[Delmololo]

You should be able to calculate it out by telling the user to press a button and after this, not rotating the camera away.

Right?

Might just not be practical at all.

On the other hand, shouldn't the earth rotate fast enough to figure this out in a short timeframe while the photographer starts looking through the finder?

[mikhailfranco]

Yes, basically Method (2) with stable measurement window. Just put the camera down on a stable surface, click button. Let system wait some ms to allow click disturbance to pass, then integrate signal over some fixed time to establish the rotation, then pick up and continue...