# Time Scales

Forming a time scale and thus recording quantitatively the progressing time has two basic preconditions. One needs on the one hand an observable recurring phenomenon (oscillator) and on the other hand the counting of this oscillation. Recurring phenomena can be either astronomic observations (e.g. earth rotation, path of the earth around the sun) or the oscillations of clocks.

For a long time only astronomic observations could provide sufficient accuracy for forming time scales. Only the technical progress of the development of frequency standards, which are based on atomic physics processes (atomic clocks), gave a good precondition for a new quality of time scales and could make manifest the irregularity of the earth rotation. By this new quality of time measurement the definition of the unit second could be revolutionized in 1967 on one hand and on the other hand a uniform stable time scale, the TAI (International Atomic Time), could be formed since 1971. Consequently the astronomically calculated UT1 (Universal Time) deviates slightly from the technically realized time scale TAI.

Finally, a new time scale has been formed, which guarantees the stability of an atomic time scale and nevertheless adjusts our time order to the natural conditions, the UTC (Coordinated Universal Time). In irregular intervals leap seconds are added according to the difference of UT1 and TAI, so that the difference between these two amounts to a maximum of 0,9 s. For the time being the difference of UTC and TAI is 37 s. However there is a debate about reforming the present Coordinated Universal Time UTC and abolishing the leap seconds.

The UTC is a time scale purely formed by calculating and is formed monthly by the BIPM (Bureau International des Poids et Mesures) in Sèvres close to Paris on the basis of time comparison measurements of approx. 80 institutes involved with worldwide about 500 atomic clocks. The BEV participates in these comparisons with its atomic clocks and realizes the UTC(BEV) valid for Austria. Once a month the BIPM informs about the deviations of the times of the individual institutes UTC(k) to the UTC, whereas k is the abbreviation of the individual institute. By corrections the deviations of the time scale UTC(BEV) to the Coordinated Universal Time UTC are always kept smaller than 100 ns.

The UTC is the basis of each national time, which depends on the organization of time zones. A time zone is an area of equal time, which is aligned to a geographic meridian. The full circumference of the earth and a division of the day into 24 h thus lead theoretically to a change of the zone each 15° for an hour. The time at the middle meridian of a time zone should correspond to the local position of the sun. With regard to the zero meridian (Greenwich) the hourly time differences are positive in the direction to the East and negative in the direction to the West. In practice this organization of time zones is not kept strictly, there are also differences of half an hour in some cases. In the mean time there are seasonal deviations in many countries (summer time – daylight saving time).

In Austria the Central European Time (CET) and the Central European Summer Time (CEST), respectively, apply as standard time. The counting of hours of the CET is one hour ahead of the UTC, that one of the CEST is two hours ahead. This is regulated by the Law on Legal Time Counting. The time of the beginning and end of the Summer Time is regulated by a directive of the European Union and takes place at the last weekend of March and October, respectively, in the night from Saturday to Sunday.

Further Reading:

*The Measurement of Time*, Claude Audoin, Bernard Guinot, Cambridge University Press, 2001