Types of time

From: Ken Pizzini <ken_at_halcyon.com>
Date: Fri, 08 Sep 2000 00:51:26 -0700

A point I haven't noted being stated clearly and explicitly yet in
this discussion: what is the precise problem being solved by UTC in
the first place?

There is a use for unsegmented uniform time, already well addressed
by TAI.

There is a use for astronomical time, which (it seems to me) would be
better served by a sidereal clock (whether actual or mean) than a
mean-solar-day based UT1, so keeping |UTC-UT1| < 0.9s doesn't seem
like something that is helping the astronomical community any.

There is a use for timekeeping in celestial navigation. When using
stars other than Sol, these users would also would be well served to
use a sidereal clock as the reference, and for solar navigation it
would be more helpful to have a clock which accounted for the equation
of time and ticked off "real" solar days instead of mean solar days.
(On the technical level it is quite possible to design a clock which
ticks off the non-uniform "seconds" which would be required for this.
Even in the age of mechanical clocks, there were a few designed with
equation-of-time adjustments built-in.)

There is a use for some form of civil time, tied to some approximation
of the solar day. There is a fair bit of slop here, because the real
solar day drifts a fair bit about the mean, and civil time is (for the
most part) diced into one-hour offsets from the civil reference time,
with only a tenuous relationship between the local apparent sun and the
clock time. |UTC-UT1| < 0.9s in this context is just a joke; bigger
anomolies happen in many juristictions every year --- when daylight
saving time is observed. Leap minutes, scheduled a half-century or more
in advance, would serve civil needs quite well. (An elementary school
student could do a science fair project to show the drift between [UTC]
and the sun? Big deal --- that same student can already demonstrate the
existence of the equation of time; the fact that the mean solar day
seldom equals an observed solar day.)


Now on the one hand I can agree that the reasons posited so far by
the "abandon the leap second" crowd so far are not convincing --- the
technical problems seem to stem from the poor choice of UTC instead of
TAI for the time base, though I do appreciate the obnoxious problems
faced by programmers who need to translate between a time ticked by
a uniform time source (e.g., a cesium clock) and civil time (for the
benefit of the humans using the system). The solution used in GPS
is basically a workable one: have your clock tick TAI, and have an
authoritative source broadcast the [UTC] offset, though this does not
work nearly as well for systems (say, a microwave oven) which have no
use for receiving broadcast or [internet] communications.

On the other hand, I am failing to understand _why_ we have this cockeyed
leap second scheme in the first place. Exactly what compelling purpose
is served in having a time base which attempts to hold the mean solar
day as measured by a clock constrained to tick only whole integer SI
seconds nearly constant relative to the mean solar day as measured by
a mathematical transform of measured earth rotation? If UT1 really is
as valuable for the basis of our timekeeping as some seem to be saying,
why not just use that directly instead of this hybrid atomic/mean-solar
approximation? What is the point?

                --Ken Pizzini
Received on Fri Sep 08 2000 - 01:14:42 PDT