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Using the Southern Cross to Find South
The Southern Cross is on the New Zealand flag, and most people here can point to it. Fewer know how to use it, which is a shame, because with it and the two Pointers you can find due south anywhere in the country in under a minute, no phone required.
Why we need a method at all
In the northern hemisphere, Polaris sits within a degree of the north celestial pole, so finding north is just a matter of finding one star. The south celestial pole has no such marker. The nearest naked-eye star, Sigma Octantis, is about magnitude 5.4, so faint that it is hard to see from a suburban backyard and invisible under any moonlight. Instead, southern observers find the pole by geometry, using brighter stars that point to an empty patch of sky.
Meet Crux and the Pointers
Crux, the Southern Cross, is the smallest of the 88 constellations, a compact kite of four bright stars plus a fainter fifth (Epsilon Crucis) tucked off-centre.
- Acrux (Alpha Crucis): at the foot of the long axis, the brightest.
- Gacrux (Gamma Crucis): at the head, distinctly orange-red.
- Mimosa (Beta Crucis) and Delta Crucis: the two arms.
Close by are the Pointers, Alpha and Beta Centauri. Alpha Centauri is the brighter and slightly yellower of the two, and is the nearest star system to the sun at just over four light-years. The Pointers are the giveaway that you have the real Cross. There is a "False Cross" in Carina and Vela that is larger, fainter and has no pointers beside it.
Method one: the long axis
Imagine a line running down the long axis of the Cross, from Gacrux through Acrux, and continue it. Extend it about four and a half times the length of the Cross itself. Where that line ends is very close to the south celestial pole.
Method two: the Pointers
Find the midpoint between Alpha and Beta Centauri. Imagine a line at right angles to the line joining them, heading off toward the side of the Cross. Where this line crosses the extended long axis of the Cross marks the pole.
Using both methods together corrects the slight errors in each.
From the pole to the ground
Once you have the point in the sky, drop a line straight down to the horizon. That point on the horizon is due south. Pick a landmark, such as a tree, a fence post or a hill, and you have your bearing.
The pole's height above the horizon equals your latitude. From Tāmaki Makaurau it is about 37 degrees up, roughly two fist-widths at arm's length plus a little more. From Ōtepoti Dunedin it is about 46 degrees, and from Rakiura nearly 47. This is a handy check that you have it about right.
How the Cross moves
Because Crux lies so close to the pole, it is circumpolar from all of New Zealand: it never sets, but wheels around the pole once a day (strictly, once every 23 hours 56 minutes).
Its position at a given time of evening changes through the year:
| Evening season | Where to look |
|---|---|
| Autumn (April to May) | High in the south-east to south, standing nearly upright |
| Winter (July) | High in the south-west, tilting over |
| Spring (September to October) | Low in the south-west, lying on its side |
| Summer (December to January) | Very low in the south to south-east, near its lowest point, sometimes hidden by hills or trees |
In summer evenings, if the Cross is hidden low behind the scenery, the method still works later in the night, or you can use the Pointers and the bright star Achernar, which sits on the opposite side of the pole.
The Coalsack
Beside the Cross, next to Acrux and Mimosa, is a dark patch where the Milky Way seems to have a hole in it. This is the Coalsack, a cloud of dust about 600 light-years away that blocks the light of the stars behind. On a dark night it is one of the most striking naked-eye features in the whole sky, and many cultures have stories about it. In binoculars its edges are soft, with a few foreground stars scattered across it.
Nearby, just beside Mimosa, binoculars show the Jewel Box, a compact star cluster with one orange star among blue-white ones.
Why aurora watchers face south
The same southern direction is where aurora watchers point their cameras. The aurora australis forms an oval around the south geomagnetic pole, which lies over Antarctica. From New Zealand the display almost always appears above the southern horizon, sometimes climbing higher on a strong night. Knowing exactly where south is, and having a clear view toward it, is the first step. Many aurora photographs from here include the Cross and the Pointers, which is a lovely way to show the direction the camera was facing.
Our notes on reading an aurora forecast cover which nights are worth heading out, and our guide to New Zealand's dark sky places suggests places with a properly dark southern horizon.
Taking it further
Planetarium software such as Stellarium will show the Cross from your exact location at any time, which is a good way to practise before a night out. The official boundaries and star names of the constellations are maintained by the International Astronomical Union, whose list of approved star names includes Acrux, Gacrux and Mimosa.
Once you can find the pole quickly, you will find it is useful for much more than direction: setting up a photography tracker, judging which objects will stay up all night, and simply knowing your way around the southern sky.