| Julian Day NE Julian Day [message #93896] |
Fri, 18 November 2016 04:30  |
andrewcool777
Messages: 27
Registered: November 2012
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Junior Member |
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Hi All,
So in converting the time stamp from an astro image to Julian Day with JULDAY, I start off with seconds of 6.31, and after doing the back conversion with CALDAT, I end up with seconds of 6.3100353!
Is this to be expected? Is it "wrong?" Is it one of those weird digital precision
things that David F. used to bang on about? (Hi David)
IDL> print,!version
{ x86_64 Win32 Windows Microsoft Windows 8.5.1 Nov 14 2015 64 64}
IDL>
IDL> Jd = julday(10,1,2016,10,10,6.31)
IDL> print,jd,format='(F20.10)'
2457662.9236841439
IDL> caldat,jd,mon,d,y,h,m,s
% Compiled module: CALDAT.
IDL> print,y,mon,d,h,m,s
2016 10 1 10 10 6.3100353
And just to show that I'm calling the inbuilt IDL routines, and not altered copies:-
IDL> help,/source
Compiled Procedures:
$MAIN$
CALDAT C:\Program Files\Exelis\IDL85\lib\caldat.pro
Compiled Functions:
JULDAY C:\Program Files\Exelis\IDL85\lib\julday.pro
I'm puzzled. And a bit concerned.
TIA,
Andrew Cool
www.skippysky.com.au (for all your astro weather forecast needs...)
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| Re: Julian Day NE Julian Day [message #93897 is a reply to message #93896] |
Fri, 18 November 2016 06:43  |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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Yes, this is a precision issue. As noted in
http://aa.usno.navy.mil/data/docs/JulianDate.php
"Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision."
and the IDL results are "equal" to within 1 millisecond.
One way to improve the precision is to use modified Julian date
MJD = JD - 2400000.5 The highest precision Astronomy routines ( http://www.iausofa.org/index.html ) break up Julian date into two double precision numbers in order to maintain precision.
-Wayne
On Friday, November 18, 2016 at 7:30:34 AM UTC-5, andrew...@gmail.com wrote:
> Hi All,
>
> So in converting the time stamp from an astro image to Julian Day with JULDAY, I start off with seconds of 6.31, and after doing the back conversion with CALDAT, I end up with seconds of 6.3100353!
>
> Is this to be expected? Is it "wrong?" Is it one of those weird digital precision
> things that David F. used to bang on about? (Hi David)
>
>
> IDL> print,!version
> { x86_64 Win32 Windows Microsoft Windows 8.5.1 Nov 14 2015 64 64}
> IDL>
> IDL> Jd = julday(10,1,2016,10,10,6.31)
> IDL> print,jd,format='(F20.10)'
> 2457662.9236841439
> IDL> caldat,jd,mon,d,y,h,m,s
> % Compiled module: CALDAT.
> IDL> print,y,mon,d,h,m,s
> 2016 10 1 10 10 6.3100353
>
> And just to show that I'm calling the inbuilt IDL routines, and not altered copies:-
> IDL> help,/source
> Compiled Procedures:
> $MAIN$
> CALDAT C:\Program Files\Exelis\IDL85\lib\caldat.pro
>
> Compiled Functions:
> JULDAY C:\Program Files\Exelis\IDL85\lib\julday.pro
>
>
> I'm puzzled. And a bit concerned.
>
> TIA,
>
> Andrew Cool
>
> www.skippysky.com.au (for all your astro weather forecast needs...)
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