Finding true south

The Northern Hemisphere has the bright North Star as its guiding light to astronomers, navigators, surveyors, and Boy Scouts everywhere. Not so the Southern Hemisphere. The place in the sky toward which Earth's south polar axis points appears virtually starless.

Except, that's not exactly true. Sigma (o) Octantis, which can just be seen with the unaided eye in a dark sky, is designated Polaris Australe, the South Pole star. Sigma, which resides in the dim constellation Octans the Octant, presently lies 1.03° from the pole, but unlike the north celestial pole star, it's not getting any closer. By 2010, this distance will have increased by about two arcminutes. Sigma Octantis was nearest the south celestial pole around 1830, when it was only 45 arc-minutes away.

There are, however, nearly half a dozen other stars that lie still closer to true celestial south. Problem is, they're too faint to be seen with the naked eye. The brightest is about magnitude 7, the others fainter still.

So how do you find true south without a bright stellar beacon to guide you? This time of year, Crux, the Southern Cross, is high in the evening sky in the southeast. By using the longest axis of the cross - marked by

Finding celestial south using Ci|ux the Southern Cross. Looking south April 2, 9 p.m., from 33°S.

Greatest Eclipse

the stars Gamma and Alpha Crucis - as both pointer and measuring stick, you can hop your way to the south celestial pole.

The apparent angular distance between Gamma and Alpha Crucis is 6°. Polaris Australe lies about 27° away, or four and a half times the length of the Cross. This amounts to about three fist widths held at arm's length. A quick check of this region in binoculars should reveal a star field similar to the one illustrated here, which you can use to pinpoint the south celestial pole. (You will likely have to tilt the chart to orient yourself to the stars' positions as they appear in your field of view.)

When the Southern Cross is not above the horizon, say around early November, the task becomes a bit more challenging because there is no prominent star pattern to use as a guide. In this case, you must look for the 2nd-magnitude star Ankaa in Phoenix and the slightly fainter Beta (P) Hydri in Hydrus. These two are your pointer stars. A line extended from Ankaa through Beta Hydri, and thence onward another 12° (slightly over one extended fist-width), puts you very near the pole.

Just as precession shifts Earth's axis away from pointing at the North Star in the Northern Hemisphere, so too will it induce the south polar axis to incline away from Polaris Australe. By the year 5700, 3.3-magni-tude Omega (ra) Carinae will be the new South Star. By 7700, Iota (i) Carinae assumes that position, followed by the brightest south pole star in the entire precessional cycle, Delta (8) Velorum, in the year 9000.

Also this week:

• The blue-white star Spica in Virgo rises after sunset as seen in the northern sky.

April 5-11

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