Remember to put your clock forward by 1 second at midnight tonight. It's leap-second day. Time measurement in Physics was traditionally based on the movement of astronomical bodies - that's a very human thing. Unfortunately, the cycles don't quite match up. A day is one rotation of the Earth about its axis. A year is the journey round the Sun. There isn't an integer number of days in a year; hence the leap-year. But the rate of rotation of the Earth has been slowing down for billions of years. This is due to the effects of tides. There never was a problem because clocks were set by 12 noon being the time at which the Sun was highest in the sky. But the need for ultra-precise timing meant that atomic clocks were introduced. These define a second as a particular large number of oscillations of light from a particular energy level jump in caesium-133. An energy level jump means that there is only one very precise frequency involved. Most timing devices now run from atomic clocks. But human "Sun-time" falls out of step because the rotation of the Earth is gradually slowing. They first put in a leap-second in 1972 and I think they said this was the 27th leap-second added. In other words, atomic clocks would have had midday out by nearly half a minute now if uncorrected.
