[LEAPSECS] Leap Seconds schedule prior to 1972

[John Sauter]

On Mon, 2016-04-25 at 09:40 -0400, Brooks Harris wrote:
> 
>  Hi John, 
> "understood and widely used ", yes. Standardized by an international
> standards organization, I'm not sure. Anyone know of one? There's a
> lot of things in timekeeping that are done on a "common practice" or
> "de facto standard" basis. In some cases these are not as commonly
> understood as one might wish.

I also don't know of an ISO standard for the Julian Day Number, but it
has been used by astronomers for about 400 years, and everybody seems
to agree on its definition.

> >  Doing something non-standard just to create a unique time scale
> > doesn't seem like a good enough reason.
>  It can avoid any ambiguity of interpretation if its clearly defined
> especially its alignment to 1972-01-01 
> 00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC).
> 

To be sure, but it is also possible to avoid any ambiguity of
interpretation by using a well-understood and widely-used method for
specifying days.

> Julian Day has an epoch of "12 noon 1 JAN -4712 (4713 BC)". Beyond
> that you've got to go to a "proleptic Julian Date" which is not
> exactly "standard". A negative 86400 second day number extends to the
> arbitrarily distant past depending on how many bits you decide to
> carry.
> 
> Julian Day may be OK. But somebody might ask when, exactly, did the
> Chicxulub meteor impact? I know that's beyond your scope but your
> timescale extended further as need arose.
> 

I suspect negative Julian Day Numbers isn't "a standard" because there
is little need for them.  I myself don't have any problem with negative
Julian Day Numbers.  The meteor hit at approximately Julian Day Number
-24,105,840,000.  Maybe someday we will know when it hit to the day, or
even the second.

A really good time scale would start with the Big Bang and count time
using a fundamental unit something like Planck time, about 10 to the
-43 seconds.

> > 
> > I am happy for programs which read the data file to compress it to
> > suit
> > their needs, but TAI-UTC won't fit in 11 bits if you want to go
> > back to
> > the year -1000, which has a DTAI over 25,000.
>  
> Right. Depends on how far back you want to go. 11-bits TAI-UTC gives
> you 2048 Leap Seconds, so, by your table 1, back to year 1000 or
> there abouts. That would be good enough for a lot of historical
> events. Who uses it for what would drive the implementation choices.
> 32-bits is very lightweight. Its just an observation - your target
> range is bigger.
> 
> -Brooks
> 

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