The Seine has carried river traffic for two millennia, but its transformation into a stage for illuminated sightseeing began in 1949, when Bateaux-Mouches launched the first purpose-built glass-canopied vessels from Port de la Conférence. Today, over three million passengers per year glide beneath the thirty-seven bridges that link the Left and Right Banks, each crossing lit by sodium-vapor arcs that turn stone into amber after sunset. The river itself bisects Paris along a gentle northwest-southeast axis, tracing a course that predates Haussmann's boulevards by centuries and offering unobstructed sight lines to monuments built deliberately along its quays: the Louvre's western façade, the Musée d'Orsay's Beaux-Arts clock tower, Notre-Dame's flying buttresses silhouetted against the Île de la Cité, and the Eiffel Tower's iron lattice, which sparkles for five minutes at the top of every hour from dusk until 1:00 AM. A boat trip Paris night reveals the capital's architectural chronology in reverse, moving upstream from the Belle Époque grandeur of the Grand Palais past the Renaissance symmetry of the Pont Neuf—completed in 1607 and still the oldest standing bridge—toward the medieval ramparts visible at the Conciergerie's waterline. The city's floodlighting program, inaugurated in 1989 for the bicentennial of the Revolution, now illuminates over three hundred monuments nightly, and the river offers the only vantage from which multiple landmarks occupy a single frame: the Assemblée Nationale's columned portico framed by the golden Pont Alexandre III, the twin towers of Saint-Sulpice rising behind the Latin Quarter's slate rooftops, the glass pyramid of the Louvre catching reflections of the Pont des Arts' pedestrian deck. Evening departures on the Seine concentrate between 20:00 and 22:00, when the transition from twilight to full dark provides graduated visibility—monuments emerge under artificial light while the sky retains enough luminosity to define their silhouettes. The water itself acts as a thermal stabilizer, moderating temperature swings that make autumn and spring evenings ten to fifteen degrees warmer on deck than along the boulevards above. Standard sightseeing cruises last sixty to seventy minutes and follow a looped route: downstream past the Eiffel Tower to the Statue of Liberty replica on the Île aux Cygnes, then upstream to the Île Saint-Louis before returning to the departure quay. Longer dinner formats extend the circuit to ninety or one hundred twenty minutes, adding a second pass beneath the illuminated spans of the Pont de Bir-Hakeim and the Pont de l'Alma, where the Zouave statue has measured flood levels since 1910.