[time-nuts] Re: time-nuts Digest, Vol 12, Issue 15

Mike S mikes at flatsurface.com
Thu Jul 7 07:11:20 EDT 2005

At 12:06 AM 7/7/2005, Tom Clark wrote...

>   It was said
>       (3) It is called UT1 these days.
>Don't be pedantic. The point is that there are organizations which depend upon 
>a time coordinate system which is closely linked to astronomical time. UTx/xMST
>, whatever.
>   Wrong! UTC is the stepwise (i.e. with leapseconds) approximation of
>   the atomic time rate to the actual rotation rate of the earth. Changes
>   in UTC are all integral seconds.
>   UT1 reflects the ACTUAL rotation of the earth with respect to an
>   external reference frame (i.e. as would be observed with a sundial ;-)

It's not clear what you're claiming is wrong. The fact is that UTC is linked to astronomical time (UT1) by definition. Yes, that link is stepwise, but it is always supposed to be maintained within 0.9 seconds. You statements simply support those facts.

I suspect you're just choosing a different interpretation of the imprecise term "closely" to create an argument.

UTC is "closely linked" enough for legal purposes (which in many locales is defined in terms of mean solar time), and apparently also "closely linked" enough that the IAU mandates it's use. Perhaps it is not close enough for your use.

To satisfy the pedants out there, UTC is not stepwise (or is time quantum?), and does not change by integral seconds; it is continuous. It is the _offset_ from UT1 which is stepwise integral seconds.

UT1 is not the "ACTUAL" rotation (in unqualified terms). It could just as validly be claimed that "actual" rotation is sidereal. As you obviously know, our use of the earth's rotation for timekeeping is typically solar or stellar relative, resulting in UTx or xST, hence my reference to UTx/xMST as coordinate systems of interest to those who need to follow astronomical time to some (undefined) degree of precision. UTC and UT1 simply offer different degrees of precision. TAI and a UTC without leap seconds do not attempt to maintain any relationship whatsoever to astro time. Such a UTC would (eventually) break many uses to which UTC is currently put. 

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