As the clocks 'fall back' in the UK this Sunday and we make the change from BST to GMT, HistoryExtra's Elinor Evans visits the Royal Observatory Greenwich where the work of the first Astronomer Royal, John Flamsteed, laid the foundations for the accurate timekeeping systems that would evolve into Greenwich Mean Time. Discover how astronomers, mathematicians, and global powers collaborated to create GMT in the full video, available now on HistoryExtra's YouTube channel. (Images by Getty/Alamy, ae