# Re: Sun and Instant Track - errors - unclassified

Hi Peter:
Good observations on the accuracy of IT in getting North near noontime.
What you really want is what is called "local apparent noon" which is most
easily done with IT.  I've even challenged the second mate on merchant
ships in this calculation using IT!.  What I was able to do was use
something like a 0.1 minute interval and as long as you present LAT/LONG
is entered, find the point where the elevation is maximum.  After all,
this is all that a seagoing navigator does with a great deal of effort!
Usually this is within fifteen minutes of noon, and of course this is far
more accuracy than needed for a sundial, but I understand the effort.
For sundial work, Peter, all that your son need do is use IT to find
sunrise and sunset.  Divide that day in half and use that time to set his
sundial.  Works great and is the best method known to set NORTH as the
shadow presented is directly North without any corrections whatever!
Cliff K7RR

On Mon, 16 Feb 1998 Peter.ELLIS@dao.defence.gov.au wrote:

>
>
> Not much to do with satellites, or is it???
>
>
> Over the weekend I was using IT to predict the azimuth and elevation of the
> sun at various hours UTC.
> (My eldest son, just a few weeks into his High School career, was designing
> a sun dial...)
>
>
> It became apparent that the azimuth and elevation figures from IT were
> sometimes different if I:
> (a) let the systems run up to the hour, or
> (b) used the T function to enter an hour and calculate an observation (i.e.
> a one-time calculation).
>
> It seemed that the one-time calc (T function) gave the 'correct' azimuth
> (see method under). This was most apparent when IT was free-running at a
> minute or two either side of the hour then I entered T and the hour/minute.
> The angle would sometimes jump by 5-8 degrees.
>
> My concern is that IT (or my computer) may be creating a regime of errors.
>
> Has anyone any ideas on what is going on here?
>
> Thanks,
> Peter
>
>
> Method:
> - We established a 'sun dial' with the axes aligned true N-S / E-W.
> - On the hour, we observed the sun azimuth and marked the shadow of the
> gnomon (the post of the sundial).
> - We measured the angle from true north with a protractor.
> - We used IT to calculate / check / predict the angles.
>
>
>      _  .      Peter R. Ellis, Amateur Radio Op. VK1KEP
>    _-' |_|\         at Canberra, Australian Capital Territory
>   /        \        email: peter.ellis@dao.defence.gov.au
>   \_.--._ C/