the legacy of ephemeris time

From: Steve Allen <>
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., ) 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

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

This archive was generated by hypermail 2.3.0 : Sat Sep 04 2010 - 09:44:54 PDT