Astro::Montenbruck::Time::DeltaT - difference between UT and TDT.
Delta-T indicates the difference between UT and TDT (Terrestrial Dynamic Time), which used to be called Ephemeris time in the last century. While UT is not a uniform time scale (it is occasionally adjusted, due to irregularities in the Earth's rotation), TDT is a uniform time scale which is needed as an argument for mathematical theories of celestial movements.
Formulae used by "delta_t( $jd )" subroutine, are based on NASA Technical Publication "Five Millennium Canon of Solar Eclipses: -1999 to +3000". They are valid for any time during the interval 2000 B.C. to 3000 A.D. See NASA Eclipse web site.
Returns approximate Delta-T in seconds for a given Julian Day.
Delta-T = ET - UT
For a historical range from 1620 to recent years, we interpolate from a table of observed values. Outside that range we use formulae from Astronomical Algorithms by J.Meeus, second edition.
$jd — Standard Julian Date.
Delta-T in seconds