[ntp:questions] A Suggestion For Abolishing the Leap Second

sla29970 at gmail.com sla29970 at gmail.com
Tue Jun 5 01:38:34 UTC 2007

On Jun 4, 4:18 pm, Quadibloc <jsav... at ecn.ab.ca> wrote:
> It isn't the center of the Universe. It's just the place where all of
> humanity lives and does its work.

The surface of the earth is where all the well-interconnected, well-
maintained atomic clocks are.  It is also where all the politicians
allocating funding for the operation of those clocks are.

> But people who are working on a Mars
> trip certainly need a source of time centered on the Sun.

People on a Mars trip, or anywhere, need to determine their needs for
clock  uniformity and synchronization and buy a clock suitable for
that purpose.  UTC, TAI, or any other time scale may not suffice for
your purposes.  Given the non-linear relativistic variations, neither
UTC nor TAI makes much sense off the surface of the earth.

> In my ignorance of these matters, I would have thought that if the Sun
> were the immovable center of the Universe, and the Earth were
> revolving around it in a perfect circle, time (at least at the poles,
> assuming no axial tilt) would progress at a uniform rate on the Earth
> as viewed from the Sun, just changed by a *fixed percentage* due to
> relativistic effects.

We don't live on that Earth, and even if we did the answer is still
No, because of tides.  Again I'll refer to this article.
Clocks tick slower in the daytime than at night because they are
deeper in the sun's gravity well in the daytime.  The effect has an
amplitude of about 2 ps.  The first team to measure it will win a
Nobel prize.

> The Sun's proper motion is approximately linear, so its reference
> frame is inertial, meaning its motion changes nothing.

(Disregarding the possible use of an ensemble of millisecond pulsars)
there are no precision clocks anywhere but on the (diurnally tidally
heaving) surface of the earth.  There is little point defining a
practical convention based on something for which there is no means of
verifying the measurements.  Even getting funding to try to measure
such things is difficult.

> Is the problem due to the fact that the Earth's orbit is elliptical?
> If that's the only cause, the effect must be a rather small one.

For measuring pulsar time arrivals that effect is relevant.

Small enough is a matter of taste -- de gustibus non disputandem --
or, more practically, a matter of your need for precision and
uniformity of timekeeping.  To within some engineering limit you can
throw money at the problem and buy or build a better clock for your
use.  At the picosecond level your laboratory will never agree with
someone else's laboratory.

TAI is a best effort, statistically-based, politically acceptable,
practical implementation of a coordinate time based on current levels
of human cognition. UT1 is the same sort of measure of earth
rotation.  When clocks reach the picosecond level the globally-
averaged TAI may not suffice for your needs. Mean solar time is
already accurate enough at a level of 1 second.  In either case this
may or may not be big enough to cause problems for you, and someone
else's needs will almost certainly be different.  The only national
government which has expressed support for UTC continuing to match
mean solar time is the UK.  Every other government's position is
either to abandon them, or silence.

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