Re: [LEAPSECS] Risks of change to UTC

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Sat, 21 Jan 2006 15:46:09 -0700

On Jan 20, 2006, at 10:17 PM, M. Warner Losh wrote:

> Any watch that is smart enough to decode those signals would be
> smart enough to add this minor correction as well.

A viable time scale could be constructed from any periodic (or "near"
periodic) waveform - there's nothing magic about the tick, tick, tick
of delta functions (or step functions if you prefer to think of it
that way). A viable watch could present a different representation -
12 hour/24 hour, sexigesimal/decimal, local/"universal" - every time
it is consulted. It wouldn't even have to be monotonic, as long as
enough "temporal metadata" is provided for context. That metadata
(such as DUT1 and DTAI) could be provided in as circuitous and
obscure a fashion as can be imagined - encrypted, proprietary,
steganographically. Heck - time signals might even arrive as little
blips on shortwave radio - as hard as that might be to believe.

There's viable - and then there's viable...

> The mechanical watch might be a bit of a problem, but DUT1 doesn't
> change enough to introduce navigation errors similar to what we
> have today over the course of a year and can easily be looked up
> like someone would lookup what the weather was going to be like.

...and we're back to the confusion between periodic and secular
effects. There seems to be some thought that mean solar time is
nothing but a polite (or lately, sometimes impolite) fiction.
Greenwich Mean Time is real enough to have built the British Empire.
You're also working both sides of the equation. A navigator observes
local apparent solar time onboard and compares it to GMT (or mean
time on any other known meridian) transported via chronometer. DUT1
is a mechanism to correct mean solar time as reported by the clock.
The equation of time, on the other hand, is used to convert shipboard
apparent time to local mean time. Subtraction does the rest.

Rob Seaman
NOAO
Received on Sat Jan 21 2006 - 14:46:19 PST