IN STARRY SKIES

Evening Post, Volume CIX, Issue 119, 22 May 1930, Page 22

IN STARRY SKIES

ASTRONOMY FOR GIRL GUIDES

By ("Omega Centauri.") The aim of a Girl Guide is to acquire efficiency. She must not only have sound knowledge, but she must bo able to use it. She is expected moreover to have a wide range of activities. The ■badges that are. awarded for success arc valuable incentives to study, and later afford a sure indication that proficiency has been achieved. It was a wise'provision to include* amongst these insignia a Star Badge. At tho present moment a number of Girl Guides arc studying for this particular distinction, so it seems a fitting timo to look at some of tho facts of the science'from their point of view.- Girl Guides naturally demand something from the stars before they consent to give: them their full allegiance. In particular they expect the stars to toll them on any clear night the exact North- and South line, ■ and so to make them independent of ■the compass. They also require the stars, when called upon, to take the place of a watch and tell them truly how the hours go by. But no- Guide will be content to stop here. Whilst \ asking direction in her travels from, the sun, the moon, the planets, or the stars, she will wish to learn .something

about their real , nature; and their strange li.fe-historjes. Let us then, without delay, get , out into the open air and look carefully at tile evening sky. We shall suppose that tho night is very dark and the moon .'far below the horizon. We choose a spot far away from street lights where there are no trees, houses, or near hills to obstruct our view. Let us turn first to. the-1 .East .and Eclect for observation, amongst the twinkling points of light, some brilliant mtar or group of stars close to the horiKon. We shall not have long to wait Wore we realise that it appears to move. It is rising, but not vertically. As it rises it moves towards the left. In AVellington its apparent path is inclined nearly 40 degrees to tho horizon. Now turn to the West and again select a brilliant star. , It quickly sinks towards tho horizon, but again' riot vertically. It moves towards the left, and its path makes tho same angle with the Horizon that we observed in that of the rising star. If we hatched all night we should see that tho star which rises exactly in the East never reaches the zenith but passes about 41 degrees to tho. north of it.

Next let us turn due North. A star Bear the horizon there appears to move directly from east towards the west. Its path seems horizontal, but if we examine it carefully we shall find it to be.concave to the horizon. The star appears above the horizon a little to the east of north, and disappears the same distance towards the west. The brilliant star Capella behaves in this way. To see it at all wo must look exactly at the right.time, for it docs not ..linger long above our horizon. Now turn and look directly South. The scene is changed. Close to the horizon the stars appear to move from west towards the east, but their paths are co.nvex to the horizon. Those to the west of south aro sinking in curved lines, those to the oast of . south are rising. Those above tho Pole arc moving from the east towards the west. Prom the horizon almost to the zenith the southern heavens form a hugo clock face. The stars serve as hands, but

they move once around the Pole in 2-1 hours instead of twelve. This celestial clock is something move than a timekeeper; it acts as a compass as well. The centre about which the- stars appear to turn is the exact South Polo of tho heavens. If you are ablo to determine this point you will aiways be certain of your bearings on a clear starlit night. Unfortunately, tlicre is no bright star to mark the point. There is a star called Sigma Oetantis, which is excellently placed,' almost exactly, at tho pole. But unfortunately it is faint and not easily recognised with the naked eye. With field glasses, however, it is easy to pick it up. But this is seldom necessary, as there is- an easy way in which wo can Jix the point we arc looking.for with sufficient accuracy. On our southern celestial^ clock face, with its shimmering illumination, eight brilliant stars stand out conspicuously. Four ■of these form the well-known .Southern Cross, two more the still more brilliant pointers. Tho latter are called Alpha and Beta Centauri, Alpha being the brighter one, which is further from the Cross. Tlie four stars of the Cross are Alpha, Beta, Gamma, and Delta Crucis. In all constellations tho brightest star is named Alpha and tho others tako the remaining letters of the Greek alphabet in order according to their brightness. Now, if to draw a line from Gamma Crucis, the head of the Cross, through Alpha, the foot,'and then continue this Jine ten times as far, we come to Aehcrnar or Alpha Eridani away on the opposite side of the pole. We must be sure to recognise this star, as we are

about to make use of it. Tho middle point of the line joining Achcrnar to Beta .Centauri is . very close' to the South' Polo of the heavens. Thus our celestial clock serves as a compass which is always' available at. night' whenever the ', sky is clear. But wo have not yet mentioned the brightest gem on our clock face. 'This is Canopus, or Alpha Argus, the second bright^; est: star in the sky. It irtoves for ever round the pole about three hours in advance of the Cross. It is worth noting that air but Delta Orucis of the eight stars /wo have, mentioned are amongst the 23 brightest stars in the heavens. Alpha Centaur.i -is excelled only by Sirius and Cahopus. Achernar comes ninth, Beta Gentauri tenth, and Alpha, Beta, and Gamma Crucis thirteenth, twenty-first, ana twenty-third respectively. ; It is a little harder to read the time by our clock than to1 find our bearing. Tho reason is that the stars arc not exactly in the same position at the same time on successive nights. Suppose wo start observing in the summer and,look out at our stellar clock every evening at 8. On Ist December we see the Southern Cross upside down near tho horizon. By the Ist of January at the same hour it will have moved thirty degrees towards tho east and a little upwards, and in its da.ily rotation it will now pass any point two hours earlier than it did a month ago:' Our 'clock thus seems to gain about four minutes a day. If we make allowance for this we shall have no difficulty in reading the time by the southern stars. It will be-a great help, however, if we use a niodel clock. Make first a circular map or the stars that never sot, all those, say, within forty degrees of the South Pole. Divide the outer margin into twelve parts, one for each month of tho year. Start, by placing the Ist of November opposite Alpha Centauri, and put all the months in;order anticlockwise. Mount this, circle so that it can turn in front of a- slightly larger circle divided clockwise into 24 hours, with noon at the top and midnight at the bottom. Our model will then be ready for use. -