NAME

Time::TT - Terrestrial Time and its realisations

DESCRIPTION

Terrestrial Time (TT) is a time scale representing time on the surface
of Terra.  Specifically, it is the proper time experienced by a clock
located on the rotating geoid (i.e., at sea level).  It is indirectly the
basis for Terran civil timekeeping, via its realisation International
Atomic Time (TAI).  It is linearly related to (and in fact now defined
in terms of) the astronomical time scale Geocentric Coordinate Time (TCG).

This module represents instants on the TT time scale as a scalar number
of SI seconds since an epoch.  This is an appropriate form for all manner
of calculations.  The TT scale is defined with a well-known point at
TAI instant 1977-01-01T00:00:00.0.  That instant is assigned the scalar
value 599616000 exactly, corresponding to an epoch (scalar value zero)
near the TAI epoch 1958-01-01T00:00:00.0.  This matches the convention
used by "Time::TAI" for instants on the TAI scale.  Because TAI does not
match the rate of TT perfectly, the TT epoch is not precisely equal to
the TAI epoch, but is instead around 600 us earlier than it.

There is also a conventional way to represent TT instants using day-based
notations associated with planetary rotation `time' scales.  The `day'
of TT 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 TT 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.  This notation does
not match the similar day-based notation used for TAI.

There is another conventional way to represent TT instants, using a larger
unit approximating the duration of a Terran year.  The `Julian year' 
is a nominal period of exactly 365.25 `days' of exactly 86400 SI seconds
each.  The TT instant 2000-01-01T12:00:00.0 (MJD 51544.5) is labelled as
Julian epoch 2000.0.  Julian epochs are used only with TT, not with any
other time scale.  The Julian epoch numbers correspond approximately to
Gregorian calendar years, for dates within a few kiloyears of the epoch.
Because TT is not connected to the Terran orbit, and so has no inherent
concept of a year, the year-based notation is somewhat misleading.
Conversion between this notation and the linear count of seconds is
supported by this module.

Because TT is a theoretical time scale, not directly accessible for
practical use, it must be realised using atomic clocks.  This is done
by metrological agencies, each with different imperfections.  To achieve
microsecond accuracy it is necessary to take account of these differences.
This module supports conversion of times between different realisations
of TT.

INSTALLATION

	perl Build.PL
	./Build
	./Build test
	./Build install

AUTHOR

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

COPYRIGHT

Copyright (C) 2006, 2007, 2010 Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

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