Guanabara Bay is not a bay in the strict sense. Portuguese navigators who entered it on 1 January 1502 mistook the four-kilometre mouth for a river estuary and named the place Rio de Janeiro — the river of January — a cartographic error that became a city of six million people. The water they misread is the second-largest bay on the Brazilian coast, roughly 380 square kilometres, ringed by granite and gneiss monoliths that geologists date to the breakup of Gondwana. Sugarloaf Mountain rises 396 metres at the western entrance. Corcovado stands behind it at 710 metres, crowned since 1931 by Christ the Redeemer. The Rio-Niterói Bridge, opened in 1974, crosses the northern reach in a 13.29-kilometre span. From the deck of a boat, these landmarks arrange themselves into a single sightline that no viewpoint on land can reproduce. Sailing here is older than tourism. The bay served as an anchorage for the Portuguese fleet, then as the colonial capital's commercial harbour, and by the late nineteenth century as the training ground for Rio's yacht clubs. Marina da Glória, the departure point for most sailings today, was built on landfill in the 1970s as part of the Aterro do Flamengo park designed by Roberto Burle Marx and Affonso Eduardo Reidy. Free entry to Guanabara Bay applies at the waterfront itself; the marina opens 09:00–18:00 daily, and boat parties require separate tickets. What draws crowds to a rio de janeiro sunset boat party is a matter of geography and light. The sun sets behind the Serra do Mar to the west, so vessels turning back toward Glória face the illuminated granite while the eastern shore falls into shadow. The effect lasts roughly forty minutes. Catamarans and sailboats hold position off Urca and Praia Vermelha to catch it, engines cut, sails slack, the city's ferry traffic threading past toward Niterói. The bay has also become a working laboratory. Decades of industrial discharge prompted a clean-up programme whose results are still contested, though water quality along the southern channel has improved enough to support regular regattas and the swimming route used during the 2016 Olympic sailing events. Rio de Janeiro landmarks remain best read from water level, where scale is honest. A rio de janeiro sunset boat party, in that sense, is less a novelty than the oldest way to look at this city — from the deck of something moving slowly across it.