IN STARRY SKIES

Evening Post, Issue 149, 20 December 1935, Page 12

IN STARRY SKIES

THE PLANETS IN 1936

..(By "Omega Centauri.") i At the; beginning of the year 1936 Mercury will be in Sagittarius, Venus will, be passing from Libra into Scorpio,-Mars will be about to leave Capricornus, Jupiter will be, nearly opposite Antares in Scorpio, whilst Saturn will be still in A'gnarius. •s The giving their times of rising and setting tells when we can se^ them to the best advantage. 'U, : ; MERCURY. 1 Mercury pays flying visits alternatively to the evening and the morning sky.; It is so rapid in its movements that in-ancient times it was called the "Messenger of the Gods." Although it "is-brighter than a first magnitude.star, lit "often escapes notice because it always "Appears in the brightest part of this sky-Must after sunset or before sunrise. ;With the exception: of some of the,asteroids, it has a more .eccentric orbit, and one more inclined': to the ecliptic than, any 11 other .planet. Its distance from the sun ranges from 28J to.nearly 43J million miles, -the sun being nearly 74 million, miles from the centre'-of its orbit. It "sweeps' round the sun in 88 days at a speed which varies %between 23 and 36 miles per second, but it takes on an average! ■ about 116 days to catch up one complete

revolution - with .regard to the. earth.; | Three' "times this 'synodical- period- is i 17 days less than a year.. Mercury ! appears therefore at least three times .' in the;evening and three times in the morning sky, each; year. • Jts-greatest -\ elongation from the sun ranges from i about 18 to 28 degrees, so some of .its appearances are much more favourable for obseryationx than others. , Its ■ dis-; . tance'i from the earth varies from about 50 to. 136 :million miles. It exhibits, phases likeVth6se!';,,of,;.the moon, arid changes .in'brightness from slightly less than that of Alpha Crucis, to-:a little greater; than .that of Canopus. The diagram shows that during 1936 it makes three visits ''to the morning sky and four to {he evening one. But' "one only in, each of the ; groups, is favourable for observation. Mercury should be,clearly visible in the morning sky during February and March, arid in'the evening sky during August and September.; During the latter.visit, for a fortnight, it will not set until two hours or moire after the sun.; ■ ')'i. ■'' - ; ■■.. .. VENUS. ' "'".■" : With the exception of the sun and moon, Venus, the sister planet of the earth, :'is the; brightest object in the heavens, usually exceeding Mercury by from ftwo -to five magnitudes. It is almost as large as' the/ earth,- having' a diameter - nearly. 2J-times as great as -that of 'mercury. Its orbit is- nearly : circular^ Its mean distance from the sun is ; dyer '67' million miles, and;the total variation in it is less than one million. With an orbital speed of 22 miles ) per second it takes 225 days to make'one journey round the sun, but as the earth is travelling in the .same direction it takes 584 days to catch up one complete revolution with regard to the earth. It follows that Venus remains .an evening or amprning star, for nearly ten at a time. Its distance from the earth ranges from .about:26 to. 160 ■: million miles, and its apparent diameter .from 10 to 64 seconds. Galileo was the first to observe; that''it; shows' phases- like the ■■; moon/ When furthest from us .it is full, when.nearest it turns its dark side ; tdwarjis us. . Its greatest brightness occur?iabput 36 days,before,or after inferior cqhjunction, when it appears like ;the2mooh' at an age of five days. After, superior conjunction, when Venus a^pea^'in,the;evening sky on the far sjde of jits orbit, it is slow in separating ■ from -the sun; , > But after attaining greatest elongation it moves rapidly towards iinferior ..conjunction.. Actually the interval between superior .conjunction and greatest ;elongation on either side isj 220 days, whilst that between inferior conjunction and greatest elongation jis, only 72. This -is well., illustrated in the .'-diagram." -Venus -was at its greatest "'.western elorigJitibnC oh November;,. 18, and , will : not... reach siiperWr xonjunction until about June 25. Itiwill.be prominent in the evening sky during the last 'three: months of ••the year;v; . i•.•■•'. ';. ~- ;•■■. ••■ '■■■;■ l^'l ■ ;;|v-;;'.-;~., ma)b5.;.;,■.„■.;,...... ; ';. • ,•■■' , \ iFor| the first^en, ■months [. of ■ -1936 ; Mars -jwill 'be1' too nearly inline with the s^ri^td'^;, be; seen r tp\ advantage, .its /siaereal^peribd 'of ; - -revolution is 687 : d^ys,!butr it:-taWesv7Bo; days- for the

