Understanding the motion of time and tides


In olden times it was commonly believed among seafaring folk that death could never come on a flowing tide.

The superstition is well documented by Dickens in David Copperfield in a conversation about the last hours of Mr Barkis: "People can't die along the coast," said Mr Peggotty, "except when the tide's pretty nigh out. They can't be born neither, unless it's pretty nigh in - not properly born, till flood. Barkis is a-going out with the tide; if he lives till it turns, he'll hold his own till past the flood, and go out with the next tide."

The word "tide" is derived from the Saxon tid or "time". The connection can be seen in the word "tidy", which originally meant "in the right season" or "at the right time". Things done in their proper season were likely to be accomplished in an orderly and proper manner, and hence by association tidy came to imply methodical, neat and well arranged.

The word is still used in the sense of "time" in a few seasonal names like "Christmastide" and "Eastertide". In Dickens's lifetime, the basic tidal mechanism was well understood, but over the centuries many creative explanations had been devised as to what might be the cause.

The Chinese, for example, saw sea water as the blood of the living Earth, and the tides as 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.

Another more romantic theory invoked an angel striding over the surface of the Earth: when he - or she for all we know - placed a foot into the water, the flow of the tide began, and when the angelic leg was raised, the tidal ebb soon followed.

In more modern times, the 17th-century French philosopher Descartes leaned in the direction of Aristotle's theory of the elements. 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 Earth, it compressed this ether in a way which transmitted pressure to the sea and caused the tides.

In the year Descartes died, 1642, Isaac Newton was born, and it was he who was to put an end to tidal speculation. He applied his formulation of the law of gravitational attraction to explain the influence of the moon and sun, and the riddle of the tides was solved.