[time-nuts] The TAI zero epoch of 1958

[jimlux]

On 10/24/19 12:37 PM, Scott Stuart wrote:
> Hello!  I'm new to the list and I have a question about time systems.
> 
> The context is that someone is going to be communicating timestamps to me
> in TAI formatted as the number of elapsed seconds since 1958 Jan 1
> 00:00:00.  To make sure that I understand how this is being calculated, I
> reviewed the history of the establishment of TAI and UTC in the late 60s
> and early 70s.  I understand why 1972 Jan 1 00:00:00 UTC == 1972 Jan 1
> 00:00:10 TAI and why TAI is now 37 seconds ahead of UTC.  I can also see
> that propagating backwards from the 1961 time conversion formulas to 1958
> Jan 1 gives TAI - UTC very close to 0 (within about 2 milliseconds).  So,
> it makes sense to use 1958 Jan 1 00:00:00 UTC == 1958 Jan 1 00:00:00 TAI as
> the 0 epoch for TAI in seconds.

  I'm going to guess that since UTC and TAI were roughly synced then, 
that's the primary reason. There's some folks on the list who are time 
scale historians, and they'll give you the real answer.

I suspect that 1 Jan 1958 was chosen because it was in the middle of the 
International Geophysical Year (IGY) - perhaps the promulgators of TAI 
did it as part of IGY?

I have heard that NASA uses 1 Jan 1958 as the basis of time scales 
because there are no "in orbit" times before that (yeah, a bit of cold 
war there, I think..) and NASA was created in 1958 (July), so it was 
"the beginning of the year in which NASA was created)".

I don't necessarily believe the latter, though, because spacecraft have 
been operated with "local spacecraft time" (mission elapsed time, in 
some form) until very, very recently, and ground events are usually 
reckoned in Central Time (for US Human flight, because that's where 
Houston is) or GMT or UTC or TAI (depending on when...)







> 
> But, I also came across this statement:
> 
> *Loran-C*, Long Range Navigation time, is an atomic time scale implemented
> by the atomic clocks in Loran-C chain transmitter sites. Loran time was
> zero at 0h 1-Jan-1958 and since it is not perturbed by leap seconds it is
> now ahead of UTC by 27 seconds.
> 
> Loran-C time seems to be currently ticking synchronously with TAI but is 10
> seconds behind TAI.  Does that mean that when TAI and UTC were
> defined/synchronized in 1972, that the extra 10 seconds were not added to
> Loran time?  And that the statement above should be that 1958 Jan 1
> 00:00:00 UTC == 1957 Dec 31 23:59:50 Loran?
> 
> Thanks,
>    Scott
> 
>