Second

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== 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

Modern atomic clocks are used to measure the second with extreme accuracy. These clocks use the vibrations of caesium atoms to keep time. The precision of these clocks is crucial for various applications, including GPS, telecommunications, and scientific research.

Applications

The second is used in various fields:

• In physics, it is essential for measuring time intervals in experiments.
• In astronomy, it is used to calculate the positions and movements of celestial bodies.
• In engineering, it is crucial for synchronizing systems and processes.
• In medicine, precise time measurement is vital for diagnostics and treatment.

Subdivisions and Multiples

The second can be divided 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 include the minute (60 seconds) and the hour (3,600 seconds).

Related Concepts

• Minute
• Hour
• Day
• Time
• Frequency
• Atomic clock

See Also

• International System of Units
• Timekeeping
• Chronometry
• Standard time