Herbert Hall Turner

“ We cannot perhaps make so good an imitation as we did of Galileo's experiment, by dropping balls from the roof, but we will make as good an imitation as we can. When astronomers measure the distance of the Moon or of any other object in the sky, they use precisely the same method as surveyors use on Earth, and indeed the method which every one of us uses nearly every moment of our wakeful lives. With our two eyes we are continually trying to estimate how far away men and things are from us. ”

“ If we bring the screen back towards the lanterns, the spreading beam recovers its brightness, and finally it ends by being brighter than the other. So in spreading out in that way the light of a star is always losing itself; and it is wonderful that it should get to us across those vast distances without being so faint that we cannot really see it at all. The light even of some of the brightest stars must travel hundreds of years before we can see it. And we have seen that there may be other things which might diminish its light on the way, dark nebulae which may screen or stop its light. ”

“ It is all very well, he reflected, to talk of the pull altering with the distance, but how are we to measure distance from a large body like the Earth or the Sun? Are we to measure it from the nearest point of the Earth, or from its centre, or from some point in between? When the whole Earth is far away, it does not much matter: but when we are dropping a weight as Galileo did, from the top of a tower, as at T in Fig. 4, then part of the Earth near F, the foot of the tower, is pretty close to us, and we have just seen how enormous the pull is when we come close to the attracting body. ”

“ Some of you have been at sea and seen a ship on the horizon; you know how small it looks when it is five or six miles away, and how close it must come before you can see the people on its decks. You can well imagine that at 240 miles you might not see the ship at all with the naked eye—even a very big ship. We have as yet been unable to see any signs of life on the Moon, but that does not mean that there is no life; still less that there has never been any life. ”

“ It is only when a great crowd of them are massed together to make the Earth that the pull becomes easy to feel. And when we get an enormously greater crowd still, such as the Sun, the pull is enormously greater. Why, then, does not the Sun pull us away from the Earth? Well, there is more than one reason; but one at least is that he is so much farther away, and this brings us to the next fact we have to notice about Gravity—that it depends on the distance. ”

“ Now when Jules Verne's travellers opened the trap-door and put the poor dog out—probably with a fairly good push, because they would be in a hurry not to let more air escape than they could help—when they pushed the dog away from the projectile, would it fall back again on to the projectile as we fall back on to the Earth when we push ourselves away by jumping? We know that the Earth pulls things to itself, but do other bodies also pull other things to themselves in the same way? We can scarcely believe that they do because we never feel the pull of anything but the Earth. ”

“ Many or all of you may have seen a total eclipse of the Moon, when the Earth comes between the Moon and the Sun and cuts off the Sun's light from it; the Earth is so much bigger than the Moon that it casts a shadow wide enough to envelope the Moon for an hour and more. You remember Jupiter's big shadow and how the little satellites disappeared into and remained in it for a long time before they came out on the other side. The Earth's shadow is much smaller than that of Jupiter, but it is still large enough to swallow the Moon entirely. ”

“ Hence we can regard all the planets and satellites as a tiny little bunch travelling with the Sun, and we can talk of the Sun alone as travelling, carrying us with him in his pocket so to speak. Now how is our Sun behaving in this matter of migrations? Is he one of the stars that pass to and fro through the centre, or does he remain at a distance from it? The answer seems to be, from the best evidence we can get at the moment, that he travels to and fro, and at the present moment he has just been through or near the centre—only about a million years ago. ”

“ I have actually seen people stand up for more than eight minutes at the end of the journey, with bags and umbrellas in their hands, so little do they reckon of eight minutes. But even travelling by telescope, at the enormous speed of light, we should take years to get to the stars—very different from the eight minutes to the Sun. Hence you see how close the Sun is compared with any other star. He is so close that we can see details on his surface, whereas even in our most powerful telescope the stars are mere points of light. ”

“ We have just read in the newspapers that an aeroplane has ascended to a height of nearly four miles, and the question arises whether this is as far away as we wish to go. If we have any idea of visiting the Moon, then it is not nearly far enough, for the Moon is not four, but 240,000 miles away; an aeroplane must beat the present record by a considerable margin if it is to take us to the Moon. It is not altogether easy to realize what such a distance means, even when we remember that it is about ten times round the Earth. ”

