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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

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

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