Look at where the island sits and the answer is immediate. Osmussaar lies 7.5 kilometres off the Estonian mainland at the very mouth of the Gulf of Finland - the gate through which everything bound for Tallinn, Helsinki, Vyborg and St Petersburg has to pass. It is also almost invisible from the water: a flat limestone slab five kilometres long and barely more than a kilometre wide, rising only a few metres, ringed by shoals and by the shallow banks of an ancient impact crater a short way offshore. A low island in a busy strait is a trap, and the islanders knew it well. Salvaging wrecks was for centuries part of how the community here made its living. The lighthouse exists to end that particular source of income.

The first stone beacon went up in 1765. In 1804 it was raised and fitted with a proper lantern room, its light produced by a catoptric apparatus - twenty-four oil lamps, each sitting in front of a polished reflector that gathered the flame's scattered glow and threw it seaward. That structure collapsed in 1814 and a temporary wooden tower stood in for it. A new stone lighthouse followed in 1850, and in 1875 it received the technology that had by then transformed lighthouses everywhere: a dioptric Fresnel apparatus, whose stepped glass rings bend light rather than bouncing it, wasting far less and reaching much further. Electricity arrived in 1902. In under a century the same headland went from twenty-four flames in tin dishes to an electric lamp behind precision-ground glass.

What ended the 1850 tower was not weather but strategy. In June 1940 the Soviet Union cleared Osmussaar of its inhabitants - about 130 people, overwhelmingly Estonian Swedes whose families had farmed and fished here for some seven centuries - and turned the island into a coastal artillery position, with heavy gun batteries and a sixteen-metre fire-control tower to spot for them. For roughly eighteen months Osmussaar was a gun platform guarding the gulf. When the garrison finally pulled out in December 1941, the sappers took the lighthouse with them on the way. The island stayed in Soviet military hands until 1993.

After the war a temporary wooden tower carried the light again until something permanent could be managed, and in 1954 the current lighthouse was finished on the island's northwestern point. Reinforced concrete, 35 metres tall, focal plane at 39 metres, painted in the broad horizontal black-and-white bands that make a daymark legible against a grey Baltic sky. It is not an old or a beautiful lighthouse by Estonian standards - this is a country that still lights ships with Kopu on Hiiumaa, completed in 1531, and with the 52-metre tower at Pakri from 1889 - but it is the tallest thing for many kilometres in any direction, and it does exactly what the 1765 beacon was built to do.

The island below the light has gone quiet in a way its builders never anticipated. Since 1996 Osmussaar has been a landscape protection area of some 486 hectares, safeguarding its limestone cliffs, its beach ridges, and the extraordinary breccia boulders thrown out of the Neugrund impact. Some 158 bird species have been recorded here, and the island sits on the East Atlantic flyway, so in migration season the traffic overhead vastly outnumbers anything on the water. Around five people live here year-round. Up to three thousand visitors come out across a summer to walk to the ruined chapel, look at the boulders, and stand under the striped tower. The lighthouse zone remains one of the reserve's restricted areas - still working, still counted as infrastructure, on an island that has otherwise been handed back to the birds.