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

• I
• Patrick Aberdeen
In summary, the SI definition of a second was originally based on 1/86,400 of a day, but due to the Earth's unsteady and changing rotation, it was redefined as a specific number of cycles of an atomic clock. This change occurred around 1950, and by 1952, the day chosen for calibration was the beginning of 1900 (Dec 31, 1899). The exact rotational velocity of the Earth on that day is not specified.

#### Patrick Aberdeen

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?

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 :)

• scottdave

## 1. What is the SI definition of a second?

The SI definition of a second is 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 cesium-133 atom.

## 2. How was the SI definition of a second established?

The SI definition of a second was established in 1967 by the 13th General Conference on Weights and Measures (CGPM) as the internationally accepted definition of the unit of time.

## 3. What is the reference year for the SI definition of a second?

The reference year for the SI definition of a second is the year 2000.

## 4. Why was the year 2000 chosen as the reference year for the SI definition of a second?

The year 2000 was chosen as the reference year for the SI definition of a second because it was the closest year to the establishment of the definition by the CGPM in 1967.

## 5. Are there any plans to change the reference year for the SI definition of a second?

At this time, there are no plans to change the reference year for the SI definition of a second. However, the CGPM may decide to revise the definition in the future if there is a need for a more precise measurement of time.