[Boatanchors] SI units and the rest

[J. Forster]

The second was originally defined so 60x60x24 of them = 1 mean solar day.
That is dependant on the time it takes the earth to rotate once.

However, the rotational period varies some, so it was eventually redefined
by clocks then atomic means.

If a civilization started on another planet, there is virtually zero
possibility they would define their unit of time in the same way.

The length of a second is arbitrary, and the number of Cs cycles is an
approximation to the traditional value. It is fairly stable. Nothing more.

-John

================

>
> 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
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