Visual signalling : as far as the eye can see

For thousands of years, people have used visual signalling systems such as flags, semaphores, lights, fires, smoke and sunlight reflected by mirrors (the heliograph).

If the signal is big or bright enough it can be seen over 20 miles away - the maximum distance is achieved when both the sender and receiver are standing on tall buildings or hills (the natural curvature of the earth is the limiting factor).

But that is on a clear day. In heavy rain, snow or fog visibility may be restricted to only a few hundred metres. (Messages sent by optical telegraph from Whitehall were often sent by horse to the first station outside London, the visibility in central London was so bad!)