1. Until 1960 second was (60•60•24)^-1=(86400)^-1 of day. (Mean solar second flucutated by 10 ppb.)

2. Until 1967 was corresponding fraction of tropical year. (Ephemeris second good to few ppb except for orbit decay.)

3. Since 1967:
   The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
   
   (Accuracy of 10 ppt. Then comparable to Hydrogen Maser at 1 ppt.)

4. In 1997 the International Committee on Weights and Measures (CIPM) affirmed this was at 0K.
   
5. Is now accurate to 2x10^-15 or 2 parts per quintillion and still improving at power of 10 per decade since 1960.

6. Pressure to stabilize or move standard from 9 GHz
   • with: hydrogen masers used since 1976 with cesium fountain clock in space
   • to: single ¹⁹⁹Hg⁺ at 1 064 721 609 899 143 (10) Hz or 282 nm
   • to: 10^{7   40}Ca at 455 986 240 494 158 (26) Hz or 657 nm
   • with: sapphire-rutile (TiO₂)
7. Effects of velocity, gravity, temperature, etc. loom large (and measureable/predictable).

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