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However, sundials are useless unless the Sun is shining.

This method evolved into the sundial, or shadow clock, with markers along the shadow’s path dividing the day into segments. One of the earliest known timekeeping methods – dating back thousands of years – involved placing a stick upright in the ground and keeping track of its moving shadow as the day progressed. Timekeeping is simply a matter of counting these oscillations to mark the passage of time.įor much of history, the chosen periodic phenomenon was the apparent motion of the Sun and stars across the sky, caused by the Earth spinning about its own axis. Over the millennia a myriad of devices has been invented for timekeeping, but what they all have in common is that they depend on natural phenomena with regular periods of oscillation. But what did people do before clocks came about? How did they measure time? The reason I know this is easy – I joined on 26 October 1998 and, with the help of clocks and calendars, I can measure the time that’s passed. On 1 November 2018, when this article was first published in the print edition of Physics World, I had been working at the UK’s National Physical Laboratory (NPL) in Teddington for exactly 20 years and six days. Helen Margolis looks at how we reached our current definition of the second, and where clock technology is going next