Asked by: Steve Purves, Preston
Time used to be determined using the position of celestial objects. The Sun was due south at midday, and the shadow of a sundial pointed due north. But the Sun isn’t due south at the same moment everywhere. For example, it’s due south in Bristol around 10 minutes after London. This became an issue with the development of railways in the 1830s, when clocks needed to be accurately synchronised over long distances in order to ensure passengers departed on schedule and to avoid train collisions.
The introduction of the electric telegraph in the 1830s ushered in the modern approach to synchronising clocks, in which precise time signals are sent to them at light-speed. Over the years, the railways managed to get towns to adopt a universal ‘railway time’ but some places refused to do so for years. In 1880 the entire UK was finally using the same time everywhere.