THE MOON.

Taranaki Daily News, 3 December 1921, Page 10

THE MOON.

AS AN OBJECT FOR TELESCOPIC SCRUTINY. (A paper read before the Executive Council of the New Plymouth Astronomical Society by Oscar Blundell, Hon. Director.) After reading about the moon in various books, and especially in the ’ English Mechanic, and having once attended a rather quaintly illustrated lecture dealing with the subject, it was a memorable occasion to the present writer when, in the month of "August, 1902, he was able for the first time to direct a good telescope towards our satellite and actually behold some of the lunar formalions of which he had heard. To enjoy such an experience is to feel a like wonder to that of Gallileo, who. first of human beings, was privileged to see something of the mountains of the moon; but this experience is only thoroughly entered into when the mind is in some degree prepared to recognise what the eye sees. Many persons look into the eye-piece of a telescope for the first time with the vaguest idea of what they are looking at. To appreciate and enjoy a first view of the inooiF to the highest extent let the beginner read and study such a book as Naysmith and Carpenter’s ‘‘Moon;” or the chapter on the same subject in Webb’s “Celestial Objects.” Fortified with knowledge so acquired the use of a good telescope upon the moon enables the observer to enjoy the delightful experience described by the poet Keats—- “ Then felt I like some watcher of the skies “When a new planet swims into his / ken.” It is indeed a great privilege to be able to study the topography of another world, and to seek to discover if possible, new or unknown physical features among its mountains, valleys and plains, and especially if any changes are in process there. In a world where most evidently tremendous forces have at one time been at work, it is hard to believe that these activities have altogether ceased; that every .sublunarian force has become spent, and that the multitudinous craters give, in no single instance, any indication that changes are still in progress. The first observers expected to find signs of life upon the moon, and for long it was believed that, given adequate optical power, cities and even individual buildings might be detected. When it became clear that such things were not to be expected, it was still hoped that volcanic activity would be seen and that changes in surface coloration might indicate vegetational growth. None can commence to study selenography without certain hopes and even

convictions that such things will be discernible. Yet since Gallileo first surveyed the moon in his little telescope in 1609, and although instruments of a size and defining power such as early modern astronomers hardly conceived of have been employed for years in this direction, .it has seemed to many astronomers that the evidence for change upon the moon is largely, if not altogether negative. Spite of this disappointing conclusion, however, there still are many who continue to keep watch upon the visible surface of our satellite, and to believe that their dilligence is not vain.

Nor are they without certain substantial grounds upon which to base their hope. Professor E. A. Pickering, who enjoys very high privileges in respect to instrumental equipment and steady atmospheric conditions, is perfectly satisfied that surface changes, such as may bo ascribed to some kind of vegetational growth, and which may -be seen with quite moderate telescopic aid, are regularly proceeding upon the moon. He has also pointed out that two craters, known as Messier and Messier A., situated in the Mare Foeeunditatis, present peculiar changes of form as the lunar day advances, as if they became covered with immense masses of some substance like ice or snow which accumulates during the long lunar night, and gradually melts as the sun rises upon it.

The best known instances of an actual change having taken place in a lunar object is that of the crater Linne in the Mare Serenetatis. It was first noted as quite a considerable formation, being 54 miles in diameter and of great depth. It is represented on Riecioli’s map, which was published in 1653. In November, 1788, Schroter discovered that a dark spot hid the place of this crater. In 1523 Lohrmann- saw it as a distinct crater, and shows it as such on his map. He describes it as one of the brightest spots on the whole surface of the moon. Beer and Madler. whose map appeared in 1837 show it distinctly as a crater, and describe it as very bright and very deep. In 1858, and for 7 years after, photographs by De la Rue and Rutherford show Linne as a very bright spot. In October, 1866, howeveri Schmidt reported that the crater Linne had disappeared, and photographs taken at that time only show a light spot scarcely one-third of the brilliance seen in De hi Rue’s and Rutherford's photographs. On December 13, 1866, Webb, the renouned author of “Celestial Objects,” could find no crater anywhere near where Linne was supposed to be, but only “an ill-defined whitishness on the floor of the mare.” Subsequent observations fully confirmed the existence of this spot, and careful measurements showed it to be twice as large as the crater which stood there previously. Then in February, 1867, Secchb of Rome, observed in place of the large 5J mile crater figured on the lunar maps, a very small crater in the midst of the bright area, which, under favorable conditions can still be seen. The present writer has seen it very distinctly when just on the morning terminator. From the evidence it would seem that this crater erupted in 17SS and again in 1866. On the latter occasion it would appear to have become filled to overflowing with lava or some such substance, whien spread over the surrounding country, and, reflecting considerable light, appeared as a white spot on the dark surface of the mare. Finally a new but

