Name

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    JULDATE

Purpose

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    Convert from calendar to Reduced Julian Date

Explanation

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    Julian Day Number is a count of days elapsed since Greenwich mean noon
    on 1 January 4713 B.C. The Julian Date is the Julian day number
    followed by the fraction of the day elapsed since the preceding noon.
    This procedure duplicates the functionality of the JULDAY() function in
    in the standard IDL distribution, but also allows interactive input and
    gives output as Reduced Julian date (=JD - 2400000.)
    (Also note that prior to V5.1 there was a bug in JULDAY() that gave
    answers offset by 0.5 days.)

Calling Sequence

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    JULDATE, /PROMPT ;Prompt for calendar Date, print Julian Date
              or
    JULDATE, date, jd

Input

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    DATE - 3 to 6-element vector containing year,month (1-12),day, and
              optionally hour, minute, and second all specified as numbers
              (Universal Time). Year should be supplied with all digits.
              Years B.C should be entered as negative numbers (and note that
              Year 0 did not exist). If Hour, minute or seconds are not
              supplied, they will default to 0.

Output

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      JD - Reduced Julian date, double precision scalar. To convert to
              Julian Date, add 2400000. JULDATE will print the value of
              JD at the terminal if less than 2 parameters are supplied, or
              if the /PROMPT keyword is set
     

Optional Input Keyword

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      /PROMPT - If this keyword is set and non-zero, then JULDATE will prompt
              for the calendar date at the terminal.

Restrictions

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      The procedure HELIO_JD can be used after JULDATE, if a heliocentric
      Julian date is required.

Example

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      A date of 25-DEC-2006 06:25 UT may be expressed as either
      IDL> juldate, [2006, 12, 25, 6, 25], jd
      IDL> juldate, [2006, 12, 25.2673611d], jd
      In either case, one should obtain a Reduced Julian date of
      JD = 54094.7673611
  PROCEDURE USED:
      GETOPT()

Revision History

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      Adapted from IUE RDAF (S. Parsons) 8-31-87
      Algorithm from Sky and Telescope April 1981
      Added /PROMPT keyword, W. Landsman September 1992
      Converted to IDL V5.0 W. Landsman September 1997
      Make negative years correspond to B.C. (no year 0), work for year 1582
      Disallow 2 digit years. W. Landsman March 2000