Help me understand the watch! Please.

Firstly let me just say I loved this film. I watch a lot of horrors/thrillers and am easily disappointed, but I greatly enjoyed this film. Edge of your seat stuff.

The one thing I did wonder about was the watch. I know he lined it up with the sun, but I just didn't see how that would work. I had to pause the film and really think, before deciding that I was too stupid to understand it. Anyone care to explain to a thicky?

Thank you in advance!