ASTRONOMICAL NOTES.

Press, Volume LI, Issue 15369, 28 August 1915, Page 9

ASTRONOMICAL NOTES.

FOR SEPTEMBER. BY THE REV. P. W. Faxrclotoh, F.R.A.S.) In this month the increaso of the length of the day rises to its maximum and equal hours of night and day aro reached. On the 21th the sun enters the sign of Libra and spring is said to begin. Jupitpr now rises early in the evening and is well placed for observation. A pair of fiold-glassog will show his moons, that is, three or four of them, as minute stars close to the planet. Mercury is an evening star, and on the 11th will be about four degrees north of the new moou. Towards the end of tho month Venus will pass bohind the sun into tho evening but will not be noticeable for a considerable time, becauso she will not only be distant but also nearly in a lino with tho sun. Those who are out about four o'clock in tho morning will see Mars and Saturn close together in tho east. They will bo in conjunction on tho 11th. . At least six comets were discovered last year, but flono of them was suflt- | ciently bright to attract the popular | eye, though "oomct 'b' 1914" snow.ed a tail 9 degrees long in photographs. The elements of that comet's orbit showed a striking likeness to those of comets that appeared in 1790 and 1911, and it is concluded that all threo belong to tho same family. Comet ''c" 1914 was remarkable for having tho most remote perihelion point yet discovered except one. Usually comets pass fairly close to the sun, that is within the earth's orbit, or not greatly beyond it. But this comet passed its perihelion at three and three-quarter times the earth's distance from the sun. The/ other distant comet appeared in 1729, and was just over four times tho earth's distance. Dr. Crommlin points out that there is also a curious likeness between the two orbits, and concludes that they belong to the Bame family. ] Encko's famous comet, which was supposed to indicate a resisting medium in space, was "d" last year. This comet is too faint for the naked cyo. Its perihelion distance is about one-third of the distance of the earth from tho 6un, and its remotest point is well over four "units J' or solar distances, the period being threo years and four months. It has hitherto been considered .good enough to pick this faint object up as it approached its perihelion, but this time Mt. Wilson, with its greatJ3o-inch reflector, determined to try to pick yip the comet at its remotest point. An exposure of 3J hours on September Ist. ,1913, showed a spock close to the spot indicated by calculation I It is quite evident that the modern reflector is likely to arm the astronomer with spaco penetrating powers undreamt of a few years ago. Comet V was interesting through tho disputed ownership question. It was independently discovered by three different observers in parts of the world far removed from one another. Dr. Lunt, of the Cape, got his report in first, and received credit for the discovery. Then it came to ho known that Mr Westland. of New Zealand, had discovered tho comet eight hours earlier, so it became tho WostlandLunt comet. Later still the records of Arequipa in Peru about lo minutes priority to New Zealand. TV'e were eight hours before tho Cape, but 16 minutes after Peru. Arequipa had singular luck that night, for just 75 seconds after snatching tho "Camp* boll-Westland-Lunc' 1 comet they discovered the returning Encke. Readers will observe the modern tendency to talk of comets being in "families." This expression originated from the fact that groups of comets retreat to the orbit of each of the great planets. Those camo to be called the families of tho planets. Any comet with a period of about six years would be said to be of Jupiter's family. If tho period wero a Bout 70 years, then Neptune would bo the patriarch. From this custom of grouping has arisen that of calling all groups of comets "families" whether they are associated with a planet or not. If there'are similarities in the main elements of the orbits they are taken to indicate som© relationship in origin or history. Another modern tendency is to conceive of planets as travelling with immense clouds of outriders and skirmishing cavalry whose appearanco a few millions of miles away from tho main body need cause no surprise. Halley's comet passed at a vast distance' from the earth, yet two meteor streams that invado our atmosphere aro believed by some to bo related to Halley. The mutual perturbations of tho planets afford ouo of tho most difficult mathematical problems in astronomy. Some of the facts dealt with aro theso. "Tho long period of Jupiter and Saturn" is 918 years, and is the cycle in which the planets repeat all their positions in relation to one another. Their mutual perturbations run through their ebb and flow, and como back to their starting point in that period. Jupiter is displaced by 21 and Saturn bv 49 minutes of arc in this long cycle. The magnitude of this disturbance maj be gathered from the fact that the moon is 31 minutes in diameter. _ A much longer period is that in which the nodes of Mercury revolve. Tho nodes are tho points at which his orbit intersects the ecliptic anj the perturbations of the other planets cause these points to revolve —once in 166,000 years! That is. however, a moderate time, for the corresponding change in the case of Neptune takes 540,000 years! y , Tho mean distance of the planets from the sun can, however, never be altered, and their eccentricity and tho inclination of their orbits to the ocliptio vary onlv within narrow limits. It was proved by Gauss that if the planets were broken up and spread in dust evenly arojnd their orbits they would influence each other just as they do now, and the rings would alter precisely as the orbits do. Professor Doolittlc has recently solved the perturbation problem as far as the four inner planets arc concerned, with an exactness never before attained. Ho finds, a s others have found, that there is a slight excess of motion over what the law of gravitation will account for. Thus Mercury gains 46

seconds of arc per century—about 1-lthoftho apparent diainoter of tho moon. Venus gams more than 10 seconds, and Mars nearly niue. Theso "residuals," though so small, aro many times greater than tho probable error. Hence they together with tho acceleration of tho moon provido tho crux of modern astronomv. which cannot be satisfied with anything short of absolute accuracy. Newton's law of tho inverse square* ought to be accurate. It is tho law according to which a beam of light spreads with increasing distance. If the beam covers on© square foot at tho distance of one yard from its source, it will cover four square feet and be four times fainter at two yards; it will cover nine square feet at three yards, sixteen feet at four yards, and so on. In fact, Newton's law is the law for the increase of tho surfac© of a sphere with tho length of its radius, except that increased area means weakened power, and hence tho pull of gravity js said to diminish as tho squares of tho. distance increase. To bring the law into stop with tho facts various' suggestions have been made, but nono of them is established j far. The first suggestion was that of a planet nearer to tho sun than Mercury. Quito a literature has gath- ! ered round this idea, but it may U? regarded as exploded. Photography during total eclipses is supposed to prove that no planet even of tho Bth magnitude exists between Mercury and tho sun. The Bth magnitude would, mean 30 miles in diameter and millions of such bodies would bo required to cause tho observed effects. Readers of Jules Verne's "Voyage to the Moon" will remember that tho great shell passed a terrific meteoric body, which was the earth's second satellite. This also was a devico for expediting the moon by a fraction. Another suggestion i 3 that gravitation acts with a finite velocity. The law assumes that it acts ouslv at any distance. This, however, is a subject that verges on metaphysics, and we pass on. Other suggestions are that tho law requires to be restated with the correction of a minute fraction, or that the earth cuts off from tho moon during an eclipse a small fraction of the sun's attraction.