# Timing

Digital display of the time, according to the cesium atomic clock "CS 4" of the Physikalisch-Technische Bundesanstalt, in the old town hall in Braunschweig

Time measurement (time determination) is the specification of measured quantities of time in clear reference units ( units of measurement ). It often requires the increasingly sharp definition of a time system and includes

A time measurement is always a comparison of reference points, as there is no direct way to measure time based on external influences, such as the current strength or the wind speed.

A distinction is made between digital time measurement (with digital display) and analog time measurement , in which the time is displayed by pointers on a dial.

## Earth rotation and stars

A natural timer is the earth's rotation with its regular alternation between day and night. Since the Babylonians , the day has been divided into 24 hours, which - like the 7-day week - has since prevailed on all continents. The regulations for years and months, however, differ in different cultures.

The civil day ( solar day ) follows from the rotation of the earth relative to the sun , from which the 365.24 days of the year result. In reality, the earth rotates 366.24 times a year. Their angle related to the stars is called sidereal time .

The sidereal time at the location is best determined by measuring star passages in the north-south meridian plane - for example with a meridian circle or a passage instrument . This is also possible approximately with a rotatable star map . The difference between the sidereal time of two places corresponds to their geographical difference in length, whereby the geographical longitude is defined as the difference between local sidereal time and that at the prime meridian . The Greenwich Meridian (more precisely: the central meridian of the Royal Greenwich Observatory ) has been the prime meridian since the 18th century .

To get from sidereal time to solar time (or vice versa), you need the date and the longitude . But there are still small corrections to be made because of the irregularity of the earth's rotation ( dUT1 ). For several decades they have been monitored by a global network of atomic clocks and measuring stations.

## Technical time measurement

In more general terms, time measurement includes the methodology and the measuring devices that are used to determine the absolute time and the duration of a specific process. Timepieces are commonly referred to as watches . The simplest timepieces, such as those used thousands of years ago, are sundials , for example . Precise clocks are called chronometers .

In physics and technology , it is often "only" measurement of time - differences - such as measuring short distances , or in the laser measurement to the moon. Such short time differences are determined with so-called interval counters.

In companies, on the other hand, in addition to technical aspects, time measurement can also be used to determine working hours (see time clock ), but also to improve work preparation (see time organizer ).

### history

The first detectable clock was - apart from sundials - a water clock or Klepsydra , as it was around 1380 BC. Was used in Egypt . It was later used by the Greeks and Romans to keep time in judgment. The first mechanical clock is one around 1250 at the court of Ludwig IX. Device developed in Paris . However, it is likely that oscillation processes like the pendulum were used to measure time thousands of years ago. From the 14th century the hourglass was used as a simple, non-mechanical time measuring device alongside the mechanical wheel clock .

For scientific purposes and short periods of time, researchers of the 16th ( Gerolamo Cardano ) and 17th centuries ( Libertus Fromundus , Galileo Galilei , Johannes Kepler ) used their own pulse for (estimating) time rather than the still imprecise wheel or hourglasses.

More precise measurements began with Jost Bürgi's clock from 1580 for the Kassel observatory, which had a second hand for the first time - and above all with the development of precise pendulum clocks . The escapement ( balance with spiral spring) patented by Christian Huygens in 1657 improved the daily clock rate to around 10 seconds. The Nuremberg egg was small and useful for everyday use, but about 100 times less accurate. The first portable ship's clock was developed by John Harrison around 1720 ; Thanks to temperature compensation, they achieved one second per day, which enabled English navigators to stop the celestial heights precisely for the first time and determine the geographic length of the ship's position.

At observatories around 1800 the exact grandfather clocks achieved daily accuracy of a few tenths of a second through evacuated pendulum boxes . With the eye-to-ear method , the required star locations could be determined with an accuracy of arc seconds and precise time systems could be established. Transportable watches of this precision did not appear until 50 years later.

In 1821 Nicolas Rieussec developed the first watch with a practical stop function, which, however, was still the size of a shoebox . As with Jost Bürgi, the hobby of a nobleman gave the impetus: the horse races of King Ludwig XVIII. An ink pen was linked to the second hand, from which the word “chronograph” arose: from ancient Greek chronos (time) and graphein (to write). The Austrian Joseph Thaddäus Winnerl took the step towards a pocket watch accurate to the second in 1831 in the form of the “Chronoskop” stopwatch . It did not have a time display like today's “chronographs”, but in addition to the stop hand, it even had a drag pointer for measuring lap times.

Around 1880 the Riefler pendulum improved the time systems of the observatories even further in the range of a few 0.01 seconds and in 1921 the Shortt clock in milliseconds ; at the same time, the worldwide synchronization of precision clocks was made possible by radio technology ( time signal transmitter ). Later, television signals and the on-board clocks of navigation satellites were also used for precise time comparisons . In the 1970s, temperature-stabilized quartz watches were already reaching the microsecond , and today's atomic clocks have an accuracy of 10 −15 , which corresponds to 1 second in 30 million years.

### Horology

Horology (Greek ώρα, "hour, time"; and λόγος, logos, "word, speech, sense, teaching"), also the science of time, is the study of the measurement of time.

## Time differences and time scales

Ultimately, however, almost every time measurement comes down to determining time differences . With a time scale (continuous time, as described above), the current time is usually added up by integrating elementary time steps - for example

and even

## literature

• H. Bock: The clock. Basics and technology of time measurement. 2nd Edition. Leipzig / Berlin 1917.
• Wolfgang Deppert : Time. The foundation of the concept of time, its necessary split and the holistic character of its parts. Steiner, Stuttgart 1989. ISBN 3-515-05219-4 , ISBN 978-3-515-05219-1 .
• Rudi Koch (Ed.): BJ-Lexikon. Clocks and timekeeping. 2nd edition, Leipzig 1989.
• Trude Ehlert (Ed.): Time Concepts, Time Experience, Time Measurement. Paderborn / Vienna / Zurich 1997.
• Markwart Herzog (Ed.): The dispute about the time. Time measurement - calendar reform - counter time - end time. Irrseer dialogues. Interdisciplinary culture and science. Vol. 5. Kohlhammer, Stuttgart 2002, ISBN 3-17-016971-8 .
• Willibald Katzinger (Ed.): Concept of time. Time measurement and understanding of time in an urban context. Contributions to the history of the cities of Central Europe. Vol. 17. Linz 2002, ISBN 3-900387-57-5 .
• Marit Rullmann , Werner Schlegel: Time - Eternal Cycle or Frenzied Standstill. Think differently in women. Frankfurt / M. 2000, ISBN 3-518-39654-4 .
• Hermann Brinkmann: The watchmaking school, a series of specialist books for vocational training . Wilhelm Knapp Verlag, Düsseldorf 2005.