In 1972, the first leap second day, one second was added to the world's time in order to keep the super-accurate atomic clocks in step with the Earth's rotation. Since the adoption of this system in 1972, firstly due to the initial choice of the value of the second (1/86400 mean solar day of the year 1900) and secondly to the general slowing down of the Earth's rotation. By 2015, it has been necessary to add 26 seconds to Coordinated Universal Time (UTC).[Image: part of the master clock system at the U.S. Naval Observatory.]