ASTRONOMICAL NOTES.

Press, Volume LXVI, Issue 20099, 1 December 1930, Page 8

ASTRONOMICAL NOTES.

FOR DECEMBER, 1930. •'•PICIAIXy VTBITTBS rOB THE PBESS.) !By E. G. Hogg, M.A., F.R-A.S-] ihe sun will enter the zodiacal sign Caprieorntis on December 23rd, which will be the longest day in the Southern Hemisphere, tho duration of daylight iii-inj; 15 hours ~'7 minutes. On that • late the meridian altitude of tile sun tor tho latitude of Christchurch ■ will tic oHdeg .OlJniin. The planet Mercury sets on December lit at S.'Jo p.m., and on December loth at 9.35 p.m.; Venus rises on December Ist at 4.3 a.m. and on December 15th at 3.1G a.m., and Mars rises ou these dates ut 0.20 a.m. and 11.32 p.m. respectively. Jupiter rises u!l December Ist at 10.37 p.m., aud on December 15th at 9.36 p.m., and Saturn tet.s ou these dateo at 10.9 p.m. ami r>.2o respectively. Planetary Notes. Mercury will be a conspicuous object in the south-western sky at the beginning of December, when its apparent magnitude will be —0.5; its brightness will decline somewhat rapidly during the month until, at tho close of the year, the apparent magnitude will be Venus is now a brilliant object in ihe eastern sky, it having passed bei ween the earth and the sun on ftuv<=mber "22nd; its apparent magnitude, which is now —3.6. will .increase up to - -1.4 by the end of the month; it will attain" its maximum brightness on December 22nd. Mars is rapidly approaching the earth, its distance from which will diminish during the month by about 18 million miles; its apparent magnitude at the beginning and end of. the month will bo 0.0 and —0.6 respectively. T Tho apparent magnitude of Jupiter during ibis month and the following will be —2.2; it will be in opposition with the sun on January Gth. There will bo an occultatlon ot Uranus bv the moon on December 29th, the earth, moon, and planet being m a line at 3.21 a.m.

1530 A.D. —1630 A.B. Wlien Ptolemy died towards the end of tho second century ho left a system of astronomy which dominated ail •scientific thought for tho next fourteen hundred years, and though the attempts made to P re di c p means of this theory tho places , m the heavens of the sun, moon, and the wayward planets, met Wlt 1 little success the truth of the Ptolemaic doctrine was not seriously jmpugned until the day of Copernicus (U<d--1543), by which time the system ot cycles and epicycles, -which strove to explain the motions of the heavenly bodies had become so complicated as" to draw forth from Alpbonso the Wise the remark that if ho had been, consulted at the creation, he could have given so mo useful hints. It 'must be borne in mind that t lie geocentric theory of Ptolemy appealed to the common-sense of mankind tne earth did appear to bo the solid and immovable base of all things e ' se ~ •where there was flux and motion, but the earth stood firm; moreover _it bad behind it all" the authority ot Aristotle, ■whose word in science was accepted. even bv those who had como to doubt his philosophy; the theory more«rer seemed a necessary adjunct to tlie belief that man himself' was-"the centra] fact of creation, and that a i nature existed but to subserve needs and aspirations.

Copernicus. It will thus bo seen that both courage and originality were required in anyone who ventured an explanation of the universe in °RP°* lt ! on . :° the feeling and teaching of that da}. Th® revival of the interest in Gr_eeK philosophy and science, which is one of the most important aspects of that intellectual movement we call tne Renascence, was, however, silently work, permeating men s minds ami raising a spirit of critical enquiry destined to bear fruit m other fields than that of astronomy. The ideas of Plato were welcome to a generation weary of Aristotle, and to it Pythagorean clement, wluch del.gli to conceive the universe m terms of a mystical harmony of aumbers or geometrical arrangement of units ofspacc In such an atmosphere as this modern astronomy had its birth. . Copernicus had learnt from his teacher, Novara, Professor of Mathematics and Astronomy at Bologna, that • the Ptolemaic system was too cumbrous to satisfy the principle of harmony, and during his six years residence in Italy he studied the writings of all the philosophers whose books he could obtain in search of light. In his letter to Popo Paul 1U he writes: "According to Cicero, tas had thought tluit the earth was moved . . . according to Plutarch that certain others had held the same opinion . . . "When from this, therefore, I had conceited its possibility, I myself also began to meditate upon the mobility of the earth. ... I found at length by much and long observation that if the motions of the other planets were added to the rotation of the earth and calculated as for tho revolution of that pi-met, not only the phenomena of the others followed from this, but that it so bound together l.oth the order and magnitudes of all the planets and the spheres and the heaven itself that in no single part could one thing be altered without confusion nniong tho other part© and in all tho universe." Copernicus believed that in tho conrbinntion of heliocentric motion and diurnal rotation of the earth lie had found "a wonderful symmetry , in the universe, and a definite relation of harmony in the motion and magnitude of tho orbs of a kind it is not possible to obtain in any other way.'' About 1-530 Copernicus finished a treatiso setting forth his scheme, and ho published that year a short abstract in popular form ; a larger work dealing with his theory in full detail was issued later, the first printed copy reaching the aathor on his death-bed in 1543. There were many phenomena—planetary and atellnr—for which Copernicus failed to account, and his. hypothesis only won -its way very slowly, but we are justified in saying that four hundred jrears ago was struck the first of a series of blows which eventually demolished tho Ptolemaic doctrine. Tycho Brake. Three years after, the death of Copernicus was Tycho Brake, who, though a firm believer all his life in the Ftolemaio system, was destined to piay an important part in bringing about its downfall. We cannot stay to deal with ■ his long and varied career, devoted to the persistent study of the heavens; it must suffice to say that with wonderful skill and using instruments far superior to any employed before, he compiled a magnifinent record of observations of the positions of the planets, which have justly entitled him. to tne admiration of all succeeding astronomers. Lata in life he was H| forced to leave the wonderful observa--9H tcpr he had founded at Xlraniborg and H retired to Bohemia, where he was apH imperial mathematician; lie H| established a» observatory at Prague,

