The MICA Delta T menu option tabulates the calendar date and time, day of the week, Julian Date, and the Delta T value. Delta T (ΔT) is the difference between Terrestrial Time (TT) and Universal Time (UT): ΔT=TT-UT1. Variations in Delta T are caused by changes in the rotational speed of the Earth. MICA Version 2.0 utilizes an internal table of historical and predicted Delta T values. The Delta T value determined by MICA on a specific date is derived by a linear interpolation between the adjacent Delta T table entries. For dates before 1972, the historical values of Delta T were derived from McCarthy and Babcock. Delta T values after 1972 to the present (or date the Delta T table was updated) were derived from the 'finals.data' and the 'cummulative number of leap seconds' files on the USNO IERS Bulletin A web site. Predicted values of Delta T to 2050 were supplied by Johnson. Predicted Delta T errors range from about +/-0.004s in early 2005, to +/-24s in 2050. Updates to the Delta T table will periodically be released on the USNO MICA web site in order to improve the accuracy of the Delta T computations.
DELTA T: TT-UT1 Date Time Day Julian Date Delta T (UT1) (UT1) d h m s d s 2003 Oct 08 00:00:00.0 Wed 2452920.500000 64.554