Second

Clock-pendulum

== Second ==

A second is the base unit of time in the International System of Units (SI). It is defined by taking the fixed numerical value of the caesium frequency, Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s⁻¹.

History

The concept of the second has evolved over time. Historically, the second was defined as 1/86,400 of a mean solar day. This definition was based on the Earth's rotation, which is not perfectly uniform. In 1967, the second was redefined in terms of the radiation emitted by a caesium-133 atom, making it much more precise.

Measurement

The second is measured using highly accurate atomic clocks. These clocks use the vibrations of atoms, such as caesium or rubidium, to keep time with extreme precision. Atomic clocks are essential for various applications, including GPS, telecommunications, and scientific research.

Applications

The precise measurement of the second is crucial in many fields:

• In astronomy, accurate timekeeping is essential for tracking celestial objects.
• In navigation, systems like GPS rely on precise time measurements to determine locations.
• In telecommunications, synchronization of data transfer relies on accurate timing.

Subdivisions and Multiples

The second can be subdivided into smaller units such as the millisecond (1/1,000 of a second), microsecond (1/1,000,000 of a second), and nanosecond (1/1,000,000,000 of a second). Larger units of time, such as the minute (60 seconds) and the hour (3,600 seconds), are also based on the second.

Related Concepts

• Minute
• Hour
• Day
• Time
• Atomic clock
• International System of Units

See Also

• Time
• Atomic clock
• International System of Units
• Global Positioning System