WHY IT IS DARK AT NIGHT

Otago Witness, Issue 3829, 2 August 1927, Page 77

WHY IT IS DARK AT NIGHT

By Professor J. Arthur Thomson. When we look up into the star-strewn sky on an unclouded dark night we get an impression of immense numbers. But Spenser was right when he said in his “ Faerie Queeno ” that there were far more different kinds of animals in the sea than there are visible star’s in the sky. There are many tens of thousands of different species of animals in the sea—each itself and no other—but there are only about 5,000 stars visible to the naked eye, and only about 3,000 at any one time.

Yet our first impression is in a way right, for with an opera-glass we can detect in the course of time over 120,000 stars, while with a comparatively small telescope we can theoretically make out seven millions. And with a really great telescope, like that at Mount Wilson (with an object glass of 100 inches) hundreds of millions of stars can be seen. To which have to be added more millions which cannot be observed as such, yet are revealed by stellar photography. And so the number rises to about a thousand millions.

But recent research still further increases our estimate of the multiplicity of worlds. Everyone is well aware that the earth and the other planets form a system revolving around that planet star which we call the sun; and that the sun is a member of a system of stars—those that we see clearly on a starry night. But the modern view inclines to the conclusion that there are other systems of stars. For there are many nebula; which are very far away compared with the stars of our system, and each of these nebulae is a vast system of hundreds of millions of stars.

These distant nebulae are very numerous (perhaps two millions of them); they are all somewhat similar in size; and they are to a considerable extent evenly distributed through space. Thus the astounding picture grows, that the earth is a member of a solar system whose central star is one of many in a vast stellar system, which is in turn one of many other stellar systems! What is man? and yet again, What is man not?

When the part of the earth where Britain lies has its face, so to speak, to the sun, the heavens are full of light, unless it happens to be a very cloudy day. When the rotation of the spinning earth carries us round twelve hours afterwards to a diametrically opposite position, it is pitch dark, with the familiar exception of the very short summer nights in the North of Scotland, and supposing also that it is not a date when we are illumined by the reflected light of the moon. This is insultingly elementary, yet is it not worth asking the question why space should bo dark at all if there are millions upon millions of stare, most of which are pouring forth light? The main part of the answer is that the stars are so far away in the immensity of space that the amount of their light energy that the earth intercepts is almost unthinkably minute, only one six-millionth part of our sunlight. Moreover, the light of a star cannot be reflected from other glowing stars in the way the sunlight is reflected from our dead satellite, the moon.

The inconceivable distances of the stars should be recalled for a moment. For while the sun is 93,000,000 miles away, and we see it by the light that left it eight minutes ago, we see the nearest “ fixed ” star (Proxima Centauri) by the light that left it about four years before. Most of the stare that we see 'with the unaided eye we see by the light that left them when Galileo Galilei was watching them in Italy with his then new telescope, in the early years of the seventeenth century. Light travels at the rate of 186,300 miles per second, and a “ light-year,” the distance covered by light in a year, is six million million miles. And now the modern astronomers are telling us that the. most distant nebulae are more than a hundred million light-years away. Any of the numerous good introductions to astronomy will explain how the distance of a star may be determined by the parallax method.

It is a remarkable fact that before the beginning of this wonderful century, that we are all in some ways so proud of belonging to, no one knew the source of the light and heat that the stars give forth. They were supposed to be gigantic furnaces, stupendous crucibles, burning themselves away, and sometimes, as was observed, going out, as if they had exhausted all their fuel. One of the difficulties in the. face of this theory, however, is that the stars have lasted so long. The true explanation, both for star and sun, came with Becquerel’s discovery of radioactivity, for the fountain and origin of the greater part of the radiant energy of the stare and the sun is to be found in the disintegration of heavy atoms. The power of the light is the liberated power of the dust. No doubt this raises the still deeper question how complex atoms like those of uranium came to be built up. For we

cannot take as our starting point atoms which seem to bo themselves like miniature constellations. According to the leading authorities, the heaviest known atom, that of uranium, has 92 outer electrons whirling round in ellipses, while the centre or “ sun ” of the system consists of a core of 238 hydrogen-nuclei or protons and outside these 146 inner electrons. And speaking of protons, we must mention another theory in the forefront of discussion just now, that part of -the energy of sun and star is due to electrons and pro tons mutually destroying one another by headlong collision, and thus producing electro-magnetic radiations which startoff on a journey through space. —John o’ London’s Weekly.