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

The radius of the Earth is 6367 kilometres.

An observer is on top of a lighthouse which is 25 metres high.

**How far from the foot of the lighthouse, measured on the surface of the Earth, is the horizon that the observer can see?**

It is 32 kilometres (21 miles) across the Channel between England and France and you can see France from the cliffs of Dover.

**How high must the cliffs be for it to be possible to see that distance?**