One of the most practical skills in Astronomy is the ability to find your bearings. Once you know where North is, you know East, West, South and all directions between. There are many ways to find these directions, but the easiest and most accurate is by using a gnomon, and the sun.
Knowing where your cardinal points are is important in astronomy. Directions to find a particular object are either given in absolute coordinates (RA 11h56m25s, Dec -25°33’21”), or relative to a landmark (about 3 full moons west of Sirius). The latter is obviously much simpler and more intuitive for our back yard needs. If you want to set up a telescope on an equatorial mount, the mount has to be aligned with the earth’s axis, which means lining it up exactly on a North-South line. If you want to match a star chart with the actual sky, it is much easier if you start by pointing the top of the chart towards North rather than trying to compare the real stars with the chart by eye.
So how is it done? Quite easily. Start with a precisely vertical gnomon (Take a sundial, and remove everything except the bit that makes the shadow. You now have a gnomon. Or, stick a thin pole into the ground, so that the sun casts a shadow. You now have another gnomon). Wait until exactly local noon, and draw a line on the ground, along the gnomon’s shadow. This line is precisely parallel with the earth’s axis which means that, in the Southern Hemisphere (where I live), the line points exactly South (if you’re looking away from the base of the gnomon). In the Northern Hemisphere, it points North.
The only tricky part of this is finding local Noon – it is defined as the instant when the sun is at it’s highest point in the sky above where you are, and is almost guaranteed to NOT be when your watch says 12:00. This is because your watch is set according to time zones, which are just an average time which we all agree on for every place within that zone. There are two ways to find local noon: Find your precise longitude, work out how far ahead or behind you are from the centre of your time zone, check an almanac for corrections due to variations in the Earth’s orbit, and do a little maths… or just wait for the point when that Shadow is at it’s shortest point.
So there you have it. South (or North, if you live on the wrong side of the world). Provided you chose your time right, you found it extremely accurately in just a few minutes with nothing more than a stick.