# [time-nuts] Re: UTC

Rob Seaman seaman at noao.edu
Tue Jul 26 16:23:02 EDT 2005

```I think I will trouble the list with just one more message on this
topic, as I'm in danger of repeating myself - certainly the folks I'm
replying to have started to do so.  It is also remarkable how many
ways there are to make my arguments for me.  Too expensive to revise
documentation?  So you're saying that a major change to civil time
will be instituted such that UTC no longer approximates GMT and
nobody should be told about it?  Worse yet, that the documentation
that does exist will be allowed to lie?

Poul-Henning Kamp says:

> The almost universal adoption of timezones and daylight savings
> time disproves this definition:  Civil time varies up to 30 degrees
> or two hours from "time-of-day" in many areas of the world.

All true.  All irrelevant.

Pop quiz!  What is the length of the day?  No tricks - no gimmicks.

Launch a tee-shirt to that guy in the last row.  Right you are!  A
day on Earth is 23h 56m 4s.  Normal SI seconds.  And a simple 360
degree rotational period with respect to a reference grid of fixed
extragalactic sources.

PHK confuses periodic effects (such as the analemma traced out by the
equation-of-time) with secular effects - a monotonically diverging
DUT1 in particular.  There are many co-existing solar time scales:
local apparent time, local mean time, standard time, daylight saving
time, and yes - Coordinated Universal Time to tie all the localities
together.  This multiplicity of time scales does not weaken the
argument for having UTC continue to track time-of-day - it
strengthens it.  Universal Time is the modern formulation of the
concept of Greenwich Mean Time that has served us so well.  UTC is
that concept joined to International Atomic Time via leap seconds.
It is only the fact that UTC continues to approximate GMT that brings
sanity to the long tally of solar time scales.

It is very telling that arguments such as PHK's always seem to be
based on solar time.  After all, the spinning Earth forms a steady
clock relative to the fixed stars, not the Sun.  The length of the
day "really, truly" is 23h 56m 4s.  Why is this never pointed out?
Because it simply isn't so for most purposes.  A day in the life is a
solar day.  It is the solar day our clocks should characterize.  That
the Earth's orbit is elliptical, its axis tipped, and that it is
circled by the Moon simply underscore the need for civil time to
follow mean solar time and thus bring clarity to our timekeeping.  It
is precisely the overwhelming dominance of the Sun in our everyday
lives that illuminates the clear choice of basing civil time on solar
time.  It is just as wrong for the precision timing community to
embrace the split second (but growing) errors of raw TAI as it would
be to permit the nearly four minute daily error that sidereal clocks
would impose on our lives.

Rob Seaman
National Optical Astronomy Observatory

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