Lunar gravitation produces two "bulges" on the world's oceans, one almost directly under the moon, the other on exactly the opposite side of the world. Halfway between these two mounds of water, on either side of the Earth, lie two regions where the sea is a good deal shallower than it ought otherwise to be.
As Earth completes its single revolution beneath the moon every 24 hours or so, the zones of high and low water retain their position with respect to the moon, so two high tides and two low tides occur at most points on the surface every day.
Now this twice-daily rising and falling of the world's oceans can be looked at as a potential source of enormous quantities of energy. With the rising tide, a store of energy is built up by raising the level of a vast mass of water. When the lunar attraction is removed, this store is released as the water tries to return to its own level. A small fraction of the power stored in the world's ebbing tides would be sufficient to meet the entire needs of humanity, if we could but tap it.
Of course, it can be done. As high tide approaches, water surges into all our estuaries; all you have to do to harness its energy is to build a big wall across the mouth of the estuary. Sluicegates in the wall - or a barrage, as it is often called - allow the water free access, but at full tide the gates are shut, and as the tide recedes, the trapped water is forced to escape through turbines, which are used to generate electricity.
But there are problems associated with this kind of tidal power. The most obvious is the enormous capital cost of a barrage of any useful size. Once built, barrages are economical to run and fuel is free, but they come with an initial price tag of many hundred million pounds.
And there are also environmental concerns. Conditions that make an estuary ideal for generating power - a large ebb and flow of tide - also give it a rich intertidal feeding area for birds. A barrage dramatically reduces the difference between the high and low water, shrinking the habitat of the aquatic life on which many birds feed.
Further damage may be caused by changes in salinity brought about by the barrage. And there is also the danger of a build-up of pollutants in the estuarine sediment, since agricultural and industrial waste products will not be flushed away by the tides as efficiently as before.
So I guess we'll have to think it out again.