complimentary way, the sun is totally undistinguished in all respects—in weight, in size, in temperature and in candle-power.

Clearly, however, we get very little knowledge of the general nature of the stars from a mere mention of extremes and of one average star. We should not know much about the English population if we had only been told the heights and weights of the shortest dwarf and the tallest man, and that a particular man 5 feet 9 inches high was a good average Englishman in all respects. We want a more detailed knowledge as to the classification of the stars by size, candle-power and weight.

Suppose that all the entrants to a dog-show broke loose and ate their labels, and had to be reclassified.

We have already seen how important the force of gravitation is, both to astronomy and ourselves. It keeps the moon tied to the earth, and maps out the paths of all the planets and other members of the sun's family; it raises the tides in our oceans, and, we believe, raised those far greater tides in the sun which, some 2000 million years ago, brought our earth, and so ultimately ourselves, into being. Finally, it keeps us alive, by making the earth stay near the sun instead of running away into the icy depths of space.

Let us try to understand a little more as to what this force is.

The Force of Gravitation

No man can lift a ton weight; he is prevented by the force of gravitation—or gravity, as we usually call it when it acts on earth. This pulls the weight to the ground, and proves too strong for him.

Again, we find it impossible to throw a cricket ball for a mile; we are prevented by the same force, which continually pulls the ball towards the ground, and invariably succeeds in getting it down before it has travelled a mile. We can easily throw the ball out of our hands at twenty miles an hour, and if gravity did not draw it earthward, it would cover a mile every three minutes, and after a year it would be far out in space, 175,000 miles away from the earth.

A century ago astronomy was concerned with little beyond the sun, moon and planets—the small colony we have described as the sun's family. To-day it is mainly engaged in studying in detail the various other stars and colonies of stars, such as the three stars which form the system of Alpha Centauri, our nearest neighbours in space. The aggregate of all such stars and colonies constitutes the Galactic System, the vast conglomeration of stars whose rim is the Milky Way. At the same time, astronomy has discovered that even this huge system is only one of a vast number of somewhat similar systems. The present situation may be perhaps summed up in the three statements:

1. (1) The earth is only one member of the sun's family.

2. (2) The sun's family is only one member of the Galactic System.

3. (3) The Galactic System is only one member of the system of star-cities in space.

This is the furthest that astronomy has travelled so far, but we may well wonder what the situation will be, say, a thousand years hence. Will the above three statements still suffice, or will they have been supplemented by more statements of the same kind? In other words, shall we find that the whole system of star-cities only forms one unit in a still vaster assembly, and this assembly perchance a mere unit in something vaster still?