Greenwich (Royal Observatory & Prime Meridian)

Every timezone on Earth is measured from a line in the courtyard of this small hilltop observatory. The Prime Meridian — zero degrees longitude — runs through Greenwich because in 1884 an international conference in Washington DC decided that this was the centre of the world, or at least the centre of timekeeping. The line was actually established in 1851 by George Airy, the seventh Astronomer Royal, using his Transit Circle telescope to define the precise north-south line. Around 600,000 observations were made through this instrument over the following century.

Charles II founded the Royal Observatory in 1675 with a single, urgent purpose: to solve the problem of longitude at sea. Ships kept crashing because nobody could reliably calculate east-west position. John Flamsteed, the first Astronomer Royal, set to work mapping the stars from this hill, and the quest eventually led to John Harrison's revolutionary marine chronometers, which solved the longitude problem and saved countless lives.

There's a wonderful irony buried in the courtyard: the actual Prime Meridian, as calculated by modern GPS, runs 102.5 metres to the east of the brass line that tourists queue to straddle. The shift happened in 1984 when satellite-based measurements replaced the old astronomical observations. Nobody moved the line. The tourists are technically standing in the wrong place, but the photo opportunities are too good to let accuracy get in the way.

Greenwich is a UNESCO World Heritage Site, and the view from the observatory hill is one of the finest in London — the Queen's House, the Old Royal Naval College, the Thames, and the glass towers of Canary Wharf aligned like a timeline of British ambition from maritime empire to financial capital.