ASTRONOMICAL NOTES

Evening Star, Issue 20989, 31 December 1931, Page 3

ASTRONOMICAL NOTES

THE SKIES IN JANUARY [Written by A. G. C. Crust, M.Sc., for the ‘Evening Star.’] POSITIONS OF STARS. Local sidereal time, 6h; latitude, 46dcg S. The following positions, hold four minutes earlier per day from midnight, January 2-3—e.g., at 11 p.m. on January 17, and 10 p.m. on February 1: Alpha Orionis, the giant red variable star, is due north, elevation 35deg.; Sirius, N.N.E., 59deg; Pollux, 46deg. lower than Sirius and in the same direction; Procyon, N.E. by N., 35deg; Regulus, N.E. by E., lldeg.; Alpha Crads (Southern Cross), S.E. by b., 38deg. Canopus, now the highest bright star, is S.S.E., 82deg, while the bright yellow pointer, Alpha Centauri, the nearest bright star to the earth, is sSdeg below Canopus. Aehernar now lies S.W. 49deg, and Fomalhaut is B. W. by W., lldeg; 'while Rigel is N.N.W., 51deg, with Aldebaran 26deg below it. The Moon is at last quarter on the afternoon of January 1, and in the morning sky passes the planet Mercury on the 6th. New Moon occurs on the forenoon of the Bth, and the planet Mars is passed by the Moon the same evening, being now extremely close to the Sun, while Saturn is also passed by our satellite on the morning of the 9th, and the brilliant Venus on the evening of the 10th. The evening of the 12th finds the Moon 20deg north of the star Fomalhaut, and the faint planet Uranus is passed on the afternoon of the 15th. first quarter falling on the morning of the 16th. At 11 p.m. on the evening of the 17th ; which is 6h S.T., the Moon will bo situated N.W., lldeg, and will pass lOdeg north of the star Aldebaran on the morning of the 20th, while at 10.40 on the evening of the 22nd the Moon will be N. by E., 15deg. Passing only 2deg south of Pollux on the forenoon of the 23rd, the Moon will become full early on the 24th, pass Jupiter, the brilliant planet of our late evening sky, on the morning of the 25th. Regains the same evening, and at 10.20 p.m. on the 27th will be situated E., Ideg, having just risen. Last quarter again occurs on the evening of the 30th. The Earth will be nearest to the Sun on January 2. The planet Mercury will be stationary in the morning sky on New Year’s Day, and at greatest elongation west on the 11th.

Venus, now a brilliant object in our western sky in the early evenings, still sets before 6h S.T., and therefore is not given in our positions. On January 13 Venus passes the star Delta'Capricorn (Cap.), mag. 3.0, and this star will be seen only one degree south_ of the briglit planet. Delta Capricorni ia thirty-live light years distant from the earth, and is really six times as bright as the Sun. The spectroscope shows that it is attended by a small companion star revolving round it in a period of only one day. Tho colour ot this star is pale blue, and the spectral type A 5. . . Mars will be in conjunction with faaturn on the evening of the 11th, beincr only o.9deg south of the ringed planet. Unfortunately both bodies are very near the Sun. Jupiter, now a brilliant object in the constellation Leo, near the bright stai Regulns, is retrograding during the month, his position at 6h S.T. being N.E. by E.,.l2deg to 13deg. On January 2 the star Omicron Leonis, mag. 3.8, will be sdeg S. of Jupiter, and on January 25 the planet will be 4deg i\. of XI. Leonis, mag, 5.1. Omicron Leod is one of a peculiar class of stars, those with “composite spectra.’’ The spectrum is quite obviously a mixture of two types, as shown by the dark lines —namely, F 5 and A 3 and tiie effect is duo to two stars, nearly equally bright, and each with its own distinct spectrum. Ihey revolve in a period of 14.5 days, and tho light from them, pale blue in colour, takes 160 years to reach the earth. The pair are actually sixtyfive times as bright as the sun. but they are surpassed in brilliancy by the single yellow giant Xi Leo., which we see not as it is now, but as it was in the last years of the reign of Queen Elizabeth.' It pours into space eightytwo times as much golden light as does our own sun.

The positions of. the three stars mentioned are all in the Equatorial Zone, and arc as follow; Xi Leo, 142,19. Omicron Leo, 145,20, and Delta Cap, 326,47. In mapping stars greatly different in magnitude, it is wise to use a different symbol for each magnitude, for example, a conventional fivepointed “ star ” may be used for_ the first mag., four strokes crossing in a point for tho second, three strokes similarly crossing for the third, a mall cross for the fourth, and five strokes meeting in appoint for the fifth. If possible the symbols for tho fainter magnitudes should also be made progressively smaller. Saturn will be in conjunction with tho sun on January 10. From the Queen’s Gardens, Dunedin, at 6h S.T., Alpha Orionis appears at a moderate height near First Chuvch, Jupiter and llegulus low over tho Early Settlers’ Hall, Canopus nearly overhead, and Alpha Centauri rather low in tho direction opposite to Burlington street, Fomalhaut low over High street, Rigel moderately high and Aldebaran rather low over Burlington street. THE GIANT PLANET JUPITER AND HIS SYSTEM OF MOONS.

