Use your iPhone/iPod/iPad on its edge, as a Hand Bearing Compass.
Press the screen to take a sight.
After 3 sights, go to the chart table !

The Magnetic variation is calculated, depending of your position, using the WMM algorithm and data.

Notes :
To save battery life, position is refreshed with an accuracy of 1NM.
Bearings are displayed in Magnetic (M). You have to apply Magnetic
Variation to determine the True North (T).

References :
- WORLD MAGNETIC MODEL : http://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml

- Moon Calculations : ELP2000 theory

- Jean Meeus. Astronomical Algorithms