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

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The definition of a second was originally based on 1/86,400 of a day, but due to the Earth's unsteady rotation, it was redefined using atomic clock cycles. The specific number of 9,192,631,770 cycles of a Caesium atomic clock was chosen for the definition, with calibration linked to the start of 1900, although calculations were finalized in 1952. The transition to atomic timekeeping significantly improved accuracy over time. The discussion also touches on the historical context of the second's definition, including earlier references to the tropical year. Further inquiry into the Earth's rotational velocity on that specific day was expressed.
Patrick Aberdeen
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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?
 
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Patrick Aberdeen said:
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 :)
 
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