2. Time and frequency

2.1 Astronomical and atomic references

The metrology of time and frequencies intimately blends the notions of time unit, which enables us to measure durations or intervals of time, and time scale, a reference frame that enables us to situate events in time.

Time has long been measured by methods inherited from astronomy, notably, for civil applications, on the basis of the Sun's movement in the sky, which is linked to the Earth's rotation around its axis. An important example of this type of scale is Universal Time (UT), which designates mean solar time at the Greenwich meridian and replaces the older and more ambiguous Greenwich Mean Time (GMT). In this scale, the second is defined as the fraction 1/86,400 of the mean solar day. UT has several slightly different...