The English name comes from the Greek: Chrysoceras, the Golden Horn. The horn part makes geographic sense—the bay curves sharply toward its northwestern end, like the tip of a horn. The golden part is more elusive. One explanation points to the light: on clear evenings, the Horn's surface catches the sunset in a way that can make the water appear briefly, astonishingly, to glow. Others suggest it was simply poetic, a name attached to a harbor of such strategic and commercial importance that it deserved a superlative.

The Ottomans named it Haliç—a word that means estuary in modern Turkish, though its Arabic root simply means gulf. Whatever the name, the geography defined Istanbul's existence. The Horn is the estuary of the Alibeyköy and Kağıthane Rivers, which European travelers once called the Sweet Waters of Europe—a term that referred not only to the pleasant rivers but to the entire atmosphere of the Ottoman pleasure grounds that lined their banks in the eighteenth century. Istanbul almost certainly would not have grown into a great city without this sheltered harbor, its calm water a refuge from the unpredictable Bosphorus currents outside.

In the early eighteenth century, the Ottoman court discovered leisure with an enthusiasm that bordered on the frantic. The Tulip Era—Lale Devri, from 1718 to 1730—saw the banks of the Kağıthane River lined with palaces and mansions surrounded by enormous tulip gardens. The suburb of Sadabad, meaning the happy city, was built for the express purpose of parties. The Sweet Waters appellation was partly a metaphor: the waters were sweet because the life on their banks was sweet, at least for those fortunate enough to participate.

It ended with characteristic Ottoman abruptness. The Janissary-led Patrona Halil Revolt of 1730 burned buildings in Sadabad and brought the Tulip Era to a close. The gardens were eventually lost. The industrial revolution arrived in the following century, and by the mid-twentieth century the Horn—once the primary harbor of one of the world's great empires—had become an open sewer. The factories that had generated the wealth also generated the pollution. The recovery that began in the late 1980s took decades to accomplish.

At the far western end of the Horn, where the estuary bends and the city thins, Eyüp stands apart from the rest of Istanbul. Its mosque complex—built over the tomb of Eyüp Sultan, a companion of the Prophet Muhammad who died during the Arab siege of Constantinople in the seventh century—is one of the holiest sites in Turkish Islam. The surrounding streets retain the texture of Ottoman piety: cemeteries with marble headstones in the distinctive tulip and carnation patterns of different eras, shops selling wooden toys and halka (salty ring-shaped cookies), souvenir stalls catering to families bringing their sons to the mosque before circumcision, dressed in small Ottoman prince outfits.

A chairlift runs from downtown Eyüp up the hill to Pierre Loti, the café named for the French romantic novelist who wrote about Istanbul at the turn of the twentieth century and is said to have frequented the site. From the hilltop, the Horn spreads below in its full curve. Electric gondolas now carry passengers between Eyüp and the opposite bank—quieter and cleaner than the diesel ferries, and perhaps more romantic, as the Wikivoyage source concedes with unusual warmth.

The Golden Horn is harder to navigate than it looks. Its meandering shape confuses compass directions: the south bank is sometimes more like west, and the north bank points east. The reliable landmark rule is this—if you can see the Süleymaniye Mosque, the Phanar Greek College with its red brick domed facade, or the Eyüp Mosque, you are on the southern bank. If the Galata Tower or the old Kasımpaşa Shipyards are on your shore, you are on the northern one.

The M2 metro crosses the Horn on the only above-ground section of that line, with Haliç station sitting in the middle of the channel connected to both shores by pedestrian bridges. The T5 tram runs along the northern approaches. Ferries from Üsküdar and Eminönü zigzag between quays on both sides and reach as far west as Eyüp. The old pontoon Galata Bridge—damaged by fire in 1992, later renovated and towed to its present location near Eyüp—sometimes opens for boat traffic, requiring pedestrians to find an alternate crossing.

The parks that now line the Golden Horn occupy what were factory lots within living memory. Eyüp Sahil, Silahtarağa, and Nilüfer Parks exist because factories were demolished and their soil remediated. The Santral İstanbul complex repurposed what was once Istanbul's main power station into a campus for Bilgi University, with an energy museum in the old turbine hall. The Feshane building—once an Ottoman fez factory, then an industrial space—has been converted for cultural use under the name Artistanbul.

The Epiphany celebration that has returned to these waters is worth watching if you're in Istanbul in January. The Patriarch of the Greek Orthodox Church, whose seat remains in the Phanar neighbourhood on the southern bank, throws a wooden cross into the Horn. Swimmers dive for it. Whoever retrieves it receives a blessing for the coming year. The ceremony is small, dignified, and very old. That it happens on these waters again—after decades of exile to the Bosphorus, forced by the pollution that made the Horn uninhabitable even ceremonially—feels like a kind of redemption.