We know that, in principle, a lighthouse needs to be seen as far away as possible. But how far is that?
Let's assume we have a very powerful light that can be seen on the moon! What is it that stops our lighthouse from being seen on the other side of the Earth? Well, the fact that the Earth is curved, of course. The horizon is what limits the distance at which you can see the light from the lighthouse. But the distance to the horizon depends upon how high the light is. The higher the light, the further away is the horizon. So you prefer to have a tall lighthouse than a short one. You might decide to put a short lighthouse on top of a tall cliff. The height of the tower is small, but it is the height of the light that matters and the higher the cliff, the farther away the light can be seen.
Unfortunately, things are never as simple as we would like. high ground is frequently shrouded in fog and cloud, especially in bad weather. So if the cliff is too high, the lighthouse is frequently hidden by the cloud and you can't see it at all, not even from close by.
There are many examples of where the lighthouse builders got it wrong. The old Lundy Island lighthouse was built high up on the centre of the island but seamen frequently reported that they could not see it. Today, the lighthouse is disused, replaced by two lighthouse at a lower height, one at each end of the island. On Beachy Head, the same thing happened. The old lighthouse - called Belle Tout - was built on top of the very high cliffs, but became disused when a new one was built at the base of the cliffs.
In parts of the world where the weather is variable, lights are rarely more than 70 to 100 metres above sea level and one of the world's tallest lighthouse at sea level is 75 metres high, at Genoa in Italy.
The Belle Toute lighthouse was built on the top of the magnificent cliffs at Beachy Head, but, although it had a very great range in principle, it was so often shrouded in mist and fog that in 1900 a new lighthouse was built at the foot of the cliffs (below).
