IN STARRY SKIES

Evening Post, Volume CXIII, Issue 64, 17 March 1927, Page 17

IN STARRY SKIES

"THE EARTH AND THE STARS"

AN INSTRUCTIVE BOOK

(By "Omega Centauri.")

Dr. C. G. Abbot has a world-wide reputation as one of the most skilful of the modern school of experiments 1 astronomers. In reviewing Professor Hale's latest book we noticed some of Dr. Abbot's triumphs in not only measuring the heat energy received from stars too faint to be seen without a large telescope, but even determining how this energy is distributed throughout the spectrum. Dr. Abbot's book on "The Sun" has been a standard for many years, and we welcome another work from his pen. In "The Earth and the Stars" he covers a much wider field and treats it in a more popular style. A general survey is made of the whole universe, but this work is very different from an ordinary book on popular astronomy. It deals very largely with the wonderful progress made during the last quarter of a century. Dr. Abbot has taken an active and important part in bringing about the development of the science in recent years, and is able to speak with authority. Even in controversial matters in which certainty has not yet been reached his opinions deserve thoughtful consideration. He knows the views of all the greatest thinkers of the day and has weigheC them in the light of his own experience. He is thus specially fitted to present us with a striking picture of the present state of astronomical knowledge and theory.

is remarkable how completely Dr. Abbot keeps his own achievements out of sight. When referred to at all they are disguised as the work of the Smithsonian Institute. It is only in the titles of the illustrations of one or two pieces of apparatus that Ms name is allowed to appear. This book is generously illustrated with 32 plates and 46 illustrations scattered through the 264 pages of the text.

In the preface the author points out that the wonderful progress of the last quarter century has brought about a most happy and fruitful nwrriage of astronomy with physics. We seem to follow a definite thread of reason right from the structure of the atom to that of the universe itself. Abbot adopts Newcomb's advice and recommends everyone to spend the whole of a clear moonless night alone on a high hilltop watching the silent march of the stars. Supposing a night in October is chosen, for this purpose he describes what c n be seen. He then discusses famous astronomers and famous instruments, giving a bird's-eye view of astronomical history up to the beginning of the twentieth century. The graphic picture thus given of noble men prsuing truth for its own sr.ke can hardly fail to arouse enthusiasm. Wo see the slow procession of intellectual giants, starting with HippaTchus, followed at long intervals by Ptolemy and Copernicus. Then a brilliant group includes Tycho Brahe, Kepler, and Galileo. Crossing to England wo sco Newton, Bradley, and the two Herschels. Eeturning to the Continent we glance at the work of Laplace, Bessel, Fraunhofer, and Kirchoff. Before coming to modern times Abbot gives interesting details of some notable instruments. A picture is shown of a beautiful Per-

sian astrolabe, and then, after a glance at Lord Kosse 's giant reflector, interesting details are given of the great telescopes on Mount Hamilton and Mount Wilson, endowed by James Lick and J. D. Hooker.

The third chapter is entitled "The Little and the Giant." Here Abbot emphasises the fact that everything on the earth, in the sun, in all the stars of the galaxy and even in the nebulae, appear to be made up of the same two kinds of building materials, electrons and protons. The electrons are so small that it would take fifty thousand million in line to stretch across a full stop in small print. Protons, though 1800 times heavier, are still Smaller. All chemical atoms are built up of these two. The nucleus of an atom consists of a certain number of protons associated with about half as many electrons. An electron has a definite negative charge of electricity, a proton an equal positive one. The nucleus of an atom is thus positively charged, and it attracts an equal negative charge of electrons which move in orbits round it. Whenever an electron passes from one definite orbit to another a quantum of light it either given out or extinguished. All our knowledge of the distant universe is brought to us by radiation, which is started in this way. Light waves, though extremely short, are long compared with electrons. A wave of yellow light is about a hundred million times as long as the diameter of an electron, but the 'waves move so fast that five hundred billion of them pass/a given point in a second.

_ Modern astronomy is the interpretation of the messages of light.'

The next five chapters of the book, filling 100 pages, are devoted to the solar system. Starting with our earth and her neighbour, the moon, the author passes on to the sun's family. Then he considers our star, the sun, ana devotes a special chapter to "The Features of the Sun and Solar Eclipses," and another to "Heat Power and Life from Sun Bays." The treatment in all cases is novel and arresting. There is no mere re-statement of old facts. "The strangest thing about the earth," he says, "is the people upon it.'' laving neighbours, if we have any, do not force themselves on our attention. "We do not positively know that there is another single orb in the heavens that boasts of any living inhabitants. This leads to a consideration of the conditions of habitability, and Abbot concludes that Venus alone, among the planets, seemu a promising abode for life like ours, and it is impossible to speak confidently, for Venus is too cloudy for us to see its surface. The earth is pictured as having n outer crust 35 mileß thick, consisting of granitic rocks, then a layer of silicates of iron and magnesium about 1000 miles thick, next a layer 850 miles thick in which the silicates gradually change to metallic iron, and lastly a nickel iron core, like a meteorite in composition, over four thousand miles in diameter. Methods of determining the size shape, weight, and structure of the' earth are treated in an interesting'way. Of the seven pages devoted to the' moon no loss than four aro given to "The Moon Hoax," a subject which does not seem worthy of such preferential treatment. The origin of the unique surface features of the moon is passed over in silence. The one sentence in which the subject is referred seems to show that Dr. Abbot has not seriously considered the question. "Its surface," ho says, <<is highly desolate, abounding in extinct volcanic craterß, and level looking deserts." His references to '.he moon's temperature, however, are particularly interesting. The moon, liko the earth

