SI Units – Time

The second (s) is defined by taking the fixed numerical value of the cesium frequency ∆ν_Cs, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s⁻¹.

The number of periods or cycles per second is called frequency. The SI unit for frequency is the hertz (Hz). One hertz is the same as one cycle per second. Standard frequencies and the correct time are broadcast by radio stations WWV and WWVB in Colorado, and WWVH in Hawaii. NIST delivers digital timing signals by telephone and through the internet.

Official U.S. Government time is provided by NIST and USNO. NIST also offers an Internet Time Service (ITS) and an Automated Computer Time Service (ACTS) that allow setting of computer and other clocks through the Internet or over standard commercial telephone lines. Free software for using these services on several types of popular computers can be downloaded there. Information about these services can be found on the Time and Frequency Division Web site.

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Resources for Students and Teachers

• SI Base Units Relationship Poster (SP 1247) - a colorful poster illustrating the relationships of the International System of Units (SI) derived units with special names and symbols and the seven traditional base units. (NIST)
• SI Units Card Deck (SP 1297) - this activity offers a fun way to enhance understanding of the International System of Units, including the defining constants, base units, derived units with special names, and prefixes. (NIST)
• Second: Introduction - Time is probably the most measured quantity on Earth. In science, we use time to measure and better understand countless things in our world. (NIST)
• How Do We Measure Time? (NIST HDYMI Series) – Time intervals, the duration between two events, is most accurately measured with atomic clocks.
• What is time? (NIST, Video)
• Why Do We Use Atoms to Keep Time? (NIST Video)
• How Do Atomic Clocks Work? (NIST)
• How Do We Know What Time It Is? (NIST)
• A Brief History of Atomic Time (NIST)
• Measuring Time and Frequency – Find out more about NIST's history including a brief history of atomic clocks and other time Frequently Asked Questions (FAQs). (NIST)
• How to Rebuild an Atomic Clock (NIST/JILA)
• Take A Walk Through Time – Follow the historic journey through of the evolution of time measurement. Click link to download the attached PDF file. (NIST)
• The Time - Place Connection – If you want to know where you are, you need a reliable clock. Dive into these Smithsonian resources to discover more about the connection between time and place. (Smithsonian)
• It's About Time – Check out this exhibit. Learn cool things about the accuracy of clocks, including a historical Timeline (pun intended)! (University of Wisconsin)
• Top 10 Tips for Teaching the Metric System. (NIST)

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FAQs

What is the current Coordinated Universal Time?

NIST and the U.S. Naval Observatory jointly operate a website that provides the Official U.S. Time. Readings from the clocks of these two agencies contribute to world time, called Coordinated Universal Time (UTC). Learn more... How to get the time using a telephone, computer or radio signals? What is a leap second? What are the daylight saving time rules? Visit the Time and Frequency Division FAQs for more information.

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Resources

Credit: J. Wang and B. Hayes/NIST ©2020 U.S. Secretary of Commerce. All Rights Reserved.

League of SI Superheroes – Professor Second

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. By reading the vibrations of her laser-cooled cesium atoms, Professor Second can synchronize any frequency and correct any clock. A second is the time it takes an excited cesium atom to vibrate 9,192,631,770 times.

Navigate to more SI base unit information

• SI Units
• Fundamental Physical Constants
• Length – meter (m)
• Amount of substance – mole (mole)
• Electric current – ampere (A)
• Temperature – kelvin (K)
• Luminous intensity – candela (cd)
• Mass – kilogram (kg)

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