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True North in One Sentence (again)

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Subject: True North in One Sentence (again)
From: (Bill Coleman AA4LR)
Date: Wed, 8 Jan 97 11:15:47 -0500
>From:        Dean Norris,
>At 06:53 1/7/97 -0800, you wrote:
>>"At the time (local) half way between local sunrise and local sunset,
>>the Sun, if shining, will cast a shadow exactly (true) North-South."

Actually, there's some variation in sunrise/sunset times. (ie the latest 
sunrise and earliest sunset don't occur on Dec 21. Instead one occurs two 
weeks before, the other two weeks after. Same thing for earliest sunrise 
and latest sunset.) This is due to the fact that as the earth rotates, it 
also moves around the sun. Since the orbit isn't exactly circular, the 
speed of the orbit varies (hence, the sun appears to speed up/slow down 
in its progress across the sky). 

>>Just use the local weather forcaster's sunrise/sunset times and it's
>>accurate for amateur antenna alignment for sure!

Maybe not. The summer variation can be rather significant (a few 
degrees). Best time for this is probably at or near the solstice or 

>I live in a location where the takeoff angle to horizon to the east is
>virtually zero degrees, in other words, straight out.  To the west the
>horizon is blocked by tall mountains and the takeoff angle is probably 20
>degrees.  Will this make a difference to the sunrise and sunset times.?

Not if you use the local weather forecaster's times.

The better technique is probably the shortest shadow technique. The 
objective is to determine which way the shadow is pointing at local noon. 
(Although this is probably subject to sunrise/subset variation as well)

I think going to a pilot shop, looking at a chart and finding the nearest 
isogonic line and then correcting your magnetic compass for magnetic 
variation is probably the simpliest and most accurate.

Bill Coleman, AA4LR           Mail:
Quote: "Not in a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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