“ As a young man beginning astronomy, I had the privilege of seeing this great telescope in the workshop at Hamburg in 1884, and I well remember my astonishment, not only at the size of the telescope, but at the number of beautiful devices for working it, especially at the eye end. When an astronomer is looking through a telescope, it is a great convenience to him to be able to make notes and measures of what he sees without taking his eye away: William Herschel had his sister Caroline to make the notes for him, but other astronomers have not had such devoted sisters. ”

“ I must mention here that the Sun is not burning like a fire or a gas jet: it is glowing like an electric glowlamp. You know the difference between the two cases: a gas jet or candle gets quickly used up; but the filament of an electric light bulb, thin though it is, lasts for hundreds of hours without being consumed. The light comes, not from the consumption of the filament, but from the energy supplied to it from the electric power-house. The question we are considering is—what is the Sun's power-house, what is the source of the energy supplied which makes it glow? ”

“ All the thousands of millions of stars in the sky may be suns like ours, and each of them may have many planets circling round it as we circle round the Sun, and yet Mr. Alfred Russel Wallace sincerely believes that there is not a sign of life on any of those thousands of millions! ”

“ A planet is not quite so clever and erratic as a burglar; indeed, if we have watched his or her movements long enough, we can calculate almost exactly where he or she will be at a future time. That is what Kepler and Newton have done for us by finding out the great Law of Gravity which controls the movements. But everything depends on that if; if we have watched her long enough. Ceres had only been watched for a very little time before Piazzi fell ill, and those who were accustomed to such calculations said it was not nearly long enough to enable the orbit to be calculated. ”

“ And we shall begin to-day with the first difficulty of getting away from this Earth at all.
Why is it so difficult to get away? If we jump up we come down again. We call the reason "gravity," and sometimes we are tempted to let the matter rest when we have given a name to the difficulty. But I want us to think a little more about it than this: to think about what the nature of it is, and also about the great men who found out for us what gravity is. ”

“ We will try another experiment. Instead of spreading out our light into colours, let us take light of only one colour, to begin with. We can do that by putting some common table salt into the flame of a spirit-lamp, when it will blaze with that brilliant yellow colour which we have seen is due to sodium. Now here is our bunch of ribbons of all colours, which perhaps one of my audience will kindly hold: when we burn our table salt only the yellow ribbon will show—the others will all appear black but we are not quite ready yet. ”

“ There is no doubt that if we actually took our "Voyage in Space" instead of only pretending, we should be chilled to the marrow very soon indeed. It is terribly cold in space—far colder than we can possibly imagine. ”

“ I think you will agree with me that this is a wonderful addition to our knowledge; we have been able to find out that a star which always appears just as a speck of light is really made up of two at a certain distance apart, one very bright and the other faint, partly shining of itself and partly being illuminated by the other; and that even the dark side of the fainter body is 16 times as bright as our Sun. ”

“ It is wonderful to think that we can also find the heights of the mountains and hills of the Moon, so that we can make not only a map, but a relief model. The Royal Astronomical Society has kindly lent us a beautiful relief model of a part of the Moon's surface, with a model of the neighbourhood of Vesuvius alongside it, so that we can compare the two. You see that on the whole the Earth's mountains are distinctly smaller than those on the Moon. ”

“ When, however, we take a photograph, the eye-piece is taken away and the light shines all over the plate at once, so that we are no longer confined to a small ring, but can photograph the whole object at once, faithfully: we see that these rifts are not straight but delicately curved, in a way suggesting that there are several rings whirling round the central bright nucleus. ”

“ Human beings have by this time found many ways of alleviating the winter, especially by means of fires and lights, but we must remember that we owe even these to the Sun. Without the Sun in times gone by, those forests would not have grown which to-day give us our coal: the fires which we light in the winter are in many respects the work of the Sun. Hence it does not surprise us that in the old days they used to worship the Sun as a god. ”

“ Yet a very great and unselfish man, who helped to make one of the greatest of scientific discoveries, has written a big book to try and establish this fact—that this little Earth of ours is the only place in the whole universe on which there is life. ”

“ If the Earth stood fast as our forefathers believed, the Sun and the stars revolving round it, we should retain something of the daily change in our journey: but now we know that it is the heavens which remain still, while the Earth rotates, we must make up our minds to lose the peaceful night with its invitation to sleep. ”

