IN STARRY SKIES

Evening Post, Volume CXI, Issue 73, 27 March 1931, Page 15

IN STARRY SKIES

THE HEAVENS THROUGH FIELD GLASSES

THE FULL MOON" ASPECT

(By Crux Axistralis.) Tlio time of full moon is regarded by most astronomers as a time when very little can be done in the way of observing the lunar features, the majority of which depend for their best visibility on the contrasts of light and shade presented by a rising or setting sun. This popular belief that there is nothing to sec in the full moon is quite erroneous. Although the craters and walled plains are lost to view under the vertical illumination there are other' features which are best seen, or can be seen only at this phase, _ The full moon aspect is remarkably different from that visible at other times. Gone are the deep craters with their high walls and central peaks, gone the

valleys and the mountain ranges, and the lunar disc bocomes a disc indeed, with nothing on it to indicate the spheroid wo know it to be. The full moon aspect shown in such a humble instrument as binoculars is well worthy of study. The illustration accompanying this article shows all the detail that can be seen on tho moon with a magnification similar to that of the ordinary field glasses. The photograph, of course, was taken through a much larger telescope, a nine-inch refractor at that time in uso at the Marist Fathers' observatory at ■Aleeanee, near Napier. At the time of full moon the principal features visible are the maria, or "seas," the dark doposits which cover cl large proportion of the visible hemisphere of the moon, and which at this time can be studied in their entirety. There are also visible at this time a few bright mountain peaks and the wonderful systems of bright rays emanating from certain craters. By far the most remarkablo system of rays is that which has its origin in the crater Tycho, which can be seen .^n the illustration about a quarter of the moon's diameter from tho top limb. This system of broad, radiating streaks lof crystalline brilliance can be traced over a large part of tho visible hemisphere and doubtless extends to tho south far into the invisible hemisphere. Of quite different appearance aro

the streaks or rays which surround the erator Copernicus, the bright area in the middle of the soa which fills the lower right-hand quadrant of the lunar disc. While the rays from Tyeho iire long, brilliant, and well-defined, the Copernicus rays are short and so closely arranged around the crater as to resemble a coma, or the solar corona, ii'ivo other less striking ray systems arc easily visible in field glasses, and the telescope reveals the presence of others. The origin of those streaks remains to this day one of tlio many unsolved problems of the moon. They pass alike over mountain and valley, and even through the rings and cavities of craters. Various suggestions have been made as to their constitution—that they resemble terrestrial trap-dykes, or that they are cracks divergent from a central region of explosion, filled with molten matter from beneath. The contraction of the lunar crust has even been suggested as a cause of these remarkable markings. In spite of the number of astronomers who have spent a considerable part of their time in examining the moon, and 'of the many theories that have been advanced to account for this peculiar spreading of bright streaks over almost an entire hemisphere, it can safely be said that astronomers today are no nearer a final solution of their composition and origin than -when

the streaks were seen through tho first crude telescopes three hundred years ago. Tho modern observations of a prominent lunar observer, W. H. Pickering, of Jamaica, havo added a little to our knowledge of tho appearance of the bright rays, several significant facts, beyond the powers of small instruments to reveal, being noted. According to Pickering the streaks do not radiate from the centres of their bright craters, but from numbers of minute craterlets generally not centrally placed, and rarely exceeding a mile in diameter. Some ray systems are greyish in colour, much less white than the brilliant streaks of Tycho. There are no very long streaks. Although we can trace tho bright rays sometimes for over a thousand miles of the lunar surface, such a ray is in reality composed of numbers of smaller streaks ranging from 10 to 15 miles in length, about a quarter of a mile in width at their narrowest and brightest portions. What all those details mean, and whether they will eventually assist in solving the questions that are being asked of the ray systems, must be left to tho future to decide. Tho purpose of this article will bo fulfilled if some can be induced on a fine autumn night when the fifll moon is bathing tho landscape in its brilliance to study for themselves the formations which have puzzled astronomers through many generations of lunar research.