Commanding a terrific view from the hill in Greenwich Park, with the towers of Canary Wharf spread out in its lap, the Observatory is yet another creation of Christopher Wren (from 1675), and the place from which time zones emanate. Historically the Empire’s most important site for celestial observation, it houses significant relics of star-peeping, but the paid areas aren’t worth the money. Most of the good stuff—marked on the map in red—is free, including a small Astronomy Centre and an exhibition on time. The only things admission buys you are an unremarkable ceilingprojection planetarium and the sparsely furnished, borderline-interesting Flamsteed House by Wren, which has a collection of 18th-century clocks once used to crack the mystery of measuring longitude (an advance that ushered the English Empire to worldwide dominance). Most people plunk down admission not because they care about those but to gain access to the Meridian Courtyard. The Prime Meridian, located at precisely 0[dg] longitude (the equator is 0[dg] latitude), crosses through the grounds, and hordes of coach tourists pay to wait an hour for an Instagram moment of straddling the line with a foot in two hemispheres at once—but the secret is they don’t have to. The line continues down the wall on the walkway north of the courtyard, where the Meridian is free and there’s never a wait. In the old days, the red Time Ball fell precisely at 1pm daily so that the city could synchronize its clocks; it still rises at 12:55pm and drops 5 minutes later. You could set your watch by it, but technically, you already do. If you’re also visiting the Cutty Sark, a combo ticket will save you a few pounds, but overall, the ticket structure is needlessly complicated.