The wind has a timetable

Start with geography

A wind that changes its mind twice a year

The Indian Ocean has something the Atlantic and Pacific do not: a wind system that reverses. In summer the land mass of Asia heats, air rises, and the wind blows from the south-west across the ocean toward it. In winter the process reverses and the wind blows from the north-east.

For a sailing ship this is close to a scheduled service. Sail one way with one monsoon, do your business, and sail home with the other — but only if you leave on time, because miss the window and you wait months. Indian Ocean trade was therefore seasonal, predictable and, crucially, round-trip in a way Atlantic voyaging was not. Merchants overwintered in foreign ports, married there, and built communities strung along both coasts.

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