Jupiter was probably one of tbo heavenly bodies first observed by mankind. In Greek mythology he was Zeus, the father of the gods, and probably Merodach, or Marduk, the Babylonian deity who, _ with his thunderbolts, conquered Tiamat, the dragon of Chaos, may be identified with this planet. Through the telescope Jupiter is almost as wonderful an object as Saturn. His surface is covered by massive cloud belts, delicately shaded, and full of detail for sharp eyes. But even a very small telescope will show the four bright satellites, noted in the histories of both astronomy and physics, for having revealed to Romer, in 1675, the secret of the velocity of light. We already know how the varying distance of the planets causes the curious apparent movements that were such a puzzle to the ancients, but when the eclipses of Jupiter’s moons were predicted from observations made near opposition, for the purpose of further observation near Jupiter’s conjunction with the sun about six months later, were by no means punctual in their occurrence. The delay amounted to nearly one thousand seconds, and he concluded that the thousand seconds represented the time taken by the light to cross the earth’s orbit, now known to bo 186 million miles in diameter. The planet itself is a magnificent globe with a diameter of 88,700 miles across its equator, but only 82,800_ miles through the poles. The polar diameter of our earth is 7,900 miles, and it is only twenty-seven miles wider at the equator. Every school child should see Jupiter through a largo telescope to understand what is meant by “flattening at the poles,’’ for the oval shape of

Jupiter’s disc is most striking. The two diameters of Jupiter, equatorial and polar, are 11.2 and 10.4 times that of the earth. By means of tho moons, the mass of Jupiter has been found to be 318 times that of the earth, but its volume is 1,312 times as great, and so the density is only 0.24 of that of the earth, or I. times that of water. Yet objects on Jupiter’s surface weigh two and a-half times as much as they do here. Being 483 million miles away from tho Sun, 5.2 times as far as is the Earth, Jupitei cannot be very warm unless he has a great supply of his own heat, which is doubtful. He takes 11. years to circumnavigate the Sun. Jupiter is the largest and heaviest of all the planets, and also rotates the fastest, his sidereal day being only 9h oOmin 30sec long at the equator, but increasing towards the poles. Jupiter’s four briglit moons, discovered by Galileo, are generally known by the numbers 1., 11., 111., and IV., but alsc by tho more romantic names 10, Europe, Ganymede, and Callisto. lo was the maiden guarded by tho monster Argus, with the hundred eyes, Europe the lady who was being carried across the Dardanelles by Jupiter in the form of a bull, when she fell off and -was drowned. Her name was given to the continent which she failed to reach. Ganymede was the beautiful youth carried off by an eagle to become Jove's cup-bearer; while poor Callisto, anothei of the god’s lady friends, was changed into a bear by the jealous Juno. These four moons are very rapid in their movements, eclipses and transits being very frequent, but requiring a fairly large telescope for proper observation, whereas tho moons themselves may bo seen with quite a small one. The following notes may help observers to identify them. Tho * Nautical Almanac ’ for 1932 predicts the superior conjunctions of each of the four moons—i.e., the times when each moon is directly beyond Jupiterj as seen from tho Earth, and is almost invariably eclipsed, but soon appears on the east of the planet, the right-hand side in a terrestrial telescope. The period from one superior conjunction to the next is the synodic period, and is divided into four parts, practically equal, by the epochs of east elongation, inferioi conjunction (usually passing across the disc of the planet) and west elongation. Tho distances from the- planet readied at elongation are approximately:—lo 2min of arc, Europa 4min, Ganymede 6min. and Callisto lOrnm, In January, 1932. the mean synodic periods and epochs of superior conjunction are; 10, Id 18h 26 Im, there being 17 superior conjunctions, the first at 5.04 p.m. on the 2nd the last two minutes after midnight on the 31st: Europa ? 3d 13h S.lmin, 9 superior conjunctions inclusive from January 1 at 9.16 a.m. to January 2! at 6.21 p.m.; Ganymede, 7d .03b 20.8 m 3 superior conjunctions from 12.42 a.m on the 4th tr 10.44 a.m. on the 25th Callisto has two superior conjunctiononly, on tht Bth at 4.50 p.m. and tin 2ot'h at 7.18 a.m Even the ‘ Nautica Almanac ’ can scarcely make it easy for the amateur with a small telescope to identify these moons, but the above times may enable some observers to tell which is which. The moons are very similar in brightness, but rather variable. lo revolves round Jupiter in Id 18h. Europa in 3d 13h, Ganymede in 7d 0.4 h, and Callisto in 16d 17h. Tho surfaces of these moons must be rather diversified, judging by the variations in their brightness. Europa is the smallest, 1,865 miles in diameter, and 5.1 to 6.4 in mag., while 10, 2,109 miles across, varies from 5.3 to 5.8. Callisto is 3,142 miles across, with maximum and minimum mags. 6.1 and 6.4, making it the faintest moon as a rule. And the largest, Ganymede, 3,272 miles across, vanes from 4.9 to 5.3. Ganymede is also the heaviest of the four, and Europa the lightest. In thousands of miles from the centre of Jupiter their distances are: lo 262, Europa 416, Ganymede 665, and Callisto 1,170. Their orbits are nearly circular. Probably few people realise that Ganymede and Callisto are larger than the planet Mercury, yet not as largo as Saturn’s satellite Titan. These four satellites revolve in the plane of Jupiter’s equator, which is nearly the samr as that of his orbit around the Sun, as does also tho fifth (V.) discovered by Barnard, revolving in twelve hours only 68,000 miles above Jupiter’s equator Satellites VI., VII. VIII. and IX., like Hyperion and Japetus in Saturn’s system, have highly inclined orbits, and the perturbations due to the Sun completely spoil the elliptic paths which they should pursue under Jupiter’s attraction alone. They are all very faint, mags. 14.7 to 18.6, and tho faintesi, No. IX., is only about fourteen mi'es in diameter. The periods and distances of the outer four show a curious pairing: VI. and VII., discovered by Perrine, 251 and 260 days at 712,000 and 730,000 miles; VIII. and IX. discovered by Melotte and Nicholson, 739 and 745 days at 1,460,000 and 1,470,000 miles. Comparing Jupiter’s system with that of Saturn, we see a somewhat superficial resemblance, but many differences in detail, which are liable to challenge any theories professing to explain the evolution of the solar system .