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.

Resources for Students and Teachers
Grade 3-5	Grade 6-8	Grade 9-12
Credit: ©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.  SI Units Card Deck (SP 1297) - this activity offers a fun way to enhance understanding of the International System of Units. (NIST) Top 10 Tips for Teaching the Metric System. (NIST)	Credit: NIST How Do You Measure Your Location Using GPS? 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) The Time - Place Connection	Credit: N. Hanacek/NIST Atomic Clock - (NIST) A Brief History of Atomic Time (NIST) How Do Atomic Clocks Work? (NIST) How Do We Know What Time It Is? (NIST) How Do We Measure Time? (NIST HDYMI Series) How to Rebuild an Atomic Clock (NIST/JILA) 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) Second: Introduction - (NIST) Take A Walk Through Time - Click link to download the attached PDF file. (NIST)

Videos

League of SI Superheroes, Episode 5: Late Notice

Why Do We Use Atoms To Keep Time?

League of SI Superheroes, Episode 2: Running Out of Time!

What is PNT?

How does NIST-F4, NIST's newest fountain clock, work?

What is time?

Introducing NIST-F4: The Nation's New Primary Frequency Standard

 

Credit: ©2020 U.S. Secretary of Commerce. All Rights Reserved.

Navigate to more SI base unit information

Return to Top of Page