MAREA

Twice a day the sea comes in, and twice a day it goes out. It did so on the morning you were born, and it will keep doing so long after anyone remembers your name. On some coasts the difference is small. In the Bay of Fundy in Canada the water rises and falls by more than fifteen metres, enough to leave fishing boats sitting on mud where a few hours earlier they were afloat.

The Greek explorer Pytheas, sailing past Britain in the fourth century BC, was among the first to link the tides to the moon. He was right. The moon and the sun pull on the oceans, and the Earth turns beneath the bulge of water. The friction of all that moving sea is very slowly braking the planet. Each century the day grows longer by about two thousandths of a second, and every year the moon drifts almost four centimetres further away.

Marcus Aurelius spent the last years of his life on the cold northern frontier of the empire, fighting wars he did not want. In the evenings he wrote notes to himself. Time, he wrote, is a river of passing events, and its current is strong. No sooner does something come into view than it is swept past, and something else takes its place. He wrote it to practise a kind of calm: accepting the rhythm rather than fighting it.

A life has its own tides. Periods when everything arrives at once: work, people, opportunities, noise. Then periods when it all withdraws and the shore looks bare. Most of us panic a little at low tide. We think the water has gone for good.

The tide does not hurry. It simply arrives, and in arriving, changes everything it touches.

People who live by the sea learn not to worry about it. They check the tables, plan the day around the rhythm, and use the low water to walk out onto sand that is usually hidden. Some things can only be found when the sea has gone out.

MAREA moves in that pattern: one group of instruments coming in as another withdraws, never at quite the same moment, until at last they meet. It is a slow breath, drawn by the moon.

— Marcus Aurelius, 121–180 AD