Musée D'Orsay.

Walk in and the ceiling lifts away. Victor Laloux's 1900 train shed runs 138 metres long and 32 metres high — a coffered barrel-vault the length of a city block, washed in cool zenithal light through the glazed roof. Footsteps and voices rise and bounce off the stone floor; the hush sits closer to a cathedral than a museum.

There are three clocks, not one. The famous transparent dial sits on the fifth floor, northeast façade — the Roman numerals frame Sacré-Cœur on the Montmartre skyline, and in late afternoon the sun throws those numerals across the parquet in a slow-moving stencil. Press close and the brass mechanism is visible, ticking behind the glass.

The second clock hides inside Café Campana, where you can sit with a café crème and look at it without queueing. The third — gilded, Belle Époque, attributed to Laloux himself — hangs above the main entrance, and almost everyone walks under it without looking up.

Take the escalator straight to the top. The fifth floor holds the reason most people come — Monet, Van Gogh, Renoir, Cézanne, Degas, Pissarro, Sisley, Gauguin, all hung in rooms lit by the same northern daylight the painters chased outdoors.

Curators rotate works daily, so even Van Gogh's self-portrait migrates. Grab the free brochure at the entrance; it's reprinted each morning. Linger over Gauguin's Maison du Jouir doorframe — carved wood from his Marquesas hut, chisel marks still raw — then step out onto the roof terrace for the Right Bank panorama: Seine, Tuileries, the Louvre across the water, Sacré-Cœur on the hill.

Go Thursday evening if you can. The museum stays open until 21:45, the nave glows under artificial light, and the crowds thin to almost nothing.

Most visitors miss the Salle des Fêtes on the second floor — the old Hôtel d'Orsay ballroom, gilded and mirrored and chandeliered, the most Versailles-feeling room in the building. Nearby, the Art Nouveau galleries hold Guimard, Mucha, Lalique and Gaudí furniture in sinuous wood and hammered copper, usually empty while crowds pile into the Impressionists upstairs.

Down in the sculpture nave, find Carpeaux's Four Parts of the World Holding the Celestial Sphere. Look at the figure of America: she stands on a broken chain, carved in 1872 to mark the 1865 abolition of slavery in the United States. Pradier's Sappho sits dead-centre, head bowed over a set-down lyre — said to depict the moment before her suicide. People rush past her every minute.

Look up between the paintings. The original cast-iron station signage is still bolted to the upper walls, and in service corners the plaster falls away to show Laloux's metal skeleton — rivets and trusses, the 1900 engineering trick laid bare under its Beaux-Arts stone skin.