Polar Inequities

Last night in my community education class a student asked about a southern polestar. "Do they have one like we do?" Yes they do, but it’s one not likely to catch the eye for most casual sky watchers living in the southern hemisphere.

If you extend the Earth’s imaginary axis into space it points at Polaris for northern hemisphere viewers and at Sigma Octantis for those living in the southern hemisphere. Illustration: Bob King

Our polestar Polaris is relatively bright and easy to find. You use the convenient "Pointer Stars" of the Big Dipper to point the way. Polaris is the same brightness as the brighter Dipper stars and sits at the sky’s north celestial pole, a point in the sky directly above Earth’s north polar axis. From the diagram above you can see that our inclined axis points straight out into space and by good fortune is aimed right at Polaris. As our planet rotates, all the other stars in the northern sky appear to describe circles around the polestar which remains nearly stationary atop the north celestial pole.

With the help of the Big Dipper, Polaris is easy to find. This map shows the sky around 8:30 p.m. Maps created with Stellarium

The Earth’s south polar axis also points into space in the direction of the faint constellation Octans the Octant, named after a navigational aid similar to the sextant. The brightest stars in Octans are only fourth magnitude and only dimly visible from a suburban location. Worse yet is the southern polestar. It’s barely above the naked eye limit and too faint to even garner a proper name like Polaris. Called Sigma Octantis, it’s located just as close to the south celestial pole as Polaris is to the north and shines at magnitude 5.5.

This map shows the sky from Sydney, Australia looking due south around 1:30-2 a.m. in late February. Southern observers can use the axis of the Southern Cross (Crux) to point them to the southern polestar. LMC and SMC stand for the Large and Small Magellanic Clouds, small satellite galaxies of the Milky Way.

When I flew down to Peru with my buddy Glenn back in 1986 to view Halley’s Comet I was able to see Sigma Octantis low in the southern sky from our desert observing site. Like the Big Dipper in the north, the Southern Cross, formally called Crux, points the way to Sigma. Draw a line through the north-south axis of the cross and it will take you to right to it. OK, so it’s not much to look at, but it does serve the same purpose as our polestar. From the perspective of those living down-under, all the stars in the southern sky appear to describe circles around Sigma Octans during the night. Despite its obscurity, Sigma rules!

Even if Octans doesn’t grab your attention, there’s much to see in the neighborhood. The second brightest star in the sky, Canopus, lies off to the west. Several outstretched fists above the octant you’ll find Alpha Centauri, probably the most famous star in the sky after the sun. Alpha is a triple star system and the closest star to Earth beyond our own. Then there’s the bright and compact Southern Cross — you can easily cover it with four fingers — which shines both in the sky and on Australia’s flag.

The Small (left) and Large Magellanic Clouds. Each galaxy contains many stars as well as star clusters and gas clouds like the Milky Way but packaged in a smaller volume. Each looks like a fuzzy chunk of the Milky Way set adrift in the night sky. Credit: ESO/S.Brunier

The two fuzzy objects on our map are the Large and Small Magellanic Clouds, the two brightest companion galaxies of the Milky Way galaxy. Named after Ferdinand Magellan, who with his crew were the first Europeans to sail around the globe, they belong to a group of about a dozen galaxies gravitationally bound to our own. The peoples of the southern hemisphere were long familiar with the clouds, but Magellan "discovered" them for European civilization in 1519. Unlike the grand spiral design of our Milky Way, the Magellanic Clouds are classified as irregular dwarf galaxies and only about a tenth its size. The LMC lies 179,000 light years away while the SMC is 210,000.

While driving back from an archaelogical site in the Peruvian desert our car broke down one afternoon, and we were forced to wait hours until help arrived. Glenn and I nursed the vehicle to a truck stop and had dinner by lantern. My most vivid memory of the place was an enormous painting of a topless mermaid on the interior restaurant wall. We invited the staff outside later when the sky got dark and showed them Halley’s Comet. That was the first time I got to see the Large Magellanic Cloud, and it was a thrill to see the real thing after years of studying pictures in textbooks. Me and Magellan, yeah, we were gellin’.

A map showing the satellite or companion galaxies of the Milky Way. We’re the Big Kahuna in the neighborhood. The other galaxies revolve around ours with periods of millions of years. Credit: Richard Powell

2 Responses

  1. Andrew Kirk

    Bob, I think I have never seen a photograph of the southern circumpolar whorl. It would look so cool with those Magellanic Clouds!

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