Last week we learnt that the position of the telescope, called the Airy Transit Circle, gave us the position of the Greenwich Meridian. It was built in the 19th century and was first used in 1851.
Interestingly, before that date several other instruments had been used over the years, but they had been housed in a couple of different buildings and were not on the same line. The new 1851 telescope was 43 metres east of the first location. So midday is 0.15 of a second earlier than it used to be!
But no one really noticed because many places still used their own local time and it was only the spread of the railways that resulted in the adoption of Greenwich Mean Time (GMT) throughout Britain in 1880.
However, although after 1880 the British were synchronised, with everyone using the same time, other countries were still using their own systems. The French, for example, were using a meridian passing through Paris, which meant that their clocks were just over 9 minutes ahead of GMT. All very confusing.
Eventually, in 1884, the International Meridian Conference was held in Washington, USA. 25 nations attended and it was agreed to adopt the Greenwich Meridian as the Prime Meridian for the reckoning of time around the whole world. Most countries eventually adopted a standard time that differs from GMT by a whole number of hours.