delta TSymbol: δT . From 1984, the increment to be added to universal time, UT1, to give terrestrial dynamical time, TDT. Prior to 1984 it was the increment to be applied to UT1 to give ephemeris time. δT is not constant but at present is increasing in an irregular manner: between 1970 and 1990 it changed from +40 to +57 seconds, and is predicted to be +67 seconds by 2010.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006