[amsat-bb] Re: Sun shadow for True North

Tony Langdon vk3jed at gmail.com
Fri Dec 28 16:06:04 PST 2007

At 05:29 AM 12/29/2007, Dave Guimont wrote:

>A vertical rod about 4' long (plumb it, or use a level) will have the
>shortest shadow at noon.

Not necessarily.  Ignoring DST, local solar noon can occur anywhere 
within a window of around 1 hour (give or take, depending on the 
exact borders of your local time zone), because each time zone is 
approximately 15 degrees wide (there are local variations that take 
the official zones beyond the 15 degree nominal zones).  For example, 
here in Melbourne, local solar noon usually occurs around 
approximately 02:15 - 02:20 UTC (12:15 - 12:20 standard time or 13:15 
- 13:20 DST).  In Brisbane, it occurs before noon AEST, despite both 
cities being in the same time zone.  Using the "shortest shadow at 
noon" method, you can have an error of up to 10 degrees (now THAT 
would ruin the performance of your high gain antenna!).  Here, it 
would be around 4-5 degrees, if one was ignorant of the actual time 
of solar noon.

The moving shadow to find the East - West line is a much better and 
MUCH more accurate idea.

73 de VK3JED

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