[TowerTalk] Positoners Calibration to True North

[Jim Lux]

At 09:24 PM 12/1/2005, AD5VJ  Bob wrote:
>Doesn't the location of the sun and Polaris depend on the
>month and season of the year?


The position of the sun at "local solar noon" is always on the N/S line, 
what changes over the year is the time offset between local solar noon on a 
particular day, and mean "noon" averaged over the year.  That is, the "wall 
clock time" for local solar noon varies a bit.  The difference is pretty 
substantial (minutes).

So, if you want to do the "north by shadow" technique, you either have to 
actually measure it on the day in question (by the bisecting equal shadows, 
approach, for instance), or fire up one of the websites or calculators that 
figures all this out.  I believe the USNO website has a calculator where 
you can enter lat/lon/date and get the local time of solar noon.

If you go look up Analemma or Equation of Time, there'll be more 
details.  For instance, http://www.analemma.com/ has some nifty animations 
of this.

This also comes up if you're doing some form of celestial navigation (e.g. 
using a sextant to find position)




>   73 fer nw,
>Bob AD5VJ