earth to catch up one complete round. Successive, oppositions are thus separated by two years and 50 days. The last was on April 6, 1935, so the next is not due until late in May, 1937. It expends this year on the far side of its orbit from us. Mars is larger than Mercury'but considerably smaller than Venus, the diajmeters being about 3100, 4215, and 7575 miles respectively- Mars moves in an eccentric orbit. Its mean distance from the sun is 141,500,000 miles, but the actual distance varies by more than 26,000,000 miles. It has wider range of brightness than any other planet. Sometimes it is no brighter than Beta Crucis, whilst at others it is more than a magnitude.brighter than Sirius. The most favourable oppositions occur at intervals of over fifteen years. The last was in 1924. At quadrature Mars is like the moon when three days from full. JUPITER. Jupiter, the giant of the solar system, will be well placed for observation all next winter. It takes |lij years to make one ■ revolution round the sun, but the earth catches up one round in 399 days. ■ Successive oppositions are thus little more than 13 months apart, and one occurs nearly every year. ■ The last was on May .9, so the next will be due about June 12. Jupiter has' a- diameter-,of 86,700 miles, so that. in spite of its/great distance, it'shows a disc*'varying"from" 32- seconds at conjunction .to 50 seconds: at opposition. Its distance from the earth varies from! 367,000,000 to 600,000,000 miles. Its orbital velocity is a little more than

B;miles per second. The planet rotates irvless.than 10 hours, the period being shorter at the. Equator. The disc is considerably flattened,r the polar diameter ; being nearly 4000 miles less than the equatorial. Jupiter, has nine satellites, but .five of these .are too iaint to-rbe of much interest to amateur 'Observers. The four discovered by. (Galileo are bright enough to ] be. seen' with small telescopes or good field glasses. They are all between the sixth/"and seventh magnitude.. Jupiter's' 'equator ■is inclined only 3.1s degrees to the plane of its orbit, which, ;n, turn, is inclined only 1.3 degrees toHlie ecliptic. The four bright satel-., fites 'mpye-inf orbits'clbsie' to the/plane;: of the.' planet's equator] So? that -we generally see them strung out,in line. Their names are 10, Europa, Ganymede, and Calisto, their diatnete'rs 2450. 2050, 3560,". and 3350 miles,' and their periods about 13,"3J,.7 and 16 2-3 days. Two of them, are,larger than the planet Mercury. •'" It -is most1 interesting to watch their occupations, eclipses, and transits, .arid the transits ;of their' shadows. Jupiter will rise earlier each day until June 10, when it appears at sunset. It will be a conspicuous object in the evening sky until about the end of November. . ...... ,;, :'.,..Saturn. '■■ ■'■'■:■ ..-■■'■ -For the first three months of the year Saturn will be too nearly in line with the sun -for convenient observation. During the next three it will shine in the morning sky, and for the rest of the year it;will be well in view during the evening. It occupies the place of honour in 1936 on account of the dis^ appearance of the rings,, whose plane passes through the sun, twice during each revolution. Phenomena can be observed next June and at the beginning of 1937 which will not be seen again for .nearly fifteen years. Details will be given in another article. Saturn is an immense planet, second only to Jupiter. Its equatorial, diameter is 72,430 miles. Qwing to the eccentricity: of its orbit, its .distance from the sun varies by nearly 100,000,000 miles. At. the most favourable oppositions it is about 745,00tf,000 miles from the earth,, whilst at its remotest conjunctions the distance is 1,027,000,000 miles. Its apparent diameter varies from 14 to 20 seconds. Its period is 10,759 days or nearly 29J years, but the earth catches it up in,378 days. Successive oppositions thus come nearly a fortnight: later; each year. .The last took place' on; August 31, 1935. Saturn rotates in'a little over ten hours on an axis inclined more than 63: degrees to ;the plana of. it.s orbit, and 62 degrees to the-ecliptic NThe rings and six of its ten satellites revolve in planes close to the planet's equator. This makes the satellites of Saturn ' less."easy' to identify than those of Jupiter. They are usually grouped around it, but in ,the coming.year. will be, more nearly in line. A beautiful photograph was taken :by E. <C. Slipher on March ■ 2 1921; when'Titan,.Rhea, Dione, Tethys and Mimas were in .line on one side I and . Enceladus. close to the disc on ! the; other. "Observations during the last half of the coming year will be particularly valuable. Spectroscopic ones to determine the periods of rotation, in different latitudes are amongst those that have been planned.