[tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision

[Paul Eggert]

On 7/28/22 16:07, Steve Allen via tz wrote:

> That comes from the tzdata asia file for Phu Lien Observatory.
> That entry gives Paris Meridian as 2 deg 20' 14.03" E with no citation.
> 
> Every historical value of longitude needs an accompanying citation.

Thanks. Sorry to have induced you to go to all that work in digging up 
historical measurements of the Paris Meridian, as the current datum 
doesn't depend on the exact value. It's simpler, I think, to remove that 
longitude from the comments as that will save us the trouble of picking 
and citing a longitude. Done in the attached patch.


> Please do not attempt sub-millisecond time offsets for astronomical
> time.

Sure, but if our understanding is correct, the cool thing about the GMT 
offset in Vietnam from 1906 to 1911 is that it wasn't astronomical time. 
That is, what we're calling Paris Mean Time was defined by French law to 
be 9 minutes 21 seconds east of GMT; and what we're calling Phù Liễn 
Mean Time was defined by law to be 104° 17′ 17″ east of Paris Mean Time. 
Neither of these numbers are astronomical; they're both legal numbers, 
and so their sum is a legal number, and has infinite precision (since 
French lawyers don't admit to any imprecision :-).

That being said, this is currently academic since tzcode isn't using the 
subsecond values, and I take your point that the actual offset was not 
accurate to 9 places even with the best technology of the time. So I 
installed the attached further proposed patch, which rounds to the 
nearest centisecond (and fixes one other, more important thing), and I 
hope that is good enough.
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