Delta T

Delta T = ET - UT is the difference between ephemeris time (ET), which is uniform and now defined practically by atomic clocks (Temps Atomique International, TAI), and universal time (UT, also called UT1, formerly Greenwich Mean time, GMT), defined by the rotation of the earth, which is nonuniform due to periodic irregularities and the secular acceleration caused by lunar tidal forces. When ET (ephemeris time) is selected in the Time span dialog, the time used for ephemeris computation is uniform; when UT is selected, universal time is used for ephemeris computation and the time is not uniform. The correction delta T affects the faster planets and, very strongly, the moon; it is normal to use UT for ephemeris calculation. Delta T is a complex quantity, expressed as a polynomial, that has been derived both empirically, principally from lunar occultations of stars and from solar and lunar eclipses, and theoretically, from the tidal acceleration. There are uncertainties about the value of delta T, particularly at earlier epochs.

To select a lookup table for ephemeris computation select Ephemeris -> delta T from the main menu. The delta T dialog appears with a list box, showing the names of the available lookup tables. In the delta T dialog select one of lookup tables for delta T. If the box from 1620 use internal delta T table is checked, all delta T computations for any instant between 1620 and 2020 will be performed using the internal delta T table, no matter what lookup table was chosen. If the box is unchecked, the selected lookup table is used for the entire period (including the range from 1620 to 2020). Since the internal table of delta T is based upon observations and predictions, it is strongly recommended to check the box from 1620 use internal delta T table. Click OK and all selections for delta T are retained and used for ephemeris calculation. For more information, see Sources and Accuracy.

Note: The lookup table named as 'JPL HORIZONS until rev 3.15' is added only for compatibility reasons to AE1. These delta T values were used by the HORIZONS ephemeris generator until February 2005.