From the top of Samson Hill on Bryher, I could see the Bishop Rock lighthouse in the distance. I used the idea that any little finger at arm's length subtends 1 degree. I enlarged the photograph and found that the lighthouse subtends 1/4 of a degree. 2*pi radians = 360 degrees so 1/4 degree = 0.0044 radians. Angle subtended in radians = height of lighthouse/distance to lighthouse using the arc length approximation. The lighthouse is 49 metres tall. That gives a calculated distance of 11.1km away. I got out the OS 1:25000 map and measured the distance as 10.8km. That means there is a percentage difference of 3% by my method. I'm quite pleased with that.
