Finding South Using the Stars
In the northern hemisphere Polaris, the North Star, tends to get all the attention when it comes to finding direction using the stars. There is a good reason for this: it is easy to find and is very accurate. In the southern hemisphere the Southern Cross is used to find south and Polaris is not visible. But what about finding south in the northern hemisphere? The easiest thing is still to find Polaris and then look in the opposite direction, but what if we want a method that actually shows us south itself. Here is a nice simple and very unusual method that I invented a few years ago, which you can try this evening.
First you need to find the constellation Leo. It is a nice, big and easy to identify constellation which, unlike some constellations I can think of, looks at least a little bit like how you would hope it does: ie. like a lion. If you are struggling then tonight it will be about three extended fist-widths to the left of the moon. Having found Leo, next you need to find two little-known but beautifully named stars, Chertan and Zosma, that form the rump of the lion.
In the first image, Chertan is the star that has been ringed, and Zosma is the one directly above it. When Zosma appears directly above Chertan, as in this picture, you must be looking due South. Have a look as soon as the stars come out tonight.
That is a nice, quick and simple method for using Leo to find south, but it does have one big drawback: it only works at certain times. I have devised a more advanced method for using Leo to find south that will work at any time and from anywhere that you can see Leo, but be warned this is a bit beyond beginner’s level so only read on if you are an experienced stargazer already or if you are up for a challenge!
We need the same two stars as before, Chertan and Zosma. Next what we do is extend a line from Zosma down to Chertan and keep it going until it hits the ground. If this is a vertical line, then, as we saw earlier, this will be due south. But most of the time these stars won’t form a perfect vertical line (they will wheel across the sky like the hand of a clock) and so what we need to do is continue their line to a point underground. The point underground needs to be the same angle below the horizon as your latitude. On the south coast of England my latitude is 50 degrees north and so I am interested in a point that is 50 degrees, (five extended fist widths) underground.
Wherever the extended line from Chertan and Zosma gets to this depth will be due south. All you need to do is find the point on your horizon that is vertically above this underground point and that will be due south.
Since you can find direction using Polaris in many more straightforward ways, this method is really just for fun, unless you are very unlucky and Leo is the only constellation that has not been obscured by cloud.
Why this method works:
Whenever we are using the stars to find north or south, we are looking for ways to find the north or south celestial poles, the two points in the night sky that are directly above the Earth’s North and South poles. If we look towards a point in the night sky that is above the north pole we must be looking due north. The same logic applies to the south celestial pole, but because we are in the northern hemisphere the south celestial pole is invisible, it is underground. But we can imagine where it might be and actually work out where it must be using the stars. The point on the horizon that is directly below the north celestial pole, ie. below Polaris, is due north and the point directly above the (underground) south celestial pole must be due south.