The tides are the pulse of the world's oceans. They are most obvious at the ocean rim, where their rhythmic ebb-and-flow brings about that periodic emersion and exposure of the intertidal zone which has such profound effects on the indigenous ecosystems.
And the tidal currents, accompanying the rise and fall of the water level, are by far the strongest currents in the coastal ocean: even today's large and powerful ships prefer to leave a port with an outgoing tide, and to enter on an incoming one, just as the sailors of antiquity did in times long past.
The word "tide" itself is derived from the Saxon tid or "time". Indeed the term is still used in this sense in a few seasonal names such as "Christmastide" and "Eastertide", while the modern German words Gezeiten, meaning "tides", and Zeiten ("times") preserve the connection even more explicitly.
Where necessary people adapted their lives, instinctively and without any basic understanding, to cope with the daily comings and goings of the local waters, and devised many creative explanations as to what might be the cause.
The Chinese, for example, regarded seawater as the blood of the living Earth, and the tides the beating of its pulse. The Arabs, on the other hand, correctly focused on the moon: they supposed the moon's rays to be reflected from the rocks at the bottom of the sea, thus heating and expanding the water which subsequently rolled in waves in the direction of the shore.
And another somewhat romantic theory of the tides invoked an angel striding over the surface of the Earth: when it placed a foot into the water the flow of the tide began, and when the angelic leg was raised again, the tidal ebb soon followed.
The earliest known astronomical explanation for the tides occurred 3,500 years ago in ancient India. The Aryan priests in that country compiled an exhaustive catalogue of their beliefs and practices, and recorded them in the four great books we call the Vedas.
The fourth book, the Sama Veda, assigns the origins of tides, correctly, to the influence of the sun and moon. But how this influence might be exerted was the subject of diverse opinions. Even in relatively modern times, the 17th-century French philosopher Descartes was of the view that space was full of an invisible substance known as ether, and that as the moon travelled on its journey around the Earth it compressed this ether sufficiently to transmit a pressure to the sea, and thus caused tides.