where, shortly before his death in 1601, he offered a post to Kepler—an act which had momentous consequences. John Kepler (1571-1630). Too frail in body for manual work and showing signs of considerable mental power, it was thought that a suitable sphere for the talents of John Kepler might be found in the.Church, and at the ago of seventeen he was admitted on the foundation of the University of Tiibingen, where his attention seems to have been equally divided between theology and astronomy; on tho advice of his friends ho accepted in 1594 the professorship of astronomy in the University of Gratz, but political and religious troubles deprived him of Mb pupils and therefore of his means of subsistence, and he was glad to become one of Tycho's assistants at Prague. The story goes that when the great Danish astronomer was dying he summoned Kepler to his bedside, and with his last breath besought him to spare no labour in deducing from the mass of observational material he was bequeathing him the secret of the movements of the planets. Tycho felt that his work required an interpreter and that in Kepler he had found the man he sought.. Kepler, unlike Tycho, was a firm believer in the Copernican doctrine ou account of its mathematical simplicity nud harmony. "He," writes Whetham, ''was convinced that God created the world in accordance with the principle of perfect numbers, so that the underlying mathematical harmony, the music of the spheres, is the, real and discoverable causo of the planetary motions. This was the true inspiring force in Kepler's laborious life. He was not, as usually represented," tediously searching for empirical rules to be rationalised by a coming Newton. He was searching for ultimate causes, the mathematical harmonies in the mind of tho Creator." After almost incredible toil he extracted from Tycho's observations three summaries which have survived in science in Kepler's Lawsj these re: (1) That the planets travel in paths which are ellipses with the sun in one focus; (2) that the straight line drawn from a planet to .tho sun sweeps out. equal areas iu equal times; (3) that the squares of tho periodic times which the different planets take to describe their orbits are proportional to the cubes of their mean distances from the sun. With regard to the first law wc may state that Copernicus had taught that the planets move round the sun in circles. but Kepler found that this did not agree with Tycho's observations; ho then supposed, that the orbits were ellipses and verified that his surmise was correct. It is said that the discovery of the second law gave Kepler moro delight than did the two others. Hie believed that each planet was driven by a Constant Divine Causo, and that it should therefore describe its orbit at a constant speed; hero again iacfc and theory were at variance, the planets did not move with uniform speed, but lie was able to save the principle ot harmony by showing that the areas were mapped out at uniform speedy "It may well be doubted," writes fair Robert Ball, '""whether liny joy experienced by mortals is more genuine than that which rewards the successful searcher after natural truths. When Kepler, alter seventeen years a* searching, discovered his third law his joy Knew no bounds. ..Nothing holds me,'' he said, "I will indulge in my sacred fury. I will triumph over mankind bv th© honest confession that I have stolen the golden vases ot the Egvptians to .build up a tabernacle for my God, far away from the conBncs of. Egypt. ■ ff" you- forgive• me, 1 rejoice; if you arc angy, I can bear it. The die is cast; the book is written;- to -be read either now or by posterity—l care' not which. Jt, may- well wait a century for a, reader :as God has waited six thousand years f6r an observer." Kepler's work, however,- was appreciated immediately bv his contemporaries, arid when he died in_ 1630 ne left behind him a name destined to bo held in the highest honour while astronomy has its votaries.