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<H3><A NAME="SECTION000515100000000000000">
Atomic Time: TAI</A>
</H3>
<I>International Atomic Time</I> TAI is a laboratory timescale. Its
unit is the SI second, which is defined in terms of a
defined number
of wavelengths of the radiation produced by a certain electronic
transition in the caesium 133 atom. It
is realized through a changing
population of high-precision atomic clocks held
at standards institutes in various countries. There is an
elaborate process of continuous intercomparison, leading to
a weighted average of all the clocks involved.
<P>
Though TAI shares the same second as the more familiar UTC, the
two timescales are noticeably separated in epoch because of the
build-up of leap seconds. At the time of writing, UTC
lags about half a minute behind TAI.
<P>
For any given date, the difference TAI-UTC
can be obtained by calling the SLALIB routine
sla_DAT.
Note, however, that an up-to-date copy of the routine must be used if
the most recent leap seconds are required. For applications
where this is critical, mechanisms independent of SLALIB
and under local control must
be set up; in such cases
sla_DAT
can be useful as an
independent check, for test dates within the range of the
available version. Up-to-date information on TAI-UTC is available
from <TT>ftp://maia.usno.navy.mil/ser7/tai-utc.dat</TT>.
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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