# time

Time is a physical quantity for events

, movement, growth, transience and change. The

unit for time is the second

(s), for larger periods of time the terms minutes, hours, days, weeks, months and years are used.The definition of time comes from astronomy and refers to the rotation of the earth - a day - and the orbit of the earth around the sun - a year. This time is referred to as astronomical time.More technically precise is the determination of time using highly constant atomic frequency standards, where

theradiation

fromcaesium atoms is used as a frequency reference. Since the references for the measurement units are determined by the National Measurement Institutes (NMI), in Germany this is the Physikalisch-Technische Bundesanstalt (PTB), the international atomic time (TAI) is determined from the time references of 200 atomic frequency standards from 50 countries. The central calculation is carried out by the French institute Bureau International des Poids et Measures

(BIPM). Theconstancy of these caesium standards

is about 10exp-14, which corresponds to about one second deviation in one million years.Since astronomical time falls behind TAI time by about 1 second per year, the BIPM adds a leap second

toTAI time on the last day of June and/or the last day of December

. Inaddition to these two time references, the Global Positioning System

(GPS) uses a time reference that deviates from these time references. Since the time changeover of the navigation satellites is very problematic, the GPS system only took into account the leap seconds that had accrued up to 1980. After that, no further time corrections were taken into account. TheGPS time is therefore 13 seconds behind the TAI time.

The International Telecommunication Union (ITU) is working on a time standard in working group 7A.