Re: [LEAPSECS] building consensus

From: Rob Seaman <seaman_at_noao.edu>
Date: Mon, 5 Jun 2006 14:17:08 -0700

On Jun 5, 2006, at 1:38 PM, John Cowan wrote:

> I found another spectacular illustration of how massive the difference
> between solar and legal time can be. Before 1845, the time in Manila,
> the Philippines, was the same as Acapulco, Mexico, a discrepancy of
> 9h16m from Manila solar time. This was a consequence of the
> Philippines
> having been colonized and administered from Spanish America.
> Nowadays the
> standard time of Acapulco is UTC-6; of Manila, UTC+8.
>
> Q: "What happened in the Philippines on December 31, 1844?"
> A: "Nothing. It never existed."

One might suggest that the accommodation between civil time and legal
time is of more interest. What does it mean to say that some nation
or locality uses the Gregorian - or any other - calendar, if some
date "legally" does not exist as you suggest? The sun certainly came
up on that day and rose the following day about 24 hours later. A
variety of activities occurred on that day that fell into bins like
"weekday", "weekend" or "holiday" - or if this was some red letter
day different from all others, then the authorities must have tacked
up fliers or alerted town criers or otherwise informed the populace
of the special nature of the day in question. When they did that,
what did they call it? "The day after December 30, 1844?" "Next
Tuesday?" (Which begs the question, of course.) Suspect rather that
"legal" time only applied to certain specific interactions with
colonial authorities. Would love more details.

An observing session with a ground-based nighttime astronomical
telescope typically begins on one calendar date and ends on the
next. For some observatories in the western hemisphere, the session
starts after 0h UT such that the entire session can be trivially
labeled with a single date. The problem with this is that the UT
date is one day after the date on the observing calendar. For
observatories elsewhere, the data from a single coherent session are
split between two dates whether local or UT time is used. The
solution I have adopted for the dozen telescopes in my bailiwick is
to establish a local noon pivot. All data are assigned to the
calendar date at the start of the night. This 12h difference is, in
effect, the maximum possible discrepancy between a "legal" date and a
"solar" date. As always, the question is: what is your timekeeping
application?

In any event, the case you are basically making is that in throwing
off the yoke of their colonial masters, the Philippines specifically
chose that their legal time should match their civil time and that
their civil time should agree with local solar time.

Rob
NOAO
Received on Mon Jun 05 2006 - 14:17:33 PDT