A MASTER STUDY OF THE EARTH’S SATELLITE

Press, Volume XCIII, Issue 27990, 9 June 1956, Page 5

A MASTER STUDY OF THE EARTH’S SATELLITE

T HE moon

The Mcon. By H. P. Wilk’ns and Patrick Moore. Faber and Faber. 388 pp. prom times of antiquity the moon hss been the subject of myth, legend ajjd superstition; it was worshipped jn ancient religions. The “orbed maiden” has inspired poets without E uinber. The moon is now the object o f explorers to whom the earth offers little scope for new ventures. And from antiquity to the present time, indent and modern earth-bound observers nave searched the moon for its secrets. This book, it is safe to say. contains all the moon’s physical secrets that are known to man. Selenography, as study of the moon is called, really began with the invention of the telescope which swept away old ideas—and many follies. With the new invention Galileo, Marius, Lower and others observed » the broader features of the moon earlv in the 17th century. In 1610, Galileo pot only recognised the existence of mountains in the moon but made estimates of their altitudes and conitructed the first lunar map. Tn the game year Sir William Lower, probably the first worker with a telescope in England, described the contrast between the sunlit mountain peaks and the blackness of the adjacent valleys and the rugged nature of many portions of the moon’s surface. In 1647, Hevelius published the first chart of the moon; it contained 250 named formations. The present work contains Dr. Wilkins’s famous map of the moon’s surface; the total number of formations shown is at least 90.000. This fact is a measure of 300 years’ assiduous observation and study by generations of selenographers. Study of the moon has been advanced, of course, by progress in related sciences. More and more powerful telescopes have disclosed more and more details. Lunar photography has become a specialised branch of the photographic art; with the aid of large reflectors minute details have been photographed. Mathematics and mathematical instruments have permitted computations which measure physical features. From radio waves, temperatures on the moon can be deduced; atmospheric tides have been discovered in the ionosphere and related to lunar problems. The geology of the moon has been the lubject of exhaustive study; a great deal is known, but only when the first space-ships make observation at close quarters possible will questions about the origin of the various lunar features be finally settled. The largest and most prominent cf these features are the great dark plains, called “seas” by the early stenographers. They are visible to the naked eye, and collectively form the dim outline of the face of “The Man in the Moon.” The plains vary in size, from the vast Oceanus Procellarum with ill-defined borders, and covering an area of 2,000,000 square miles, to the Mare Parvum—the “little, sea.” Mountain ranges, so typical of the earth, have their counterpart in the moon, but to a secondary extent. On the moon the craters are the predominating features. Nevertheless, ranges exist vhich include many peaks approaching 20.000 feet in altitude, and in one

or two instances, 30.000 feet. The craters, or walled plains, are approximately circular areas enclosed by massive mountain borders, varying in size from 180 miles in diameter, in the case of Bailly, to 60 miles in such formation as Piatus and Plato. Smaller and more numerous than the wallet, plains are the ring-plains, the most perfect example of which is doubtless the magnificent isolated ring of Copernicus. More numerous still are the craters proper which range in diameter from 30 miles to less than five. Circularity is. indeed, most striking in lunar geology. The clefts are a fourth interesting feature. The earlier observers discovered but few, but every time a new giant telescope has been employed, “new” clefts have invariably been found. The clefts are of the nature of chasms or cracks in the surface, rarely exceeding a mile in width but frequently extending for scores or even hundreds of miles. The most remarkable and mysterious of all lunar features are the systems of bright rays, radiating from certain crater-rings as centres and stretching for long distances. The inost extensive and conspicuous of £ll the ray systems is that grouped iround Tycho as a centre. Hundreds of rays radiate from this ring-plain. Under good conditions the rays are wen to consist of a series of dots and dashes, so arranged that as one begins to fade out another commences, and w the sequence is preserved. The origin of the Tycho and other rays systems is the subject of many theories. The authors of the present book incline to the theory that the rays are of volcanic origin, the product of activity at certain definite foci, and that the material of which they are composed was ejected from the foci and from cracks in the crust formed by the eruption; that they are composed of volcanic glass of a similar nature to that so abundantly produced at the fused lava-pit °f Kilauea in Hawaii: that the actual particles are of spherical shape, either hollow or solid and that they were deposited around the vent by the

K ses rushin g trbm the atmosphere."* e necessity a lunar feat? l rp a ™h, n ,.k e of atmosphere is one anri h ’ ln the words of B eer dl ’ enaures that the moon . py of the earth.” Its ment r innlrt SUrf ?. Ce featureK have been erarftv TL J ts temperature, surface different V ength ° f da ? are totally heat Measures of the degree of surface d cold to w hich the lunar ax P°sed have produced ISrfl« confilc J ln ® results. But great establish.lt ° £ temperature have been hS 1 S' , w hen the sun is over- . rock y surface reaches high temperatures, possibly as much as 350 e £ eentigrade. During the night snhnr absence „ of appreciable atmosphere greatly accentuates fall in du nng the long lunar night (equal to 14 earth days) ternperature is considered to approach minus 243 degrees centigrade. Life on the moon?: lunar life, it is evident, must be capable of withstanding not ®?iy ~a wme range of direct solar radiation (because there Is no protecting atmosphere), great extremes ot temperature and capable of flourishing in the absence of free atmospheric oxygen or, probably, even carbon dlox }? e g as ‘ It * s no * impossible thai on the moon there may exist, or have once existed, some form of. life peculiar to the moon and totally unnke anything ever known on earth lhe authors mention the theory of Professor W. H. Pickering that vegetation or insects account for the moving spots within the crater Eratosthenes; the existence of life may. not be the explanation, but it remains to be explained how these spots apparently move in the contrary direction to that in which shades or shadows would do. Dr. Wilkins, director of the lunar section of the British Astronomical Association, and Mr Moore, secretary of the section, have compiled this book primarily for observers. Since amateur observers comprise all but a very few students of selenography, the book will be widely received as the masterpiece it undoubtedly is. All the known information about the moon is given and fully treated; a detailed description of every visible feature is given; appendices deal with such subjects as lunar photography and the measurement of lunar altitudes; a short biographical section gives succinct sketches of lunar observers of the past and present. Dr. Wilkins’s map has been mentioned, sections of the map (itself a fascinating study) preface details of the features dealt with in letterpress in the relevant sections. Finally, 16 pages of magnificent lunar photographs must be mentioned. Some (those taken ai the Pic du Midi Observatory) are claimed by the publishers to be ‘‘the finest yet produced.” There is no difficulty in accepting this claim. “Selenographia,” the book containing the observations of Hevelius, remained the standard authority for over 100 years and still possesses value today. It is difficult to think that the work of Dr. Wilkins and Mr Mooup will be replaced for many years—perhaps not until men reach the moon.

FOOTNOTE: The mean distance of the moon from the earth is 238,857 miles (maximum 252,972 miles, minimum. 221,614 miles). Its diameter is 2160 miles, rather more than one-fourth of the earth; its mass, in weight, about oneeightieth of that of the earth. The gravitational attraction at a point on the surface of the moon is onesixth of the gravitational attraction at a point on the surface of the earth. The moon revolves round the earth in a period of 27 days 7 hours 43 minutes 111 seconds, but eccentricity of orbit and perturbations cause this to varv as much as three hours. Owing to avial revolution in exactly the same time as the period of sidereal revolution, the moon presents always the same aspect to earth. From the earth about 50 per cent, of the moon’s surface is seen—4l per cent. constantly . turned towards the earth, and 18 per cent, visible at times due to three librations by which parts near the edge of the disc are alternately visible and invisible.