[Boatanchors] SI units and the rest

[mac]

On Nov 16, 2010, at 4:30 PM, J. Forster wrote:

> The second is arbitrary also.
>
> When I was working on SETI, the question came up about what  
> frequency to
> listen on. There are an infinite number of possibilities.
>
> One sensible answer is the 1421 MHz Hydrogen line, as that is an  
> atomic
> transition and should be the same all over the universe. The second
> depends on the rotation rate of the earth, which has been changing for
> millions of years and will continue to change. (Hence "lea seconds")



Since 1967 the second has been defined to be 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 caesium  
133 atom.

Dennis D. W7QHO
Glendale, CA