What is a Second (s)?
Formal Definition
The second (symbol: s) is the base unit of time in the International System of Units (SI). Since 1967, it has been defined as 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 at rest at absolute zero. This atomic definition replaced the earlier astronomical definitions based on Earth's rotation.
The second is the only SI base unit of time, and all other time units — from the nanosecond to the year — are derived from or defined in terms of it. The second also underpins the definitions of other SI units: the meter is defined via the speed of light (the distance light travels in 1/299,792,458 of a second), and the kilogram is defined via the Planck constant, which involves seconds.
Precision and Stability
Caesium atomic clocks achieve a precision of approximately one part in 10¹³, meaning they would gain or lose no more than one second in about 300,000 years. Optical atomic clocks using strontium or ytterbium atoms achieve even higher precision — one part in 10¹⁸ — and are expected to form the basis of a future redefinition of the second.
Etymology
Latin Origins
The word "second" derives from the Medieval Latin "secunda minuta," meaning "second small part." In the medieval system of time division, the hour was first divided into sixty "minute" parts (from Latin "pars minuta prima," the "first small division"), yielding minutes. Each minute was then divided into sixty "second" small parts — the "secunda minuta" — giving us seconds. This sexagesimal (base-60) system was inherited from Babylonian mathematics.
Babylonian Legacy
The Babylonians used a base-60 number system for astronomical calculations as early as 2000 BCE. This system was transmitted through Greek astronomy (notably Ptolemy's Almagest, c. 150 CE) into medieval Islamic and European science. The division of the hour into 60 minutes and 3,600 seconds is a direct legacy of Babylonian mathematics that has survived over four millennia.
History
Astronomical Origins
For most of recorded history, the second was defined as a fraction of the day. The Egyptian division of daytime and nighttime into 12 hours each (c. 1500 BCE) was refined by Greek and Islamic astronomers. By the 17th century, mechanical clocks with second hands had made the second a practical unit. The second was formally defined as 1/86,400 of a mean solar day (24 hours × 60 minutes × 60 seconds).
Ephemeris Time
By the early 20th century, astronomers recognized that Earth's rotation is not perfectly uniform — it is gradually slowing due to tidal friction with the Moon. In 1956, the second was redefined as 1/31,556,925.9747 of the tropical year 1900, a definition based on Earth's orbital motion rather than its rotation. This "ephemeris second" was adopted by the CGPM in 1960.
The Atomic Definition
In 1967, the 13th General Conference on Weights and Measures redefined the second using the caesium-133 atom's hyperfine transition frequency. Louis Essen and Jack Parry at the UK's National Physical Laboratory had built the first accurate caesium atomic clock in 1955, demonstrating that atomic transitions were far more stable than any astronomical reference. The atomic second has remained unchanged since 1967 and is the longest-standing definition among the current SI base units.
Future Redefinition
Optical atomic clocks, which operate at frequencies hundreds of thousands of times higher than microwave caesium clocks, now achieve precisions 100 to 1,000 times better. The international metrology community is working toward a redefinition of the second based on an optical transition, possibly in strontium-87 or ytterbium-171, expected around 2030.
Current Use
Universal Timekeeping
The second is the fundamental unit of timekeeping worldwide. Coordinated Universal Time (UTC), the basis for civil time in all countries, is maintained by a network of over 400 atomic clocks in about 80 laboratories worldwide. UTC seconds are SI seconds, with leap seconds occasionally inserted to keep UTC within 0.9 seconds of Earth's rotation (UT1).
Technology and Computing
In computing, the second defines processor clock speeds, network latencies, and data transfer rates. A modern CPU operates at billions of cycles per second (GHz). Internet latency is measured in milliseconds. Financial trading systems measure execution times in microseconds or nanoseconds.
Science and Engineering
The second is fundamental to all branches of science. In physics, the speed of light is defined as exactly 299,792,458 meters per second. Acceleration due to gravity is approximately 9.81 m/s². Chemical reaction rates are expressed per second. In medicine, heart rate is measured in beats per minute (seconds being the underlying unit).
Everyday Use
Daily Timekeeping
People use seconds constantly, often unconsciously. Traffic lights count down in seconds. Microwave ovens are set in seconds. Workout intervals are timed in seconds. Egg timers count minutes and seconds. The "five-second rule" for dropped food is a popular (if scientifically dubious) cultural reference.
Sports and Competition
Seconds determine athletic results. The 100-meter sprint world record (9.58 seconds by Usain Bolt) is timed to the hundredth of a second. Swimming records are measured to the hundredth. Formula 1 qualifying gaps are often thousandths of a second. Photo finishes can resolve differences of 0.001 seconds.
Music and Rhythm
Musical tempo is defined in beats per minute (BPM), with each beat lasting a fraction of a second. At 120 BPM, each beat lasts 0.5 seconds. Music production software measures timing in seconds and milliseconds. The human perception of rhythm can detect timing differences as small as 10-20 milliseconds.
In Science & Industry
Fundamental Physics
The second is integral to the definitions of fundamental physical constants and SI units. The speed of light: 299,792,458 m/s. Gravitational constant: 6.674 × 10⁻¹¹ m³·kg⁻¹·s⁻². Planck's constant: 6.626 × 10⁻³⁴ kg·m²·s⁻¹. Every mechanical, electromagnetic, and thermal quantity in physics involves the second.
Atomic Physics and Metrology
The caesium-133 transition frequency (9,192,631,770 Hz) is the most precisely measured quantity in physics. Optical clocks now measure frequencies at 10¹⁴ to 10¹⁵ Hz with fractional uncertainties below 10⁻¹⁸, enabling tests of general relativity, searches for dark matter, and monitoring of fundamental constant stability.
GPS and Navigation
The Global Positioning System relies on precise time measurement. GPS satellites carry atomic clocks that synchronize to within billionths of a second. A timing error of just one nanosecond causes a position error of about 30 centimeters. GPS time is maintained by the US Naval Observatory and currently differs from UTC by 18 seconds (as of 2024).
Interesting Facts
A caesium atomic clock loses or gains no more than one second in approximately 300,000 years. Optical clocks are even better — they would not lose a second in the entire age of the universe (13.8 billion years).
Light travels approximately 299,792 km in one second — enough to circle the Earth about 7.5 times. This fact underpins the SI definition of the meter.
The human blink takes about 0.1-0.4 seconds. In that time, a modern computer processor can execute billions of operations.
Earth's rotation is slowing by about 2.3 milliseconds per century due to tidal interactions with the Moon. In 600 million years, a day will last about 28 hours.
The shortest directly measured time interval is approximately 247 zeptoseconds (247 × 10⁻²¹ seconds), measured in 2020 by tracking the time it takes for a photon to cross a hydrogen molecule.
Usain Bolt's 100m world record of 9.58 seconds means he averaged 37.58 km/h, reaching a peak speed of about 44.7 km/h. The difference between gold and silver was 0.11 seconds.
The leap second system, introduced in 1972, has added 27 seconds to UTC as of 2024. The decision to abolish leap seconds was made in 2022, with the change taking effect by 2035.