IN STARRY SKIES

Evening Post, Volume CIX, Issue 125, 29 May 1930, Page 23

IN STARRY SKIES

(By Omega Centauri.)

MOTIONS ,OF THE EARTH

ASTRONOMY FOR GIRL GUIDES

: We have studied the apparent motions of the stars in the evening sky as seen from near Wellington. We have /watched the Southern Cross/ the Pointers, Canopus, and Aehernar circling perpetually round a point about 41 degrees abovo tho horizon and exactly south of us. These stars have furnished us with a compass and a celestial clock. Wherever we may bo on a clear night they point out due soutli and tell us the time. We havo observed other stars rising in the east, passing in a i stately -procession across the sky and setting in the west. We have noticed a few that peep once- in twenty-four ;hours, for a few minutes only, abovo the northern horizon. Many groups of stars that we road about in books never appear to us at ail. This shows 'at once.that the details of the picture must depend on the latitude of the observer. If we couid travel from Pole to Pole we should see an interesting series of changes. 'At the South Pole no stars would over-rise or set. Sigma

Octantis would appear .at rest right 1 overhead. Our clock would no longer be a compass, and we should have to call help from some distant mountain or other landmark before- it would even tell us the time. There would, of course, be no other direction than north. In what ever direction we walk we must start northwards. If wo wish to make good progress it will never do to steer directly towards a star. Suppose, for instance, we were to: select Canopus and walk-steadily towards it at the rate of four miles an hour. We should never get more than about fifteen and a quarter miles from the Polo, and in twenty-four hours, after ' walking 96 miles,.we should he back again at our starting point. The Southern Cross, the Pointers, Canopus, and Achernar, all the familiar gems of our stellar clock, are, now' always in view, but they remain each at a constant elevation, moving round for ever: from right to left. Orion's,head is below the horizon, but his belt' would be in view just above the ice, circling the horizon once in twenty-four hours. Turning our back on Orion we see tho Scorpion rather higher in the'isky, an,d the two chase oneanother,,.round and round for ever. We soon realise that the apparent path of every star, when seen from the Polo, is a circle parallel to tho horizon. If we went to the North Pole we should see an entirely different set of stars, but they would bo behaving in much the same way. Orion's head would bo in viovv, but his feet would be below the horizon..All the stars would be circling the heavens, but they would now'appear to move1 from left to right. If now we moved our observing station to tho cquator^wo should bo able to see both sets of ; stars. Each ono would remain abovo tho horizon for twelve hours out of the twenty-four. Tho" North Pole Star would be at rest on the horizon due nor;th, and Sigma Oetantis similarly stationary due south. Other stars would rise and set, coming up and going down at right angles to the horizon. Orion being on the equator would rise dne east, pass across the zenith, and set exactly in tho west. When we consider these facts, we can well understand how tho ancient astro- • nomers were led to picture the stars »3 luminous gems fastened to the inside-'

oi: a, celestial sphere which revolved i'oiover round tho earth. But \vs know a far simpler, and much more rational, explanation. Motiojis avo relative. If, when dancing, wo spin rapidly round and rouud on our toes, all sides of tho' room flash in succession before our eyes'. But wo do not attribute tho movement to them. Knowing that stars are suns, it would be far more preposterous to suggest that they aro all revolving round, tho earth. Their apparent movement can bo readily explained by the rotation of the earth on'its axis , once in twenty-four hours. As it spins it carries us with it, and presents to our gaze in regular succession different parts of the distant universe. Still and peaceful as our planet seems, it is moving in a very complicated way and currying us through- spaco at a terrific speed. Its rotation is the slowest of its motions, and yet a place on tho equator is carried "round by tho spinning earth at a rate of over a thousand miles an hour. In Wellington we traverse a shorter path, but even here the rotation of the earth carries us inoro than 7u'O miles an hour. But this astounding speed is far exceeded by other movements of the earth. A Girl Guido prefers to find things out by observation instead of trusting to books or hearsay. If sho watches the evening sky, slie will soon notice that any particular star rises or sets a few minutes earlier each night. In six months the aspect of the heavens is completely changed. Tho same stars, it is true, still circle tho South Pole, but the clock face is exactly the other way up. Tho rest of the constellations are mostly completely changed. This was a puzzling

phenomenon/to tho ancients.'They solved it, as wo can do now, by watching the first stars that appear in tho western sky, after the sun has set, and the last that precede the rising sun. It is found that whilst the stars move as if fixed to tho celestial sphere, the sun -docs not. It appears to move amongst the stars from west to east. , The natural conclusion was that whilst everything moved round the earth, the sun revolved more slowly than tho stars, taking, indeed, nearly four minutes longer over each revolution. But here again appearances were deceptive. We know now that the sun is a huge body more than 330,000 times as massive as the earth, and with a volumo one million three hundred thousand times as great. It would indeed be remarkable if so tiny an earth -could control the movement of so mighty-a sun. It is just the other way. We may say, if we wish, that each .revolves round their common centre of gravity, but the sun is so enormqus thai it is practically the same thing to'say simply that tho earth revolves round the sun; and this explains the apparent motions completely. Kepler proved that the planets move in ellipses round the sun, which is one focus of each orbit. Newton found the law of gravitation under which such motion is possible. The earth takes a year for each revolution, and-so the sun appears to pass completely round the heavens in the same time. Tha sun's apparent, path amongst the stars is called tho ecliptic. But it is not easy to determino. this''path. Tho sun -is so brilliant that .whenever it is above f the" horizon' it blinds ns to the comparatively feeble' light of the stars. They »ro present in the sky as truly during the day as they are 'at. night. ', But though they are continually above bur heads ive see-them not. It is only when we are carried by the rotation of the earth into its shadow that we realise the glory of ■ the starry sky> If the earth turned the side on which we live perpetually towards tho sun, we should know very little of tho marvellous universe by which wo are surrounded. All messages frpm tho great "Beyond" come on the.wings' of light, ; but we caimot read their delicate indications until we are shielded from the dazzling 'brilliance of the sun.