IN STARRY SKIES

Evening Post, Volume CXVI, Issue 145, 16 December 1933, Page 21

IN STARRY SKIES

WHAT IS THE SUN ?

(By "Omega Centauri.") As soon'as, we know the immense distance that the sun is from us we can form some conception of its size, and can begin to realise what an astounding body it really is. The- moon and the sun look about the same- size, but, if the sun is 400 times as far away, its volume must be 04,000,000 times as great. This is a closo approximation to the truth. The moon, moving in an ellipse around tho earth, is continually changing its distance from us. Its mean distance is 238,857 miles, but it may be as much as 252,710 miles, or as little as 221,463 miles from us. That

is, at one time it may be 31,247 miles nearer to us than it is at another time. Similarly, since the earth travels in an ellipse around the sun, the distance between these two also is continually changing. The mean distance is about 92,870,000 miles, but the actual distance varies by fully 3,000,000 miles. One consequence of these changes of distance is.that the apparent diameter of the moon changes from 1766 to 2015 seconds of arc, whilst that of th 3 sun varies between somewhat narrower limits of 1891 and 1956 seconds. Thus the moon appears sometimes slightly smaller and sometimes slightly larger than the sun, a fact of great importance inthe study of eclipses. As we have already seen, tho mean distance of the earth:from the sun is a fact of such fundamental importance that its a task of considerable difficulty, has been attempted in a great many different ways. With instruments capable of extreme accuracy, the differences between tho results secured by different methods have boon reduced, so that at. present we can bo sure that the sun is a vast globe with a diameter just over 109, times that of the. earth, or 400 times that of the moon. Its volume is 1,301,100 times that of the earth, and over 65,000,000 times that of the moon. When ivo consider the actual masses of tho three bodies we find tho differences are not t quite so extreme. The earth happens to have tho greatest density amongst all the bodies of tho solar system. It is more that five and a half times as massive as the samo volume of writer. The moon has only about three-fifths and the sun only a quarter of this density. It would take more than 81 moons to make up the mass of tho earth, and 333,400 earths to make up the mass of the sun. It is the immense mass of the sun that enables it to control the movements of all the members of its great family. Each one is perpetually striving to fly off at a tangent to its course, but is restrained in a most mysterious way by the gravitational pull of the sun. The satellites that form such an interesting feature of the system are restrained in a similar way by the gravitational pull of the planets. It is estimated that although Neptune is more than 2,793,000,000 miles from the sun, and although the gravitational pull diminishes according to tho square of the distance, it requires the strength of a bar of steel 500 miles in diameter to keep the planet from flying off its course. Fortunate indeed it is for us that the sun prevents the earth from wandering too far from it or approaching too near to its fiery furnace. We are absolutely dependent on the regulated supplies of warmth that are furnished by the sun. Terrestrial life has become adapted to the prevailing conditions, and it1 is very doubtful whether we could live at all in any other part of the solar system. Let us compare the moon, the earth, and the sun in this one resjpeet of temperature. The moon is a land of extremes. Being less than a quarter of a million miles from the earth we might think that it should enjoy a temperature almost:exactly the same as we do. But unfortunately it has not tjeen able to retain enough air or water to form a blanket for it. Tho surface is therefore a barren waste frozen by night and scorched by day. The temperature of any spot on which the sun's rays fall vertically is estimated to be just above that of boiling water. But the heat does riot penetrate far, and at a very short distance, down, the rocks are believed to be below freezing point. For just over a fortnight of our time any particular spot on the moon is exposed to the fierce glare with never a cloud to give a moment's relief, and then it is suddenly plunged into darkness, without any twilight and immediately experiences the most intense cold. In the middle of the dark hemisphere tho temperature is said to be at least 150 degrees centigrade below free/ing point. In New Zealand we generally measure temperatures still by tho Fahrenheit scale. This is rather unfortunate as in scientific work the centigrade scale is always used. _In the •Centigrade scale, freezing point is 0, boiling point 100. In the Fahrenheit scale, freezing point is 32, boiling point 212. We can, therefore, if wish, transpose any Centigrade reading into Fahrenheit, or Fahrenheit reading into Centigrade by using tho following formulae: Let F degrees bo the Fahrenheit reading and C degrees the Centigrade one, then 9C equals five times (F—32) or 5F equals 9C, plus 160. On the surface of the earth, we have a considerable range of temperature, but tho changes are slow and mild in comparison with what we find elsewhere. . Man is able to exist temporarily even in the greatest cold experienced on the great Antarctic plateau, or in the intense heat near tho Equator. But the conditions ruling in nearly all parts of the universe except on the surface of our planet would bo almost instantaneously fatal. High above the earth's surfaco also, or down in its depths, conditions are intolerable. Not very many miles directly above the Equator a temperature as low as 85 degrees Centigrade below the freezing point is believed to rule, whilst, in the vast spaces between the stars, it is little above absolute zero. If on the other hand we descend below the earth's surfacej the. temgeraturg irises p.% jfirstj

at a rate of about 4.0 degrees per mile and then more slowly and at a'depth of 400 miles it is estimated to reach 3500 C. In distant space we should bo no better oft. The stars that we can see are all intensely hot, a.-.d probably most of the invisible ones are extremely cold." Our only hope would be to find a planet at a suitable distance from some bright star. None of tho nebulae that look so beautiful offer us aii; attractive home. The name given to them, when their true nature was quite unknown, means simply a cloud, and these cosmic clouds turn -out to be of several distinct kinds ranging from masses of glowing gas to vast galactic systems. But before passing in imagination to any of these we should glance at a small fraction of the astonishing wealth of knowledge that has been gathered by astronomers about tho particular star that wo call the sun. The very name is suggestive, being derived from an sAnglo-Saxon root meaning to shine, and we shall find

that a great part of our knowledge of the universe, to say nothing of the possibility of life,on earth, we owe to its radiation. Today we have tried -to emphasise its immense distance, from which it follows that it is of stupendous size. A favourite illustration of this is to imagine the earth at the aeritre of a great hollow sphere the

size of tho sun. The moon could then perform its monthly journey round the earth inside tho sphere without ever coming within 190,000 miles of its surface. We have also laid stress on tho sun's gravitational power wlfich keeps all the planets in their orbits, but we have still to inquire what this marvellous globe is like. What is its physical state and tho temperature of its different parts? What are the immense spots that appear but as dots on its surface, and the glorious crimson liames tljat rise hundreds of thousands of miles above its surface? What also is the mysterious corona or crown of glory, that is never seen except during the brief moments of a total eclipse? In asking these questions we aim at something deeper still, for our : sun is but a star, and' if we can find what it is like, we shall have a true sample of one of the chief constituents of the universe.