Until the mid-19th century, almost every town kept its own local time, defined by the sun. But telling the time by the sun creates a problem: not all days are the same length. Around 22nd December, noon to noon takes 30 seconds more than 24 hours, and around 20th September, noon to noon is 21 seconds less.

When railway networks expanded in the 1850s, the need for a national time standard became inevitable. So, the people agreed to use Greenwich Mean Time: the yearly average or ‘mean’ of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory of Greenwich. While around 98% of the clocks in Britain were set to Greenwich mean time by 1855, it wasn’t made the law until 1880.