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Navigating Using the Southern Cross

If you ever have the misfortune of being lost at night out in the open, the battery on your smart phone with its GPS or night sky app is flat and like Bear Grylls you just "need to keep moving".  Or more realistically you are at a party, out on the deck and are trying to impress the assembled crowd with your knowledge of astronomy.  Here are two ways of locating the points of the compass by using the Southern Cross.

 

Obviously the first thing you need is a relatively clear view of the night sky which in Aotearoa is easier said than done.

The next step is to locate the Southern Cross.  This is not quite as easy as you might think.  The constellation is smaller than many people think and the "false cross" (the Vela constellation) can be mistaken for it, especially in New Zealand where the fifth star (which appears on the Aussie flag) is not visible to the naked eye.  The best way to confirm if the group of four stars in a kite formation that you are gazing at are in fact the Southern Cross is to look for the two bright stars (Alpha Centauri and Beta Centauri) which are "The Pointers".  These appear to the left of the Southern Cross and are used in one of the methods described below to find due south.

southern-cross-and-pointers

You then need to locate the South Celestial Pole (SCP).  This is the point in the sky directly above the South Pole.  All of the stars in the southern sky rotate around this point.  Unlike the North Celestial Pole which can be identified by the North Star (Polaris), the SCP is just a piece of blank sky hence why we have to use other constellations to get our bearings when looking south.

Method 1.

Draw an imaginary line from the top of the Southern Cross to the bottom and extend it 4.5 times.  Drop a vertical line from this point, which is the South Celestial Pole (SCP), to the horizon – that place is due south.

Method 2.

Perhaps an easier way, especially if you struggle visualizing 4.5 x the length of the long axis of the cross, is to also use the Pointers.  Draw an imaginary line between the two Pointers.  Half along this line draw another at right angles and extend it until it intersects the line running through the Southern Cross.  This intersection is also the SCP.  Again drop a vertical line to the horizon and again you have found due south.

Behind you is North, to your left is East and to your right is West.  Easy!

Southern Celestial Pole

Now look around to gauge the reaction on the faces of your wowed audience.  They have all probably gone inside and started a game of Twister!  Never mind, use your great navigational skills to find your way to the fridge and get yourself another beer.  Grab me one while you're there!

 

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