sends out no visible rays of its own, but if our eyes were sensitive to rays ten or twenty times as long as those of red light, we should see her sending out radiation several times as intense as that which she reflects from the sun. Langley's bolometer, an extremely delicate electrica' thermometer, enables this radiation to be studied, and it is found that the sunlit moon approaches the temperature of boiling water. When a total lunar eclipse comes, however, it takes only a few minutes for the temperature to fall below the freezing point. The descriptions of the planets are original and full of interest. Venus, as we have said, is the only one which appears to Abbot to be fit to support life similar to ours. The only obstacle to the acceptance of this view is the uncertainty as to the time of the planets rotation. If, as some think, the day on Venus is equal_ to the year, there must be one hot side and one frigid side, both extremely unfavourable to living conditions. Abbot, however, does not accept this view. The evidence of the spectroscope is that the day on Venus must be at least as long as ten of our days, but in 1924 Pettit and Nicholson obtained conclusive. indirect evidence of moderately rapid rotation! They observed that the temperature of the dark side is nearly the same as that of the earth, and the mean temperature like that of our tropics. Conditions on Mars appear to be far less favourable. Dr. Campbell concludes that there is not one-fifth as much, water vapour on Mars as there is above Mount Hamilton iv cold, dry weather, and that is very little indeed. There is" also a great de'fieiency of oxygen.- The temperatures, moreover, are extreme. The desert soil in the daytime is probably heated to 125 degrees Fahr., but without the clouds and humidity which retard cooling on earth, the loss of heat which occurs as soon as the sun sets' is extremely rapid. Even on the Martian equator the thermometer would fall below zero Fahrenheit every night, and towards the Pole carbonic acid^ would be frozen and air would be liquefied. The other planets all pass in review, and the followed by"the minor planets, r^rerred to by Professor " Weiss as "those vermin of the skies," the comets, the meteors, the zodiacal light, and the gegenschein. We are then led back to tho centre of the system. Our sun, says Dr. Abbot, is an average star m many. ways. No star could have obeyed better the advice of Kobinson Crusoe's father to stick to the middle class. It is ten thousand times v as bright as some stars, but ten thousand times less bright than others. Its spectrum is about halfway in character between that of a blue star and that of a very rod one. Its mass, though over 300,000 times that of the earth, is about the average mass of all known stars. Its surface shows a temperature of 6000 degrees absolute centigrade, but Eddington calculates that towards the centre this rises to the astonishing figure of IS million degrees centigrade. The light which the sun emits from each square foot of its surface was found by Langley to be 5000 times as intense as the most brilliant glow from the same area of molten steel poured from a Bessemer converter. Of the radiant energy sent out from the sun the entire family of p^nets gather only one two-hundred millionth part. The source of the gun's energy has long been one of the greatest outstanding enigmas. How can it have poured out unceasingly such vast amounts for a thousand million years? The energy of the fall of its material towards the centre as the sun shrinks would supply, as Helonholtz pointed out, the present output for about 25 million years only. But perhaps stars are the crucibles of Nature, and in their interiors helium,

oxygen, iron, gold, and all other elements are built up out of protons and electrons. Hydrogen has an atomic weight 1.008. When four atoms of hydrogen become one atom of helium its weight is exactly 4. The loss is believed to represent a store of energy set tree. Physicists now go even further and assert that matter is a passive rorm of energy, and that at the tremendous temperatures ruling within the stars this may be changing into the active form of radiation. The actual amount of radiant energy emitted by the sun is one of the special subjects on which Dr. Abbot has carried out most important researches. Without mentioning his own name, he tells us many of the conclusions arrived at. ihe sun is a tremendous globe containing all or almost all the chemical elements found on earth. Although its density is 1.4 times that of water, it is entirely gaseous. Even in the comparatively cool portions of its atmosphere solids would pass into the gaseous form explosively fast. Gravity at the sun s surface is thirty times as great as on earth, and the pressure increases rapidly downwards from the surface. At the centre gravitational pressure has to counteract not only the outward force due to gaseous expansion under a temperature of some eighteen million degrees, tut also the pressure of light. The magnitude of this radiation pressure is shown by the streaming out of comets' tails. That of the comet of 1843 is said to have been five hundred million miles long. The sun, then, is an object well worthy of study, and the spectroscope, the spectro-heliograph, the bolometer, and the radiometer are enabling man to.interpret the messages borne by its light. Dr. Abbot has very much more to tell than we have yet referred to.