not sure if this is geometry

Tide

If I understand correctly what you are asking then you just need to find the distances to common horizon of both the observer and the observed point and add them. Pythagoras will accomplish that.

Tide Recognitions: Homework Help Science Advisor	If I understand correctly what you are asking then you just need to find the distances to common horizon of both the observer and the observed point and add them. Pythagoras will accomplish that.