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From: Steve Allen <sla_at_ucolick.org>

Date: Sat, 6 Dec 2003 22:44:53 -0800

Having noted that universal time (UT) was not uniform and therefore

not suited for use in calculating ephemerides of bodies in the solar

system, in 1952 the IAU decided that it was necessary to create a new

form of time -- ephemeris time (ET). But the equation that defined

the rate of ephemeris time was exactly the same equation from Newcomb

that defined the rate of universal time. When used with inputs of

star transit observations the equation produced UT. When used with

inputs of orbital motion the equation produced ET.

One legacy of the creation of ET is that the SI second was matched in

length to the ephemeris second.

For mean solar days of UT it is natural in all respects that it should

be subdivided into hours, minutes and seconds and integrated into

months and years. But ET was supposedly uniform and

rotation-agnostic, so it did not make sense to subdivide into seconds

or to integrate into years. Indeed, for ET a decimal count as the

Julian ephemeris date is probably the most natural form of notation.

Nevertheless, the other legacy of ET is that because it used the same

equation as for UT, it also used the same notation as for UT. ET

adopted the use of YYYY-MM-DDThh:mm:ss as a common form, with Delta T

as the difference.

Following the precedent of ET we have other forms of uniform,

rotation-agnostic time -- TAI, TT, TCG, TCB, GPS time, etc. And also

following the precedent of ET we find these other forms of uniform

time being expressed as YYYY-MM-DDThh:mm:ss even though the notion of

diurnal and annual cycles is contrary to the underlying concept of

these time scales.

Klepczynski's presentation at Torino resurrected an objection posed at

the IAU meeting in 1970 just after the CCIR had decreed there would be

leap seconds. It despaired that air traffic control would eventually

suffer a catastrophe because "System designers do not understand

TIME". That is hardly any wonder given the abundance of timescales (

see e.g., http://www.ucolick.org/~sla/leapsecs/timescales.html ) and

the difficulty of finding authoritative explications of them. More

recently in the New Scientist article Klepcynski worries that air

traffic controllers may be using some atomic system time and also

glancing at their wristwatches, confusing the offsets between the two,

and producing a catastrophe.

I'm mystified at how omitting leap seconds from broadcasts of time

signals is supposed to alleviate the abundance of timescales or

prevent a controller from seeing a wall clock that differs from a

wristwatch. Any time in the form of hh:mm:ss is subject to confusion

with poorly set timepieces in the environment.

For the sake of air traffic controllers, however, I wonder if the

solution is not to demand the use of a uniform time scale and to

enforce a lack of confusing by expressing that in a notation that is

suitable for a uniform time scale. I.e., rather than use the

inappropriate (and confusing) hh:mm:ss notation, why not require that

the uniform time scale be reported as 5 significant digits of decimal

fraction days or the 4 least significant decimal digits of a count of

seconds?

Date: Sat, 6 Dec 2003 22:44:53 -0800

Having noted that universal time (UT) was not uniform and therefore

not suited for use in calculating ephemerides of bodies in the solar

system, in 1952 the IAU decided that it was necessary to create a new

form of time -- ephemeris time (ET). But the equation that defined

the rate of ephemeris time was exactly the same equation from Newcomb

that defined the rate of universal time. When used with inputs of

star transit observations the equation produced UT. When used with

inputs of orbital motion the equation produced ET.

One legacy of the creation of ET is that the SI second was matched in

length to the ephemeris second.

For mean solar days of UT it is natural in all respects that it should

be subdivided into hours, minutes and seconds and integrated into

months and years. But ET was supposedly uniform and

rotation-agnostic, so it did not make sense to subdivide into seconds

or to integrate into years. Indeed, for ET a decimal count as the

Julian ephemeris date is probably the most natural form of notation.

Nevertheless, the other legacy of ET is that because it used the same

equation as for UT, it also used the same notation as for UT. ET

adopted the use of YYYY-MM-DDThh:mm:ss as a common form, with Delta T

as the difference.

Following the precedent of ET we have other forms of uniform,

rotation-agnostic time -- TAI, TT, TCG, TCB, GPS time, etc. And also

following the precedent of ET we find these other forms of uniform

time being expressed as YYYY-MM-DDThh:mm:ss even though the notion of

diurnal and annual cycles is contrary to the underlying concept of

these time scales.

Klepczynski's presentation at Torino resurrected an objection posed at

the IAU meeting in 1970 just after the CCIR had decreed there would be

leap seconds. It despaired that air traffic control would eventually

suffer a catastrophe because "System designers do not understand

TIME". That is hardly any wonder given the abundance of timescales (

see e.g., http://www.ucolick.org/~sla/leapsecs/timescales.html ) and

the difficulty of finding authoritative explications of them. More

recently in the New Scientist article Klepcynski worries that air

traffic controllers may be using some atomic system time and also

glancing at their wristwatches, confusing the offsets between the two,

and producing a catastrophe.

I'm mystified at how omitting leap seconds from broadcasts of time

signals is supposed to alleviate the abundance of timescales or

prevent a controller from seeing a wall clock that differs from a

wristwatch. Any time in the form of hh:mm:ss is subject to confusion

with poorly set timepieces in the environment.

For the sake of air traffic controllers, however, I wonder if the

solution is not to demand the use of a uniform time scale and to

enforce a lack of confusing by expressing that in a notation that is

suitable for a uniform time scale. I.e., rather than use the

inappropriate (and confusing) hh:mm:ss notation, why not require that

the uniform time scale be reported as 5 significant digits of decimal

fraction days or the 4 least significant decimal digits of a count of

seconds?

-- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 sla_at_ucolick.org Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93Received on Sat Dec 06 2003 - 22:45:27 PST

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