NAME

Time::TCB - Barycentric Coordinate Time

SYNOPSIS

        use Time::TCB qw(tcb_instant_to_mjd tcb_mjd_to_instant);

        $mjd = tcb_instant_to_mjd($instant);
        $instant = tcb_mjd_to_instant($mjd);

DESCRIPTION

Barycentric Coordinate Time (TCB) is a coordinate time scale representing time in the Sol system. Specifically, it is the proper time experienced by a distant clock comoving with the barycentre of the Sol system.

This module represents instants on the TCB time scale as a scalar number of SI seconds since an epoch. This is an appropriate form for all manner of calculations. TCB is defined with a well-known point at TAI instant 1977-01-01T00:00:00.0 at the Terran geocentre. This point is assigned the scalar value -460_080_000, putting the epoch at approximately the date at which the resolution defining TCB was adopted by the International Astronomical Union. This epoch is deliberately very different from those used for Geocentric Coordinate Time (TCG) in Time::TCG and for Terrestrial Time (TT) in Time::TT, to avoid confusion between them.

There is also a conventional way to represent TCB instants using day-based notations associated with planetary rotation `time' scales. The `day' of TCB 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 TCB 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, TCG, and TCB instants yield very similar values for corresponding instants, so care must be taken to avoid confusion.

FUNCTIONS

tcb_instant_to_mjd(INSTANT)

Converts from a count of seconds to a Modified Julian Date in the manner conventional for TCB. The MJD can be further converted to other forms of day-based date using other modules. The input must be a Math::BigRat object, and the result is the same type.

tcb_mjd_to_instant(MJD)

Converts from a Modified Julian Date, interpreted in the manner conventional for TCB, to a count of seconds. The input must be a Math::BigRat object, and the result is the same type.

SEE ALSO

Date::JD, Time::TCG, Time::TT

AUTHOR

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

COPYRIGHT

Copyright (C) 2006, 2010, 2012 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.