From ebb to flood

At first glance, it seems as though the tides are just a bit of simple physics – the Moon’s gravity tugs on the oceans, and the Sun has a smaller but similar effect. But why then, do tides in some places rise and fall roughly twice a day, while elsewhere the cycle only occurs once in 24 hours? Why does Canada’s Bay of Fundy have an enormous tidal range of 16 m whereas just 50 km away in the Northumberland Strait the range is a piddling 1.6 m?

These and other questions perplexed some of history’s greatest scientists and in Tide: the Science and Lore of the Greatest Force on Earth, author Hugh Aldersey-Williams explains how we came to understand why the oceans rise and fall, and indeed, how the course of history can turn on the tide. Attacking on an exceptionally high tide, for example, was seen as crucial to the success of the D-day landings in the Second World War. The Allies, he explains, used a mechanical tidal prediction machine designed 1872 by William Thomson (later Lord Kelvin) to calculate the tides along the Normandy coast using a hodge-podge of incomplete information.

The best part of the book though, is Aldersey-Williams’ contemplative description of a complete tidal cycle – from ebb to flood, and back again – that he experienced one warm September day on a lonely creek near the Norfolk coast. “The warming mud that has not seen the Sun for half a day raises a sweet shellfish odour,” he writes, “The seabed is coming to life.”

• 2016, Viking, £18.99, 426pp