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JBEPOCH

JBEPOCH

Name


  JBEPOCH

Author


  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  craigm@lheamail.gsfc.nasa.gov

Purpose


  Compute Julian Day to/from Julian or Besselian Epoch

Calling Sequence


  EPOCH = JBEPOCH(/B, JDAY) ;; Julian Day to Besselian Epoch
  EPOCH = JBEPOCH(/J, JDAY) ;; Julian Day to Julian Epoch
  JDAY = JBEPOCH(/B, EPOCH, /TO_DAY) ;; Besselian Epoch to Julian Day
  JDAY = JBEPOCH(/J, EPOCH, /TO_DAY) ;; Julian Epoch to Julian Day

Description



  The function JBEPOCH computes the Julian or Besselian Epoch year
  number from a given Julian day number. Epochs of this form are
  often given in the astronomical literature as B1950.0 or J2000.0,
  but they can be different.
  Besselian year numbers are measured in tropical years of about
  365.2422 days. Julian year numbers are measured in years whose
  lengths are exactly 365.25 days of 86400 second lengths. The "/J"
  or "/B" keywords identify which year numbering system is being
  used.
  JBEPOCH also computes the inverse transformation, from Julian or
  Besselian epoch to Julian Day, by specifying the /TO_DAY keyword.
  The computational logic is inspired by STARLINK (P.T. Wallace).

Inputs



  EPOCH or JDAY - If TO_DAY is set, Besselian or Julian year number.
                  If TO_DAY is not set, the Julian day number.

Keywords



  B - if set, then year numbers (input/output) are expressed in
      Besselian years.
  J - if set, then year numbers (input/output) are expressed in
      Julian years.
  TO_DAY - if set, then convert EJ (assumed to be year number) into
            Julian day number.
  MJD - if set, then Julian days are expressed as "modified" Julian
        Days, or Julian days minus 2400000.5d.

Returns



  If TO_DAY is set, then returns Julian Days.
  If TO_DAY is not set, then returns year number.
 

References



  Lieske, J. H. 1979, Astron & Astrophysics, 73, 282
  Wallace, P. T. 1999, SLALIB Software Library (STARLINK)
    http://star-www.st-and.ac.uk/starlink/
    (Routines sla_EPB2D, sla_EPJ2D, sla_EPB and sla_EPJ)

See Also



  JULDAY, CALDAT

Modification History


  Written, CM, 04 Mar 2002
  Documented, CM, 22 Mar 2002
  $Id: jbepoch.pro,v 1.3 2002/03/22 22:02:02 craigm Exp $



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