Guanabara Bay was mistaken for a river mouth. Portuguese navigators entered its 380-square-kilometre basin on 1 January 1502 and named the place Rio de Janeiro — river of January — for a river that does not exist. The error survives as the name of a city of six million. The bay is the geological argument for that city. Granite and gneiss domes rise straight from the water: Pão de Açúcar at 396 metres, the Morro da Urca beside it, Corcovado inland at 710 metres beneath the outstretched arms of Cristo Redentor. The Tupi called the entrance channel a narrow throat, and it is — barely 1.5 kilometres wide between Fort São João in Urca and Fortaleza de Santa Cruz on the Niterói shore. Everything that ever mattered to Rio passed through that gap: sugar, gold from Minas Gerais, coffee, the Portuguese royal court fleeing Napoleon in 1808, and now the sailboats that carry visitors out on a sunset dinner cruise rio de janeiro operators run from the western shore. Marina da Glória anchors that departure point. Built into the landfill of Parque do Flamengo — Roberto Burle Marx's 1965 waterfront design, 1.2 million square metres of reclaimed shoreline — the marina sits below the Outeiro da Glória church of 1739 and within sight of Niemeyer's saucer-shaped Museu de Arte Contemporânea across the water. Its pontoons were rebuilt for the 2016 Olympic sailing regattas. The 13.9-kilometre Rio–Niterói bridge crosses the bay to the north, and Ilha Fiscal's green neo-Gothic customs house, site of the empire's last imperial ball in November 1889, stands mid-channel. The light is the reason the boats go out when they do. Rio faces east, so the bay does not receive the sun so much as reflect its withdrawal: the granite of Sugarloaf holds a copper cast for roughly twenty minutes after the disc drops behind the Tijuca massif, and the Christ statue's floodlights come up while the water is still bright. Sailing conditions cooperate. A thermal breeze builds across the bay through the afternoon and eases at dusk, which is why the yachts on a rio de janeiro sunset sailboat tour cut engines and raise canvas around the same hour each day. The bay has been a working harbour for five centuries and a protected seascape for far less time. Cleanup programmes continue. Guanabara remains, on any clear evening from the deck of a sunset dinner cruise rio de janeiro tour, the most theatrical body of water in the Americas.