The current and formal definition in the International System of Units (SI) is more precise:The second [...] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

[1]This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks.

"Minute" comes from the Latin pars minuta prima, meaning "first small part" i.e. first division of the hour - dividing into sixty, and "second" comes from the pars minuta secunda, "second small part", dividing again into sixty.

An everyday experience with small fractions of a second is a 1-gigahertz microprocessor that has a cycle time of 1 nanosecond.

Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today.

Starting in the 1950s, atomic clocks became better timekeepers than Earth's rotation, and they continue to set the standard today.

The effect is due chiefly to the obliqueness of Earth's axis with respect to its orbit around the Sun.

It rarely makes sense to express longer periods of time like hours or days in seconds, because they are awkwardly large numbers.

Moreover, most other SI base units are defined by their relationship to the second: the meter is defined by setting the speed of light (in vacuum) to be 299 792 458 m/s, exactly; definitions of the SI base units kilogram, ampere, kelvin, and candela also depend on the second.

A strontium clock with frequency 430 THz, in the red range of visible light, during the 2010s held the accuracy record: it gains or loses less than a second in 15 billion years, which is longer than the estimated age of the universe.

[citation needed] Sundials and water clocks were among the earliest timekeeping devices, and units of time were measured in degrees of arc.

There are references to "second" as part of a lunar month in the writings of natural philosophers of the Middle Ages, which were mathematical subdivisions that could not be measured mechanically.

By the 1730s, 80 years later, John Harrison's maritime chronometers could keep time accurate to within one second in 100 days.

MKS was adopted internationally during the 1940s, defining the second as 1⁄86,400 of a mean solar day.

[13] This resulted in adoption of an ephemeris time scale expressed in units of the sidereal year at that epoch by the IAU in 1952.

[14] This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion.

The second was thus defined as "the fraction 1⁄31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time".

[15] Even the best mechanical, electric motorized and quartz crystal-based clocks develop discrepancies from environmental conditions; far better for timekeeping is the natural and exact "vibration" in an energized atom.

[16] Since 1967, the second has been defined as exactly "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom".

The current generation of atomic clocks is accurate to within one second in a few hundred million years.

the footnote was added at the 86th (1997) meeting of the CIPM GCPM 1998 7th Edition SI Brochure A future re-definition of the second would be justified if these idealized conditions can be achieved much easier than with the current definition.

In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory.

The second, so defined, is the unit of proper time in the sense of the general theory of relativity.

The CIPM has adopted various secondary representations of the second, based on a selected number of spectral lines of atoms, ions or molecules.

SI Brochure 9 In 2022, the best realisation of the second is done with caesium primary standard clocks such as IT-CsF2, NIST-F2, NPL-CsF2, PTB-CSF2, SU–CsFO2 or SYRTE-FO2.

These clocks work by laser-cooling a cloud of Cs atoms to a microkelvin in a magneto-optic trap.

The Rydberg constant describes the energy levels in a hydrogen atom with the nonrelativistic approximation

Another hurdle involves improving the uncertainty in QED calculations, specifically the Lamb shift in the 1s-2s transition of the hydrogen atom.

A consistent method of sending signals must be developed before the second is redefined, such as fiber-optics.

Instead, certain non-SI units are permitted for use with SI: minutes, hours, days, and in astronomy Julian years.