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JBEPOCH

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)
(Routines sla_EPB2D, sla_EPJ2D, sla_EPB and sla_EPJ)

JULDAY, CALDAT

Modification History

Written, CM, 04 Mar 2002
Documented, CM, 22 Mar 2002

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