| Re: Julian Day Question [message #48875 is a reply to message #48874] |
Fri, 26 May 2006 07:05   |
Mike Wallace
Messages: 25
Registered: May 2006
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Junior Member |
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> 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)
I should have added that I always specify hours minutes and seconds when
working with julday() for this very reason and because I specify the
time within the day, I get what I expect. Come to think of it, julday()
without the hours minutes and seconds also gives me what I expect,
however what I expect is a weird number because some astronomer thought
he was being smart by having the day boundary be at a time when he
wouldn't be observing. I guess it has to deal with your expectations
and since I've looked at Julian Day numbers and other weird time systems
for years now, I've become conditioned to it. After working with
things like Ephemeris Time and Barycentric Dynamical Time, Julian Day
seems pretty easy. :-)
-Mike
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