Astronomers were historically the custodians of time, but the discovery that the period of Earth's rotation is not constant meant that they needed to reassess their basic units. On a separate path, physicists developed atomic clocks and held out the prospect of a timekeeping system rooted in physical constants.

These two stories came together in 1955 when the unit of astronomical time, the ephemeris second, was calibrated in atomic terms. For some years the debate resumed about whether timekeeping was the responsibility of astronomy or physics. In the UK, both the National Physical Laboratory and the Royal Greenwich Observatory regarded themselves as official timekeepers.

From 1972, the world formally adopted atomic time, but it now looks as though astronomy is making a comeback and millisecond pulsars are strong candidates to make better clocks...