Tides: how the Moon lifts the oceans

Twice a day, along most of the world's coasts, the sea rises and withdraws. It is the most tangible sign of the physical bond between the Earth and the Moon, observed by sailors since antiquity and explained only at the end of the seventeenth century. Understanding tides means realising that gravity does not act in the same way on every point of a body as large as our planet.

Two bulges

The tide arises from the difference in the attraction the Moon exerts on the different parts of the Earth. The side facing the Moon is pulled more strongly than the centre of the planet, and the centre more strongly than the opposite side. The result is a stretching of the oceans into two bulges, one towards the Moon and one on the far side. As the Earth rotates, each coast passes under both, and there are normally two high and two low tides in just under 25 hours.

The average interval between two passages of the Moon across the same meridian is in fact about 24 hours and 50 minutes, which is why tides shift by some fifty minutes every day. In reality the shape of coastlines, the depth of the seabed and the resonance of basins greatly modify the ideal pattern, and in some places there is only one high tide a day.

Sun and Moon together

The Sun also produces tides, but its tidal force is a little less than half that of the Moon, because the effect depends on distance more than on mass. At new moon and full moon, when Sun and Moon are aligned, the effects add up and produce spring tides, with the greatest range. At first and last quarter the two attractions act at right angles and produce neap tides, which are weaker.

Tidal ranges vary enormously from place to place. In the Bay of Fundy, in Canada, the difference between high and low tide can exceed fifteen metres, while in the Mediterranean, an almost enclosed sea, it is generally a few tenths of a metre. In the northern Adriatic the astronomical tide, combined with wind and low pressure, contributes to the Venetian phenomenon of acqua alta.

From the ancients to Newton

The link between tides and the Moon was already known in antiquity. The navigator Pytheas of Massalia observed it in the Atlantic in the fourth century BC, and Seleucus of Seleucia devoted theoretical reflections to it in the second century BC. Pliny the Elder, in his Natural History, attributes tides to the Sun and the Moon and notes that the greatest occur at full moon and at new moon.

The physical explanation came with Isaac Newton, who in the Principia of 1687 traced tides back to universal gravitation. A few decades earlier Galileo, in his Dialogue Concerning the Two Chief World Systems of 1632, had proposed a different theory, based on the motion of the Earth, and had dismissed as a kind of superstition the idea of a lunar influence upheld by Kepler.

Scylla and Charybdis

In the Strait of Messina tides produce one of the most famous phenomena of the Mediterranean. The Tyrrhenian and Ionian seas have tides out of step with each other, and the difference in level generates alternating currents that can reach speeds of several knots, with eddies and upwellings well known to sailors. Many scholars have recognised in these waters the origin of the myth of Scylla and Charybdis told in the Odyssey, where Charybdis swallows and spews out the sea three times a day.

Tides are thus a meeting point between physics and story: the regular rhythm of the Moon is translated into a movement of water that has shaped harbours, trade and fears. From the Homeric monsters to Newton's calculations, the history of tides shows how patient observation of the sea led to one of the fundamental laws of nature.