Current local time in London

Annual daylight in London

Did You Know?

For 104 Years, One Family Sold London the Time

To drive a black cab in London you must pass an exam called The Knowledge, and it has been running since 1865.

To pass it you must memorise every street within a six-mile radius of Charing Cross. That is roughly 25,000 streets. You must also learn the locations of some 20,000 landmarks — museums, embassies, theatres, clubs, police stations — and 320 set routes, canonised by Transport for London in a document known as the Blue Book.

Candidates ride around the city on mopeds with a clipboard strapped to the handlebars, up to fifteen hours a day, saying street names out loud as they go. It takes most of them three to four years. The fastest anyone has done it is eighteen months. Around half never finish.

The exams are called appearances. An examiner names two points at random and the candidate recites the shortest route between them. The only map permitted is the one in their head.

And it does change the head. In a study at University College London, eighty trainees were brain-scanned before they started, alongside a matched control group. There was no difference to begin with. Four years later, the trainees who qualified showed a measurable increase in grey matter in the posterior hippocampus — the region associated with spatial memory. Those who trained and failed showed nothing. Neither did the controls. When cabbies retire, it shrinks back.

The clock: which is the second time London has solved a problem by putting the answer inside a person and having them walk around with it.

In 1836 the Astronomer Royal at Greenwich had a problem: clockmakers kept knocking on the Observatory door to ask what time it was. So he set up a subscription service and put an assistant, John Henry Belville, in charge of it.

Belville was given a pocket chronometer made by John Arnold in the 1790s. Everyone called it Arnold. Once a week he checked Arnold against the Observatory's master clock — accurate to one tenth of a second — and then walked it around London to roughly 200 subscribers, who set their own clocks by looking at it.

He died in 1856. His widow Maria was refused a pension, so she lobbied for the job instead, and delivered the time around London herself until she retired in 1892, aged 81. Her daughter Ruth then took over, aged 38, and carried Arnold through the London Docks, Clerkenwell, the West End and Baker Street for another forty-eight years.

Ruth retired in 1940, aged 86. The business had run for 104 years on one pocket watch.

The best part: a rival firm, the Standard Time Company, distributed time by electric telegraph and ran a campaign in lecture halls and newspapers to discredit the woman who carried the time in her handbag. They did not put her out of business. Arnold is now in the Science Museum.

In 1884, twenty-two of the twenty-five countries at a conference in Washington voted for longitude zero to run through the Airy Transit Circle at Greenwich. Every time zone on Earth — including the one you are reading this in — is measured from that line.

Which is funny, because the line is in the wrong place.

The famous brass strip that tourists straddle for photographs sits about 102 metres west of where GPS says zero is. This is not an error anyone made. Airy levelled his telescope in 1850 using a basin of mercury, which aligns to local gravity — and local gravity at Greenwich does not point exactly at the Earth's axis. Both lines are correct. They just answer different questions.

The most photographed line in the world is 102 metres off, and the entire planet sets its watch by it anyway.