Timepieces
World Clock USAAll World ClocksAll timepieces
Time Gadgets
Time ConverterEvent TimeAll gadgets
Time Zones
Timezone mapUSA timezonesEurope timezonesAsia timezones
Info
What is GMTAll guides

What is the SI unit of time?

Official definition of a second

In 1967 a new SI definition of a second was created based on the radiation from the caesium-133 atom.

It is correctly defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom."


Rate this page

Thanks for rating!

Let us know what we can do to make this page better:Send
placeFast Find - start by typingGMT Tools - check and bookmarkTime ConverterWhen to Call?USA Phone Area LookupEvent Time AnnouncerCurrency ConverterWhat is GMT?