# second

Physical unit
Unit name second
Unit symbol ${\ displaystyle \ mathrm {s}}$
Physical quantity (s) Time , span of time
Formula symbol ${\ displaystyle t}$
dimension ${\ displaystyle {\ mathsf {T}}}$
system International system of units , CGS system of units , technical measurement system
In SI units Base unit
In CGS units Base unit
Named after Latin pars minuta secunda 'second diminished part'
See also: year , month , day , hour , minute , tertie
Ten seconds shown with a long exposure of a wristwatch

The second is the unit of time in the International System of Units (SI). It is about one heartbeat (adult resting heart rate).

The division of the hour into 60 minutes of 60 seconds each can already be found around the year 1000 in a text by al-Biruni . It has been known as Secunda from Latin pars minuta secunda ('second diminished part') since the 13th century. In 1585, Jost Bürgi constructed a clock with a second hand for the first time.

## definition

One second has been the 9th since 1967 192 631 770 times the period of radiation , which corresponds to the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133 Cs. With the redefinition of the SI in 2019 by the 26th General Conference on Weights and Measures, the exact wording of the definition was changed, but the content of the definition of the second remained.

${\ displaystyle \ Delta \ nu _ {\ mathrm {Cs}} = 9 \, 192 \, 631 \, 770 \; \ mathrm {Hz}}$

it follows

${\ displaystyle 1 \; \ mathrm {s} = {\ frac {9 \, 192 \, 631 \, 770} {\ Delta \ nu _ {\ mathrm {Cs}}}}}$

By definition, the second is on a multiple of the period of a microwave oven , with a selected level transition in the cesium atom in resonance is. Hence it is called the atomic second . Atomic clocks are based on measuring this transition. Its accuracy has been increased by more than four powers of ten to 10 −16 since the definition given above .

## Earlier definitions

As long as one assumed a steady rotation of the earth , the second was the sixtieth part of a minute of the day divided into 24 hours to 60 minutes.

Around 1885, Karl Friedrich Küstner (Bonn observatory) established that the earth's axis of rotation was moving 5 to 10 meters in the polar direction . This was analyzed in more detail by Richard Schumann (TU Wien) and Seth Carlo Chandler (Harvard) and was the first indication that the rotation period itself is also changing. However, it was not yet possible to prove it with the best watches of the time (deviation 0.05 seconds per day).

This was only achieved by Adolf Scheibe and Udo Adelsberger in 1934 at the Physikalisch-Technische Reichsanstalt . After clearing up their own concerns, they published the results in 1935 and refined them using the quartz watch they had developed . From around 1950 it became clear that the continuously improved quartz clocks would be a better time standard than the earth's rotation. The astronomical day length increases not only gradually because of tidal friction , but also shows irregular changes by magma flows between the mantle and the Earth's core . Due to the slowing down of the earth's rotation, the solar day shifts compared to a completely uniform time. For compensation, a leap second must be inserted every two to five years in order to synchronize all clocks with the solar day, which is a few fractions of a second longer (see the time correction dUT1 ).

Until 1967 the second was based on astronomical measurements:

• Solar second (until the 1950s): The fraction 186 400 of the mean sunny day . This definition was introduced so that an average sunny day would be 24 x 60 x 60 seconds. This corresponds to the time after which a fictitious mean sun is again in the same place. (The solar day is about 4 minutes longer than the earth's rotation period, because the earth moves around the sun during the day and it therefore takes a little longer for a point on the earth to point towards the sun again.)
• Ephemeris second : The fraction 131 556 925.9747 of the tropical year on January 0, 1900 (= December 31, 1899) at 12 noon UT . It refers to the relationship between the duration of the year and the rotation of the earth .

## Units related to the second

The unit second is used with various prefixes for units of measurement ( SI prefixes ), for example:

Prefix notation designation Decimal
1 fs Femtosecond 0.000 000 000 000 001 s
1 ps Picosecond 0.000 000 000 001 s
1 ns Nanosecond 0.000 000 001 s
1 µs Microsecond 0.000 001 s
1 ms Millisecond 0.001 s
1 s second 1 s

Minute , hour and day are common as larger time units and are legal units of measurement in the European Union and Switzerland . Enlarging SI prefixes such as “megasecond” (= 1 000 000 s = 11 days 13 h 46 min 40 s) are permitted, but very unusual.

A second can also be divided into sixty small time units, the tertia . But this is not common today.

## Trivia

With the introduction of the French revolutionary calendar in 1792, an attempt was made to change the division of a day to the decimal system . A day was divided into ten hours, one hour into one hundred minutes and one minute into one hundred seconds. According to this, a day had 100,000 seconds compared to 86,400 seconds for conventional determination. For the smallest time unit, this decimal time had an imperceptible effect, as it was only about 15% shorter. The effects on the next two time units were much stronger. The decimal minute was almost one and a half times as long, the decimal hour almost two and a half times as long.