In case your GPS Breaks - How to Use Your Wristwatch as a Compass

Hold a watch with 12 o’clock at left. Move your arm so the hour hand points at the sun. The spot halfway between the hour hand and the 12 is south. This is for the Northern Hemisphere. In the Southern Hemisphere you point the 12 at the sun, and halfway between that and the hour hand is North.

From Wired Magazine