In this photo, one of the Outward Bound Oman instructors, who I visited recently, is being taught how to use a traditional and beautifully simple navigational instrument called a 'kamal'.
This instrument is as simple as they get: it works by forming a triangle. If you know the base of a triangle (the fixed length of twine from eye to instrument) and you know the height of the triangle (the number of fingers counted up from the horizon), then you have a fixed angle to the horizon. This is the ancestor of nearly all navigational instruments prior to electronics. (In fact the triangulation used has a lot in common with the way GPS works, but that is another story.)
How does it work in practice?
Here's the simplest example: the Pole Star (Polaris, North Star) will be the same angle above your horizon as your latitude. At the North Pole it is vertically overhead and you are at 90 degrees north. At the Equator it sits on the horizon and your latitude is 0 degrees.
On a more practical level... Imagine you set out west from a desert camp on a long journey west into the Sahara. If you measure, as accurately as possible, the number and fraction of fingers that the Pole Star is above your northern horizon then you have a measure of your latitude. You can confidently explore the desert for as long as resources allow, then, when it is time to head home you line your kamal up with the Pole Star. If the star is higher (more fingers) you are north of your camp, if there are fewer fingers you are south of it. All you do is head north or south until the right number of fingers line up with the star and then head back east. You will bump into your camp. If you have been skilful enough! (The same of course applies to any ocean journey).
There are lots of slightly more convoluted (and fun) ways of using this simple instrument, but that is the crux of the matter.
Once this makes sense it is very easy to see how the whole history of navigational instruments developed. They mostly boil down to a desire to measure first angles and then time more accurately. The backstaff, octant and sextant are just the pernickety (and more accurate) grandchildren of the kamal.
You can make a kamal at home using cardboard and string. I nearly mentioned double-sided sticky tape then, but its too simple for even that. Bad luck, Blue Peter! With a bit of luck they've already got one somewhere, one that they made earlier...
- north pole
- north star
- Pole Star