small crater burst out through the overlaying substance and soon afterwards became inactive as it now appears. Obviously this object should be watched carefully for evident change is taking place there. It soon becomes clear to the student that little can be achieved by single observations of lunar objects. It is only by patient concentration of attention to two or three points that evidence for or against change can be forthcoming. The opportunities for obtaining first class records of any object are necessarily few and far between. Either clouds or disturbed atmospheric conditions are likely to prevent adequate scrutiny of an [object just at the time when its position in regard to the terminator is most favorable for observation. An observer must feel jubilant if he obtains two or three first-class observations of the .-aine object in the course of a year au remind to siU’Jacu c-hauges uu the

moon, it should be borne in mind that even under the most favorable conditions the apparent distance of any object under observation is very great. If an aerophotograph of a portion of the earth’s surface taken -from a height of half a mile be placed 30 feet from the eye, all or nearly all its details become invisible or merged into one or two smooth tones. That- is the same as to say that seen from an aeroplane at a height of 74 miles the region in question would present hardly any details to photograph. What then if the observer were situated at 60 miles or more from the earth (which is about the distance to wliiuh our best telescopes apparently bring the moon) ? Little indeed could then be discerned even under the best lights. It is safe to say that, if we could see what on the moon appears (even with our most powerful optical aid) to be a smooth piano, at a distance of only half a mile, we should see it in reality to consist of details as pronounced as the vast craters in the southern hemisphere of the moon appear to us in ordinary telescopic vision. Grimaldi, Plato and the various marae appear dark under a high angle of illumination. For all wo can tell this might bo due to dense forest growth of a luxuriance unknown upon earth, for certainly at 60 miles vertical distance our forests would appear as smooth, dark regions. This is not to say, of course, that vegetation exists upon the moon, where neither water nor atmospheer is known to be present, but evidently there might easily be objects of great size even where, to our best views, nothing but a smooth surface presents itself.

In aerophotographs, roadways are generally the most conspicuous objects, hut could such be seen at all at a vertical distance of 66 miles? Probably the smallest objects ever clearly detected upon the moon are from half a mile to three-quarters of a mile in diameter, and there is a good deal of room for many large objects in a radius of half a mile. This means to say, of course, that not even the shadow of an object could be seen unless it covered at least half a mile of surface. W« have not yet heard what the great American reflecting telescopes can accomplish visually in this direction. Theoretically the 100 inch Hooker instrument might bear a power of 60,000, which would bring the moon to apparently 20 miles or so distant, but it may’ well be doubted if the atmosphere even at Mt. Wilson will ever be steady enough to permit of the use of poweis anything like so great. Still there ought to be occasions when several thousand diameters might be used, and judging by the wonderful photographs, which show the presence of craters as small as one mile in diameter, the favored observers at Mt. Wilson ought to be able to impart reliable information regarding fine detail upj.’i the moon. We must not imagine, however, that our own moderate telescopic equipment possesses small value as compared with giant instruments such as that at Mt. Wilson. Given a clear sky and perfectly steady air the 6-inch refractor at the local observatory will show detail whien in unsteady air do telescope on earth could reveal. On the all too rare occasions when our “seeing’' can be recorded as No. 1. the amount of detail to be seen in the 6-inch is truly wonderful. For such occasions (and they do happen) we ought to possess an e.p. of higher power than \ye have \a| present, or a Barlow lense to increase existing powers. On one happy occasion, when the crater Gassendi was on the terminator, the present winter was able to employ X 430 t,o perfect advantage, and evidently considerably higher magnificence could have been used. It is to be hoped that financial support will be forthcoming in the near future to enable the society to mount and house the 12-inch reflector which we possess, as with its larger light grasp still more could be discovered and recorded. No doubt we miss many opportunities when the seeing is good, owing to the restrictions which are inevitable to our small staff of active observers. Certain lunar objects have been selected for regular and frequent scrutiny, and these should be observed under the evening as well as the morning terminator. This, of course,’ entails visiting the observatory in the small hours of the morning. The regions chosen for this special scrutiny are as follows:

(1) Posidonius X. A curious formation lying N.E., of the great ring mountain of that name, and the remarkable serpentine ridge which bounds the Mare Serenatatis on the west.

(2) Pahis Putredis, well seen near Ist quarter. (3) The Mare in which several unrecorded objects have already been discovered.

(4) Wargentin, the unique formation situated to the S.E. of the great walled plain Shickard. Other objects will no doubt be included, but if these are thoroughly scrutinised on every possible occasion during a course of years, much valuable information is bound to be secured.

Reports will be made and sketches forwarded to the Lunar Section of Ihe B.A A. for publication and occasionally to the English Mechanic.