[LEAPSECS] Leap Seconds schedule prior to 1972

[Brooks Harris]

On 2016-04-25 11:11 AM, John Sauter wrote:
> 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.
Yes, its well known and well established. But it drives me a little nuts 
when there's not a real standard, and timekeeping is full of such 
things. Gregorian calendar is well defined in ISO 8601 for example, and 
that is something you can hang your hat on. UTC similarly, but the 
specifications are scattered throughout BIPM, IERS, and ITU-R so its not 
so easy to understand. It holds water in the end, but its pretty fragmented.
>
>>>   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.
Sure. But a negative 86400 day number is simpler to explain, understand, 
and implement - no conversion required until you need to align it to 
something else - Julian, MJD, POSIX, PTP, GPS, etc. Just a thought.
>
>> 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.
OK. Wow. Fun to think about :-)
> 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.
Yup. But "proleptic UTC" as you are doing it is a useful engineering 
approximation for "civil time" purposes on Earth. It gets a little dicey 
if you ask what proleptic UTC time it was when the impact that created 
the moon occurred. Were there Leap Seconds before that?

-Brooks


>
>>> 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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