“ It is rather like that with the Sun: losing a lot of heat makes him reduce his establishment—makes him shrink, but the very shrinking causes a development of heat—more heat, perhaps, than he lost, so that he is actually hotter after the shrinkage than before.
This way of getting heat by shrinkage does not appeal to our imaginations so readily as the idea of getting it from fierce blows from meteors; but it is known to be just as real a way, and it can be calculated how much contraction is required to give out the heat we now experience. ”

“ We get another proof of this fact if we look at the edges of the Sun, where we see great red flames shooting up to enormous heights. Perhaps we ought not to call them flames, because in our earthly fire-grates a flame means that something is being burnt, generally the gas from the coal which is being burnt up in the air. The solar flames do not represent anything burning; they glow like the filament of an electric lamp, but we have no better word to describe them than "flames." ”

“ A body close to you pulls hard: take it farther away and the pull becomes less. Mathematicians say that it diminishes as the "inverse square of the distance"; but we can put the law into simpler language in this way. Take a body twice as far away, and the pull is one quarter of what it was; bring it twice as near, and the pull is four times what it was. And now let us go back to the history of the discovery and see how Newton found this out for us. ”

“ Those who cannot sleep generally long for the daylight. As the Earth turns round they are carried towards the place where the Sun rises; the birds begin to sing, the animals get up and begin to feed; boys and girls get ready for their breakfast and go to school and enjoy themselves, and have dinner about noon and go on with the other pleasures of the day: and finally, when bedtime comes, they plead for just another ten minutes because they know that going to bed means getting into the dark. They like to keep in the light as long as possible. ”

“ It was a most exciting announcement, because we all thought that we knew practically all there was to be known about the nature of air, and it was a great shock to find that we were all wrong. The new gas was called argon; and soon other new gases were found, especially by Sir William Ramsay—helium, and neon, and xenon, and a heap of others; so that unfortunate school-boys, instead of having to learn only four things as making up air, as we did, have now to learn a large number of names. Life has a terrible way of getting more and more difficult! ”

“ That is due to the fact that besides gravity that pulls the head towards the Sun, there is another force away from the Sun; it is as though the Sun was blowing the tail of the comet away from the head. Of course there is no actual blowing as one blows with one's breath ”

“ When we make a telescope with a lens we use a property of light called refraction, a word from the Latin which means simply breaking back. You know how a cricket ball can be bowled so as to go straight for some distance, and then when it hits the surface of the ground it "breaks back" owing to the spin the bowler has given it? Well, a beam of light behaves in much the same way when it is refracted; after going straight for some distance, it strikes a surface and turns off at an angle. ”

“ Any one who gets a motor-car looks carefully at all the parts; and to-day I want you to learn about the telescope, and what has been its history. Perhaps I ought not to say telescope, but camera, for most of the telescopes astronomers use nowadays are just photographic cameras on a large scale. So we might say that we will journey by camera. But in its early history the telescope was not a camera, and so we will for the present use the name telescope. ”

“ But now I want you to think of light instead of sound, because the stars do not whistle to us, but they do send us light. You remember that we were to compare light and sound several times in this lecture? And I want you to realize that just as there are scales in sound, which you have to practise on the piano, so there are scales in light—nothing more nor less than those beautiful colours which a prism makes for us. We will throw one of those rainbow-coloured strips on the screen ”

“ This old difficulty, however, is not yet done with: it crops up again in a most tiresome way when we want to find the distance of the Sun instead of that of the Moon, because the Sun is still further away—nearly a hundred million miles: and even when our base is the biggest we can get on our Earth, the "squint" required for the Sun is very small, and the difficulty of measuring it very great. The only thing then left for the astronomer is to take advantage of special occasions when the difficulty happens to be less than usual. Such an occasion was the Transit of Venus. ”

“ So I think for a few moments we might think about the air through which we must first ascend, and which corresponds to the dock from which we start for a sea voyage.
There is one property of the air that seems very remarkable to those who have never thought about it before. Here it is all around us. It does not seem to be exerting any pressure and yet it is pressing on us all with tremendous force—15 pounds to every square inch. ”

“ I can easily make a patch with no children visible by holding a screen in front of my eye. If the screen is near enough, quite a small one will do. A threepenny bit shuts out quite a lot of you. And similarly some kind of a screen, of dark nebulous matter, say, will easily explain any of these dark patches.
Hence we come to the conclusion that not only are there stars of the last degree of faintness, but there are in the heavens bodies of some kind not shining at all—"dogs in the manger," which not only do not shine themselves, but stop the light of other stars from reaching us. ”