Second

The second ( shortened from Latin pars minuta secunda, according to the minutes of the decreased for the second time part [ of an hour] ', so the second character " ) is the basic unit of time in the International System of Units (SI).

Traditionally, the second part of the 86.400ste (24 × 60 × 60) of the mean solar day. In 1900 it was defined more precisely over the middle sidereal day ( the true rotational period of the earth from 86164.091 seconds ), or as ephemeris over the duration of the tropical year.

Since 1967, the second of the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133Cs corresponding radiation is defined as the 9,192,631,770 times the period. It is therefore also called the atomic second.

Definition

By definition, the second is on the multiple of the period of a microwave that is with a selected level transition in the cesium atom in resonance. Therefore it is referred to as a second atom. Atomic clocks based on the measurement of the transition. Their accuracy was O.A. since the Definition increased by more than 4 orders of magnitude to 10-15.

Old definitions

Until 1967, the second was based on astronomical measurements:

• Sun seconds (until the 1950s ): the fraction of the mean solar day. This definition was introduced so that an average solar day is 24 × 60 × 60 seconds. This corresponds to the time after which a fictitious mean sun is at the same place again. Note: This is to mean solar 1/365stel longer than the real rotation period of the Earth from 86164.091 seconds. The difference reflects the annual orbit of the earth around the sun, because our time is determined by the position of the sun and not the stars.

From the old to the new definition

Around 1885, noted Karl Friedrich Küstner ( Bonn observatory ) that the rotation axis of the earth performs a polar motion of 5-10 meters. More from this R.Schumann (TU Vienna) and S.Chandler (Harvard ) was analyzed and was the first indication that also changes the rotation period itself. But you could not prove with the then best watches (0.05 seconds per day).

This was only Adolf Scheibe and Udo Adelsberger 1934 at the Physical -Technical Institute. After removal of own concerns, they published the results in 1935 and specified them by the developed of them quartz watch. From about 1950 it became clear that the continuously improved quartz watches a better time than would be normal, the Earth's rotation.

For the astronomical day length slows down not only gradually due to tidal friction, but also shows irregular changes by magma flows between the mantle and core. By slowing down the Earth's rotation, the solar day shifts compared with a completely uniform measure of time. Every 2 to 5 years must be added to compensate for a leap second, to synchronize all clocks with the products of only fractions of a second, but longer and longer solar day (see the Zeitkorrektion DUT1 ).

Derived units

On the second SI prefixes can be used. However, the intentions with positive exponents of the associated magnitude are not common. Instead, those derived from the second, and not for SI units belonging minute, hour and day are used. These are legal units in Germany. Common parts and multiples are:

First clock with a second hand

In 1585, the Swiss mathematician Jost Bürgi designed for the Landgrave Wilhelm IV of Hesse in Kassel a clock with three hands. In addition to the hour and minute hands Bürgi built a clock of a second hand. This clock was the first time the new time unit second are measured. Jost Bürgi regarded as the true inventor of the second.

Trivia

With the introduction of the French Republican Calendar 1792 valid once during the time of the Paris Commune, also the most recent attempt was made to change the time schedule of a day on the decimal system, ie, a tag has been divided into ten hours in one hour in one hundred minutes and a minute one hundred seconds. Thus one days had now 100,000 seconds over 86.4 thousand seconds conventional determination. For the smallest unit of time this decimal time was by imperceptible impact as it was only shorter by about 15 percent. Stronger impact on the next two higher units of time were ( The Dezimalminute was almost one and a half times as long, the Dezimalstunde even almost two and a half times. ).