# Re: [Scheme-reports] TAI epoch

> R7RS did not invent the POSIX 1970 + 10 second epoch.
> The question is why did older libraries use this (and does
> anybody know)?

I don’t know for sure, but I’d guess that because prior to 1972 atomic seconds and astronomical seconds were different, so the difference between atomic time and astronomical time was not an integral number of seconds. The earliest epoch for which the common atomic time scale coincides with the common astronomical time scale to within an integral number of seconds is 1972, at which point the difference was 10 seconds.

> When the 10 seconds were

My understanding is that the 10 seconds were not added in 1972. Instead, prior to that atomic time and astronomical time had drifted apart by a non-integral and time-dependent number of seconds. By 1972, the difference was about 10 seconds. Therefore, when the decision was taken to make astronomical second equal to atomic seconds and add leap seconds, it made sense to define the offset to be exactly 10 seconds.

> unless there were some official list of
> exact times the seconds should be retroactively applied
> prior to 1972, it would arguably be incorrect to compute
> approximately scaled leap seconds for calendar times.

The relation between atomic time and astronomical time is fairly well defined at least as far back as 1970. Shiro referenced a table with a formula.

> So there are three possibilities:
>
> 1. The sources and our assumptions are wrong and the
> libraries actually used an epoch of 1970 + ~8 seconds,
> possibly being misadvertised since when the change
> occurred the diff was 10 seconds.
>
> 2. The epoch was really 1970 + 10 seconds due to a
> simplifying assumption of a 10 second diff for all times
> prior to the introduction of leap seconds.
>
> 3. The epoch was really 1970 + 10 seconds due to a
> historical mistake.

I don’t fully understand any of your possibilities.

My understanding is that the error is in saying that the epoch of "ten seconds after midnight on January 1,
1970 TAI” is "equivalent to midnight Universal Time”.

My suggestion would be to remove the phrase "equivalent to midnight Universal Time”.

Regards,

Alan

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