Re: [LEAPSECS] more media coverage

From: Rob Seaman <>
Date: Thu, 10 Jul 2003 16:26:52 -0700 (MST)

It is revealing that the precision timing community refers to itself
that way. Perhaps what we really need is an accurate timing community.

Real world concerns have been keeping me occupied lately, hence
the lack of my relentless nattering. I am very pleased to see the
conversation begin to turn to actual questions of actual civil
timescales. It is about time some sense of reality was brought to
the discussion. I will emphasize as always that we have literally
centuries to reach any conclusions before making any changes to the
current UTC standard.

Ed Davies says:

> The conclusion is that though, of course, there are lots of
> things on an aircraft which need their own accurate timing
> there is no pressing need for accurate synchonisation with
> global standards and no big issues with multiple different
> timescales - as far as the aircraft's internal systems are
> concerned.

I do believe that there are likely lots of systems and procedures on
aircraft that require accurate timing. This paragraph, however, refers
to various needs for precise timing. Undoubtedly the continued second
by second health of modern aircraft requires a high level of precise
synchronization between different subsystems. For many of those
subsystems, it may be more important that they be synchronized with
each other than the question of to what timescale they are synchronized.
However, I doubt whether any complete inventory has been taken of the
dependencies of aviation - or of any other technical or civil field -
on *accurate* synchronization with time of day.

One of the most embarrassing parts of the discussion to date has been
the confusion of the precision timing community between periodic effects
and secular effects. To say that any user community - especially one as
important as aviation - has no pressing need for precise synchronization
with global standards is NOT the same as saying that community has
no pressing needs regarding what those global standards are. All it
is, is a statement of the *level* of precision required for zeroing
their clocks as well as a constraint on the schedule for do so.

Aviation has many operational dependencies on time of day, not
just on international time standards. It may be that many of those
dependencies only care about time of day (the orientation of the
Earth wrt the sun) to a precision of 1 second or 1 minute or 5 minutes
or whatever. But precision is not the same thing as accuracy. Many
of the operational time dependencies likely require that civil time
continue - generally - to track time of day, rather than that clocks
drift wrt the sun. Just because this drift might be small (for some
purposes) for some number of decades following the abandonment of leap
seconds, does not imply that a dependency on time of day does not exist.
Rather, we would simply be setting an obscure trap for our children's
children to solve.

There is also the question of whether different airlines or different
nations will choose the same operational procedures for setting the
captain's clocks. If one airline or one nation makes the change to
TI - a purely human generated standard - ALL airlines and ALL nations
have to make the change. Our current civil time, since it is based
by close approximation on mean time of day, requires only that nations
(and their airlines, railways, shipping companies, phone companies,
and on and on and on) adhere to a first principles definition of
civil time.

Leap seconds are not just a question of precision.

We should understand the full implications of any proposed change to
a standard as fundamental as civil time - before making the change.
We aren't even close.

Rob Seaman
National Optical Astronomy Observatory
Received on Thu Jul 10 2003 - 16:27:03 PDT

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