TDBTCB Subroutine

  1. astro_module.f90
  2. astro_module
  3. TDBTCB

public subroutine TDBTCB(tdb1, tdb2, tcb1, tcb2, j)

Time scale transformation: Barycentric Dynamical Time, TDB, to Barycentric Coordinate Time, TCB.

Status: canonical.

Notes

  1. TDB1+TDB2 is Julian Date, apportioned in any convenient way between the two arguments, for example where TDB1 is the Julian Day Number and TDB2 is the fraction of a day. The returned TCB1,TCB2 follow suit.

  2. The 2006 IAU General Assembly introduced a conventional linear transformation between TDB and TCB. This transformation compensates for the drift between TCB and terrestrial time TT, and keeps TDB approximately centered on TT. Because the relationship between TT and TCB depends on the adopted solar system ephemeris, the degree of alignment between TDB and TT over long intervals will vary according to which ephemeris is used. Former definitions of TDB attempted to avoid this problem by stipulating that TDB and TT should differ only by periodic effects. This is a good description of the nature of the relationship but eluded precise mathematical formulation. The conventional linear relationship adopted in 2006 sidestepped these difficulties whilst delivering a TDB that in practice was consistent with values before that date.

  3. TDB is essentially the same as Teph, the time argument for the JPL solar system ephemerides.

Reference

  • IAU 2006 Resolution B3

History

  • IAU SOFA revision: 2019 June 20

Arguments

TypeIntentOptionalAttributesName
real(kind=wp), intent(in) :: tdb1

TDB as a 2-part Julian Date
real(kind=wp), intent(in) :: tdb2

TDB as a 2-part Julian Date
real(kind=wp), intent(out) :: tcb1

TCB as a 2-part Julian Date
real(kind=wp), intent(out) :: tcb2

TCB as a 2-part Julian Date
integer, intent(out) :: j

Contents

Source Code

TDBTCB

Source Code

    subroutine TDBTCB ( tdb1, tdb2, tcb1, tcb2, j )

    implicit none

    real(wp),intent(in) :: tdb1 !! TDB as a 2-part Julian Date
    real(wp),intent(in) :: tdb2 !! TDB as a 2-part Julian Date
    real(wp),intent(out) :: tcb1 !! TCB as a 2-part Julian Date
    real(wp),intent(out) :: tcb2 !! TCB as a 2-part Julian Date
    integer,intent(out) :: j !! status:  0 = OK

    !  1977 Jan 1.0 TAI = 1977/1/1 00:00:32.184 TCB, as two-part JD
    real(wp),parameter :: t77td = 2443144.0_wp
    real(wp),parameter :: t77tf = 0.5003725_wp

    !  L_B, and TDB0 (d)
    real(wp),parameter :: elb = 1.550519768e-8_wp
    real(wp),parameter :: tdb0 = -6.55e-5_wp/86400.0_wp

    !  TDB to TCB rate
    real(wp),parameter :: elbb = elb/(1.0_wp-elb)

    real(wp) :: d, f

    !  Result, preserving date format but safeguarding precision.
    if ( abs(tdb1)>abs(tdb2) ) then
       d = t77td - tdb1
       f = tdb2 - tdb0
       tcb1 = tdb1
       tcb2 = f - ( d - ( f - t77tf ) ) * elbb
    else
       d = t77td - tdb2
       f = tdb1 - tdb0
       tcb1 = f - ( d - ( f - t77tf ) ) * elbb
       tcb2 = tdb2
    end if

    !  Status (always OK).
    j = 0

    end subroutine TDBTCB