setdt Subroutine

public subroutine setdt(delt)

THIS SUBROUTINE ALLOWS FOR THE SPECIFICATION OF THE DELTA-T VALUE (DELTA-T = TT - UT1) TO BE USED IN THE CALCULATION OF SIDEREAL TIME AND THE TERRESTRIAL-TO-CELESTIAL TRANSFORMATION. IT ALLOWS THESE CALCULATIONS TO BE PERFORMED, CORRECTLY, USING UT1 AS THE TIME ARGUMENT FOR THE EARTH ROTATION ANGLE AND TDB AS THE TIME ARGUMENT FOR THE PRECESSION AND NUTATION COMPONENTS. THIS SUBROUTINE, IF USED, SHOULD BE CALLED BEFORE ANY SUBROUTINE RELATED TO EARTH ROTATION (E.G., SIDTIM OR TERCEL) FOR A GIVEN DATE. THE VALUE OF DELTA-T SPECIFIED HERE WILL BE USED UNTIL EXPLICITLY CHANGED.

 DELT   = VALUE OF DELTA-T (TT-UT1) IN SECONDS (IN)

NOTE 1: THE COMPUTED VALUE OF SIDEREAL TIME, AND THE EQUIVALENT EARTH ORIENTATION ANGLES, ARE RELATIVELY INSENSITIVE TO THE VALUE OF DELTA-T: UP TO ONLY ~3 MICROARCSECONDS PER SECOND OF DELTA-T. THEREFORE, FOR MANY APPLICATIONS, THIS SUBROUTINE EITHER NEED NOT BE CALLED AT ALL, OR CAN BE CALLED JUST ONCE FOR A WIDE RANGE OF DATES (E.G., A YEAR). IF THIS CALL IS NOT USED, A DEFAULT VALUE OF DELTA-T OF 64 SECONDS IS USED, WHICH IS APPROPRIATE TO 2000.0.

NOTE 2: THE INPUT TIME ARGUMENTS TO SIDTIM AND TERCEL (TJDH AND TJDL) ARE EXPRESSED IN UT1 REGARDLESS OF WHETHER THIS CALL IS USED.

Arguments