Re: floating prime meridian

From: Rob Seaman <>
Date: Thu, 31 Aug 2000 23:11:56 -0700 (MST)

> AFAIK the date line is not defined as such: it is simply a theoretical
> construct which depends on various nearby jurisdictions' definition of
> what day it is, and it moves when they change their minds.

Yup. That was my point - made stronger by a thirty five mile variation
as compared to the erroneous two mile number. In case folks don't know,
the old posts are available from:

Again - this is not a massive technical issue, of course. But there
are all sorts of weird legal entanglements that the initial article,
survey and report ignore.

> But what is the civil definition of the day?

You've made my case. It certainly currently includes the notion that
midnight won't drift secularly.

> What re-education? Mostly either 1) they need to be re-educated about
> leap seconds every time there is one, or 2) they have absolutely no
> interest in the subject.

Your number 1 and number 2 are the same issue. If they aren't going
to care about the absence of leap seconds - why do they care about
their current presence? In general lay people (which includes a lot
of highly technically trained folks dabbling in other fields - like
me, you and the other 53 people reading this list at one time or
another) - in general lay people don't *know* what they have an
interest in.

Yes - for many purposes a 140 second/century secular drift will be
negligible (at least for a few centuries). I myself wouldn't care
about such a change for very many reasons outside of the observatory.
(And many of the observatory issues would really be sidereal time
considerations.) But for many other purposes a 140 second/century
effect is immense. After a century (and astronomers do need to think
on those time scales) the deviation will be larger than the width of
the moon or the sun in the sky. This is many times larger than the
field of view of most astronomical instruments on - or above - the planet.

Big deal? Maybe - maybe not. Certainly all the critical astronomical
software could be rewritten. I'd like to see a better inventory of
this than I guessed at in an earlier message. Certainly any professional
applications could just be redesigned.

But every amateur astronomer on the planet will also be able to detect
the effect after only a few years. Anyone navigating by traditional
methods will have to know about the changes.

The biggest missing piece in the discussion so far is any analysis
of why such a major (philosophical, if not technical) change need
be made any time soon. Are GLONASS and "some" spread-spectrum
applications the only busted systems?

So I'll balance amateur astronomers and classical navigators against
the GLONASS community and that fraction of the spread-spectrum
applications that can't get leap second handling right (or can't
just choose to use a more appropriate unsegmented timescale).

> In any event, people who care only about civil time may well be willing
> to trade off certainty for astronomical accuracy.

"Accuracy" is a stochastic concept. We're discussing a secular effect.

Rob Seaman
National Optical Astronomy Observatory
Received on Thu Aug 31 2000 - 23:11:49 PDT

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