# SI definition of second: what day/year is the reference year

The SI definition of a second was originally based on 1/86 400 of a day - but the earth's rotation is a) unsteady and b) changing so it was changed to a defined number of cycles of an atomic clock. Was there a specific day for which the atomic clock was calibrated? Put another way, why were 9 192 631 770 cycles of a Caesium atomic clock chosen as the number to define a second?

scottdave
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jbriggs444
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The SI definition of a second was originally based on 1/86 400 of a day - but the earth's rotation is a) unsteady and b) changing so it was changed to a defined number of cycles of an atomic clock. Was there a specific day for which the atomic clock was calibrated? Put another way, why were 9 192 631 770 cycles of a Caesium atomic clock chosen as the number to define a second?
My recollection is that the second was first redefined in terms of the tropical year and then redefined in terms of an atomic clock.
https://en.wikipedia.org/wiki/Second#History_of_definition

Thanks scottdave and jbriggs - I think I found it. The day was the beginning of 1900 (technically Dec 31, 1899) - although the calculations were made in 1952. https://en.wikipedia.org/wiki/Ephemeris_time.

Now I just want to know what was the rotational velocity of Earth on that day :)

scottdave