Second
The Babylonian astronomers of the third millennium BC divided their calendar days into 24 hours. They further split each hour into 60 minutes and each minute into 60 seconds. This created a total of 86,400 seconds per day using a sexagesimal system. Sundials and water clocks served as early timekeeping devices during this era. Units of time were measured in degrees of arc rather than mechanical counts. Conceptual units smaller than what sundials could measure existed but remained mathematical subdivisions. Natural philosophers of the Middle Ages referenced seconds within lunar months in their writings. These divisions lacked mechanical measurement capabilities until centuries later.
Christiaan Huygens invented the first pendulum clock in 1656 with a swing lasting one second. The device featured a pendulum length just under one meter to achieve this timing. An escapement mechanism allowed the clock to tick every single second accurately. John Harrison developed maritime chronometers by the 1730s accurate to within one second over 100 days. Unsigned clocks depicting Orpheus from the Fremersdorf collection dated between 1560 and 1579 marked seconds on their faces. Taqi al-Din constructed a clock in the third quarter of the 16th century with marks every half-minute. Tycho Brahe redesigned observatory clocks in 1581 to display seconds despite their lack of accuracy. Jost Bürgi built a clock for William of Hesse in 1579 that explicitly marked seconds. Mechanical clocks kept mean time while sundials displayed apparent solar time varying by up to 15 minutes throughout the year.
The International System of Units adopted the caesium-133 atomic frequency standard in 1967. This definition fixed the value at exactly 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels. Atomic clocks now set the global time standard using this specific microwave frequency. The duration selected matched the length of the ephemeris second previously defined. Metrologists recognized Earth's orbit around the Sun as more stable than its rotation. Quartz crystal oscillator clocks operating near 100 kHz achieved better accuracy than mechanical devices by the late 1940s. Proposals emerged as early as 1950 to define the second based on a fraction of a year. The IAU chose the tropical year over the sidereal year in 1955 for this purpose. A redefinition planned after 2022 aims to improve upon current standards with optical lattice technology.
General relativity dictates that gravitational fields affect the rate at which time passes. A clock gains or loses less than one second over 15 billion years when measuring elevation changes of just 2 centimeters. Strontium clocks operate within the red range of visible light at 430 THz during the 2010s. These devices measure gravitational time dilation through changes in their ticking rates. Signals from primary clocks in different locations require correction for relativistic caesium frequency shifts. The definition applies to atoms free of any perturbation and at rest in an environment of 0 K. Laboratories must account for velocity relative to the reference frame to obtain proper seconds. Special relativistic corrections apply within small spatial domains where gravity remains uniform enough to neglect non-uniformity effects.
Strontium clocks achieved accuracy records by gaining or losing less than one second in 15 billion years. This duration exceeds the estimated age of the universe itself. Optical lattice clocks utilize frequencies around 1 Hz natural linewidths with Q-factors reaching 10^15 or higher. They employ single ions or optical lattices containing up to 100,000 atoms simultaneously. Current generation atomic clocks based on atoms other than caesium-133 improved precision by a factor of 100 since 1967. Laser-cooled clouds of caesium atoms reach temperatures of one microkelvin inside magneto-optic traps. These cold atoms undergo Ramsey excitation within microwave cavities before detection via laser beams. Systematic uncertainty equals 50 picoseconds per day for systems like IT-CsF2 and NIST-F2. Fiber-optics provide consistent methods for sending signals between these advanced timekeeping devices.
International Atomic Time relies on a consensus vote among atomic clocks distributed worldwide. Coordinated Universal Time inserts leap seconds as necessary to correct Earth's rotational variations. UT1 defines universal time through Earth's rotation relative to the Sun without any leap seconds. Civil time must agree with Earth's rotation despite the irregularity of its speed. The difference between apparent solar time and mean time reaches up to 15 minutes cumulatively over parts of the year. A stone falls about 4.9 meters from rest in exactly one second according to standard gravity. Sound travels approximately 343 meters in air during that same single second interval. Light takes 1.3 seconds to travel from the Moon surface to Earth covering 384,400 kilometers. Digital watches display two-digit counters while analog faces show sixty tick marks representing each passing second.
Common questions
What is the definition of the second according to the International System of Units?
The International System of Units adopted the caesium-133 atomic frequency standard in 1967. This definition fixed the value at exactly 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels.
When did Christiaan Huygens invent the first pendulum clock for measuring seconds?
Christiaan Huygens invented the first pendulum clock in 1656 with a swing lasting one second. The device featured a pendulum length just under one meter to achieve this timing and an escapement mechanism that allowed the clock to tick every single second accurately.
How accurate are modern strontium clocks compared to the age of the universe?
Strontium clocks achieved accuracy records by gaining or losing less than one second in 15 billion years. This duration exceeds the estimated age of the universe itself.
Why was the tropical year chosen over the sidereal year for defining the second in 1955?
Metrologists recognized Earth's orbit around the Sun as more stable than its rotation. The IAU chose the tropical year over the sidereal year in 1955 for this purpose.
What physical phenomena occur within one second according to standard gravity and light speed?
A stone falls about 4.9 meters from rest in exactly one second according to standard gravity. Light takes 1.3 seconds to travel from the Moon surface to Earth covering 384,400 kilometers.