NAME

Time::TCG - Geocentric Coordinate Time and realisations

DESCRIPTION

Geocentric Coordinate Time (TCG) is a coordinate time scale representing
time in the Terran system.  Specifically, it is the proper time
experienced by a distant clock comoving with the geocentre.  It is
linearly related to Terrestrial Time (TT), which is the proper time scale
underlying timekeeping on the Terran surface.  TT is formally defined
in terms of TCG: TT ticks exactly 0.999_999_999_303_070_986_6 seconds
for each second of TCG.

This module represents instants on the TCG and TT time scales as scalar
numbers of SI seconds since an epoch.  This is an appropriate form for
all manner of calculations.  Both scales are defined with a well-known
point at TAI instant 1977-01-01T00:00:00.0.  This point is used as the
epoch for TCG, having the scalar value zero.  The same instant on the
TT scale is assigned the scalar value 599_616_000 exactly, corresponding
to an epoch near the TAI epoch 1958-01-01T00:00:00.0.  This matches the
convention used by "Time::TT" for instants on the TT scale.  The use of
very different epochs for the two scales avoids confusion between them.

There is also a conventional way to represent TCG instants using day-based
notations associated with planetary rotation `time' scales.  The `day'
of TCG is a nominal period of exactly 86400 SI seconds, which is slightly
shorter than an actual Terran day.  The well-known point at TAI instant
1977-01-01T00:00:00.0 is assigned the label 1977-01-01T00:00:32.184
(MJD 43144.0003725).  Because TCG is not connected to Terran rotation,
and so has no inherent concept of a day, it is somewhat misleading to use
such day-based notations.  Conversion between this notation and the linear
count of seconds is supported by this module.  The day-based notations for
TT and TCG instants yield very similar values for corresponding instants,
so care must be taken to avoid confusion.

Because TCG is a theoretical time scale, not directly accessible for
practical use, it must be realised using atomic clocks.  In fact, it is TT
that is directly so realised, but the linear relationship between TT and
TCG means that any realisation of TT is effectively also realising TCG.
This module supports conversion of times between different realisations of
TCG, by making use of the facility in "Time::TT" for realisations of TT.

INSTALLATION

	perl Makefile.PL
	make
	make test
	make install

AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

Copyright (C) 2006 Andrew Main (Zefram) <zefram@fysh.org>

This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
