Accurate measurement of time is one of the glories of physics, and one of the most exciting frontiers of current research. For most of history, people measured time using the sun and moon. Earth’s rotation gives us the day, its revolution around the sun gives us the year, and its dance with the moon gives us the month. Sundials used the shadow cast by the sun to track the hours, but they had drawbacks; for one thing, they didn’t work on cloudy days.

Source: Wall Street Journal May 22, 2019 18:13 UTC