IN STARRY SKIES

Evening Post, Volume CVIII, Issue 80, 1 October 1924, Page 15

IN STARRY SKIES

FINDING THE SOUTH POLE

LOCATION BY NEIGHBOURING

STARS

(By "Omega Ceutauri.")

If we know the position of the Mer idian v?e can, of course, find the South Pole in the sky by looking at a point due south of us, and nearly 49 degrees above the horizon. But it is far more often useful to be able to tell which is due south by means of the stars. We have a pole star, Sigma Octantis, but unfortunately it is so faint that it is difficult to pick up with the naked eye. ' Its magnitujde is 5.48, only _ half a magnitude above the usual limit -of visibility. If you point a camera with a large stop, and a fast plate approximately in this direction, and leave the plate exposed for half an hour, shieldin« it carefully from other lights, each star will show its movement by tracing an aro of a circle on the film. All the arcs will have the same centre, but stars near the pole will make very short arcs, and those farther away longer ones. By picking a pin-hole through the beginning point of each arc on a print you can make a map of the southern heavens, and the common centre of all the arcs is the position of the South Pole. It is very close to the tiny arc traced by Sijjma Octantis. There are several ways in which we can find the approximate position of. the pole by eye. One of the very simplest, and a fairly accurate one, too, is to bisect the line from Beta Centauri, the fainter of the two Pointers, to Achernar,, the principal star of the long river Eridanus. This gives a point which is about two degrees, that is one-third of the length of the Southern Cross, from the, true Pole. Bisecting the line from Alpha Pavonis to Epsilim Argus brings •us . still closer, but these two stars are not quite so easy to pick out. Th» best way to identify the Pole star itself, with the naked eye or with field glasses, is to start from the smaller Magellanic cloud: Close beside it appears as a hazy star the great globular, cluster 47 Toucani. Bather further from it in another direction is the bright star Beta Hydri. Take a point mid-way between 47 and the centre of the cloud. Go from, there to Beta and then on as far again. This brings us to the three stars Gamma Hydri, which form a ehort curved line. We have now got more than half-way to the Pole, but the rsmaining stepping stones are rather indistinct. They are three stars unequally spaced in a straight line. The last of the three is Sigma, and it may be identified with field glasses by noticing two faint stars . coming back from it like a barb on an arrow. Now that we have found Sigma, let us make the journey again m a slightly different way, taking three equal jumps, and one tiny' Btep. The first iump_ from 47 brings us to Beta ana indicates the scale. At the end of tne second jump the three stars of Gamma are spread out on our left. At the end oi the. third jump the middle star of the unevenly spaced three is close by on our left, and Sigma with its barb pointing almost to us is but a. short step in front. Sigma can be seen by thosa with keen sight on any night that is clear and moonless but it is much mor» easily identified with field glasses. If this , part of the sky is wall known we need never be in doubt about the points of the compass as long as the stars are visible. . ■ v