\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{4822}{6 January 2000}

\IBVStitletl{The Time Coordinate Used}{ in the Variable-star Community }

\IBVSauth{Bastian, U.}


\IBVSinsto{Astronomisches Rechen-Institut, M\"onchhofstr.\ 14, 69120 Heidelberg, Germany,\\ \indent e-mail: s01@ix.urz.uni-heidelberg.de}

\IBVSkey{JD unphysical time scale}
\IBVSabs{The variable-star community uses a physically incorrect}
\IBVSabs{time coordinate for observations and light-curve elements.}
\IBVSabs{The problem is explained and a solution is proposed.}

\begintext

The variable-star community does not use a correct physical
time coordinate for recording observations and for deriving light-curve
elements. Below I describe the problem and propose a solution.
IAU Commissions 27 and 42 should take a decision on the subject.

Up to now, all observations, as well as light-curve elements, are
published in terms of ``JD''\@. The conventional Julian Date,
JD, is linked to the Universal Time, UT, which is not a
physical time coordinate. Depending on whether you precisely
refer to UT1 or UTC, its run is either wavy or has irregular jumps of
a full second. Universal Time is not a physical time coordinate
because it is defined by the varying rotation rate of the Earth.

The preferred physical time coordinate for astronomy, as
recommended by IAU Division~I, is Terrestrial Time, TT\@.
Technically, TT is simply defined as TT~=~TAI~+~32.184~sec, where
TAI is the International Atomic Time. Physically, TT is the
continuation of Ephemeris Time, ET, into the era of TAI\@.
For more details and explanations see Seidelmann~\&~Fukushima
(1992) and references therein. The representation of
TT and ET in terms of day numbers is called Julian Ephemeris Date, JED.

The mean trend between TT (or ET) and UT is about
0.6 sec/yr for the interval from 1920 to 2000 (over which
most of the presently used light-curve elements have been
determined), amounting to almost a minute over a century,
see Fig.~1.

Now, two problems arise. First, a period defined in terms
of UT is not the same as one defined in terms of TT (TAI, ET)\@.
The two differ by roughly $2\times10^{-8}$ over the twentieth century.
Since some published periods are given to 10 decimal digits, this
difference can be highly significant. In other words: A ``JD-based''
period is not given in days of 86400 seconds, but in days about
2 milliseconds longer.

Second, a variable star with perfectly constant period will
show an $O-C$ diagram with humps and bumps with a full
amplitude of about 15 sec. The mean trend between TT (or
ET) and UT was about 0.9 sec/yr (or $3\times10^{-8}$) between 1965
and 1985, and it was practically zero between 1920 and 1940,
see Fig.~1. In other words: Relative period changes of the order of
a few times $10^{-8}$ are induced by a UT-based time coordinate.

\IBVSfig{12cm}{4822-f1.eps}{The difference ${\rm TT-UT1}$ (or ${\rm ET-UT1}$) in the period 1830--1992. The values 1830--1955.5 were derived from lunar occultations, after 1955.5 directly from ${\rm UT1-TAI}$. The UTC scale follows the UT1 curve within 0.9~sec, with 1-sec jumps introduced whenever needed. This graph is a copy of Fig.~5 in Jordi et al.\ (1994), where a more complete description is given.}

\vskip-11pt plus1pt
All this is pretty small numbers, and for the majority of
all variable-star observations it is well below the observing
precision. But there are cases like HW Vir where it is
highly significant already. And these cases will tremendously
grow in numbers in the near future. In addition, the gradual
slowing of the earth's rotation will soon induce a mean
decrease of all variable-star periods over time. This will
be detectable for a whole group (e.g.\ the RR Lyrae stars) long
before it will become obvious for individual stars, and may
lead to incorrect astrophysical interpretations.

All that mentioned above is not really new, and the research communities
observing pulsars or the solar system, for instance, are fully
aware of it. The general variable-star community should --- like
these others have done long ago --- turn to correct physical
time units as soon as possible. It is now too late to organize
a Joint Discussion for the forthcoming IAU General Assembly,
but perhaps a resolution can be taken on the business meeting
of either or both commissions at Manchester.

The fairly obvious solution to the problem would be as follows:
In the future, all light-curve elements and $O-C$~diagrams must be
determined and published in terms of TT, represented in the form
of JED. As for raw observations, publication in terms of UTC,
represented in the form of JD, is still acceptable, but
publication in terms of TT/JED should be preferred. The usage of
the TT time coordinate must in any case be indicated by the
symbol ``JED'' instead of ``JD''.

The convention proposed here is both clear and unambiguous, as well
as very easy to use. The transformation from JD to JED is simply
$$
{\rm JED = JD} + (32.184+n_{\rm leap})/86400,
$$
where $n_{\rm leap}$ is the cumulated number of leap seconds
applicable at date JD\@. This number is tabulated in astronomical
almanacs. Forthcoming leap seconds are announced in the IAU
Circulars regularly. For observational epochs before 1972 (when leap seconds had not been fully in use yet) the values of $\Delta {\rm T = ET - UT}$, as tabulated in the astronomical almanacs, can directly be  used for the transformation. The light-curve elements of an eclipsing variable in the new convention would have the form
$$
{\rm JED~(Min~I)} = 2\,456\,456.4564 + 0.456 456 456 \times E.
$$

As a cautionary remark it should perhaps be noted that the proposed convention, along with the usual procedure of barycentric correction, makes
the physical interpretation of variable-star timings correct to
the level of about 0.1~sec. Below that level, more disturbing effects
become significant which require more conventions and more
complex reduction procedures. By far the biggest effect is
the topocentric light-time correction (up to 20~msec). All others,
e.g.\ the relativistic corrections due to the non-flat metric along the light path through the solar system and to the varying gravitation potential at the geoid are well below 1~msec.

\bigskip
{\it Acknowledgements:} Provision of the Postscript original of Fig.~1 by Carme Jordi is gratefully acknowledged.

\references

Jordi, C., Morrison, L.V., Rosen, R.D., Salstein, D.A.,
Rosello, G., 1994, {\it Geophys.\ J. Int.}, {\bf 117}, 811

Seidelmann, P.K., Fukushima, T., 1992, {\it Astron.\ Astrophys.}, {\bf 265}, 833.

\end{document}
