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## 'Far Horizon' printed from http://nrich.maths.org/

The observer's line of vision is along a tangent to the surface of the
earth at the far point she can see on the horizon. Imagine a right
angled triangle with the observer at one vertex, the far horizon point
at another vertex and the centre of the earth at the third vertex.

You have enough information to find the angles in this triangle and
hence the distance required.