Biblical Hebrew Forum

[James Read <J.Read-2 AT sms.ed.ac.uk>]

Hi Rolf,

this is a quick and impulsive first reaction response from my  
experience as a computer scientist.

I wouldn't pay too much attention to the similarity of the results in  
the programs. They are probably all using the same underlying  
algorithm and only really differ in the front end. So, the assumption  
is that the algorithm is correct.

Concerning errors, there are a number of potential sources. One source  
of error is whether the algorithm is correct. A good example of this  
is the evolution of our understanding of the length of a solar year.  
As our understanding has increased our algorithm has become more  
accurate.

The second, and perhaps most important, source of error for you  
consideration is the fact that computer simulation make frequent use  
of floating point numbers. The implementation of floating point  
numbers means that, unlike integers, they are approximations of  
values. The implication of this is that the more sums we make using  
floating point numbers the greater our incurred error due to rounding  
errors. A good program should calculate the boundaries of the error  
and make them known to you.

A third source of error the principle of GIGO (garbage in garbage  
out). Such a program needs an accurate starting point to work back  
from. Minimal differences in the actual present state of the universe  
could have significant impacts on the output of the program. An  
example that is often quoted is the butterfly effect. Even the best  
climate change simulations on the largest super computers on earth  
make significant mistakes because we do not know the exact present  
state of the earth to feed into the algorithm (and even if we did know  
it, it would take too much data to input let alone process). I'm not  
saying your astronomical simulations will be way off because of this.  
I'm just making you aware of the issues.

Anyway, thanks for you answer. If I get time I'll look into the  
programs you mention.

James Christian

Quoting Rolf Furuli <furuli AT online.no>:

> Dear James,
>
> While I have a very good background in Hebrew and Akkadian (having
> taught both languages for 12 years), I have little background in
> mathematics and astronomy. So here I have to rely on others. I use
> four astro-programs: TheSky6., SkyMap Lite, CyberSky 4, and TheSky X
> (for Macintosh). As for solar and lunar eclipses, all the programs
> give the same information, and this is the same as can be found at
> NASA.
>
> Some of the positions of the moon and the planets found in old
> cuneiform tablets (some being real observations, an quite a lot being
> backward calculations) have very specific measures; some go down to
> the equivalent to 1/2 degree, and others even to a few minutes. In
> these specific cases there can be some problems because the programs
> sometimes differ, in special cases up to 2 degrees. I have discussed
> the reason for this with several US astronomers, and they have told
> me that the differences probably are caused by slightly different
> mathematical formulas in the different programs. Their advice has
> been to use TheSky X or the related TheSky6, because these are the
> most accurate programs. I have a colleague at the University of Oslo,
> who is professor in astrophysics with archaeoastronomy as one of his
> special fields. He has at my request made some calculations-not using
> any of the mentioned programs-and his results are almost exactly the
> same as those found by the help of TheSky X. I therefore think that
> the positions I have found has a good foundation.
>
>
> Best regards,
>
> Rolf Furuli
> University of Oslo
>
>
> > Sounds like good stuff. That's the kind of thing I've been meaning
> > to look into for a long while.
> >
> > What sources do you use for astronomical events such solar eclipses etc?
> >
> > James Christian
>
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