| Re: Julian Day Question [message #48880 is a reply to message #48879] |
Thu, 25 May 2006 22:52   |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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Mike Wallace <mwallace.no.spam.please@swri.edu.invalid> writes:
>> Anyone have any ideas about this? Is a Julian Day number
>> a "standard" in the sense that OpenGL is a standard?
>
> Julian Day is simply the number of days past 12 noon UTC on January 1,
> 4713 BC. Nothing more. Nothing less. However I have seen people use
> the term "Julian Day" for a quantity that is not actually a Julian
> Day, but something derived from a Julian Day. There are Modified
> Julian Days and Truncated Julian Days and Reduced Julian Days among
> many others. There's also a Julian Year, but despite the name it has
> absolutely no relationship to Julian Days.
>
> I will say that the IDL Julian Day routines give me what I expect to
> see. Perhaps the others are just variations.
My experience is that the naive usage of the IDL JULDAY function does
not give me what I expect to see. For a given calendar date, say
JULDAY(5,26,2006), I normally expect this to refer to midnight at the
start of the day (JD 2453881.5), whereas IDL returns the Julian day at
noon, twelve hours later (JD 2453882). Of course, if one specifies
hours, minutes and seconds, then the proper result pops out.
I.e., I would naively expect these to be the same but they are not:
JULDAY(5,26,2006, 0,0,0)
JULDAY(5,26,2006)
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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