[TowerTalk] north

[Patrick Greenlee]

Ahh, what fun.  Be careful shaving Phill as he gets razor rash easily.  
I too am a bit into astrophotography and a little star gazing with or 
without my ancient Celestron Pacific 8 inch reflector.  I also dabbled 
in using a sextant for navigating back in my sailing days.  Was maritime 
mobile for 9
years.

You can get local noon on dry land with a sextant and a bowl of water as 
a horizontal reflecting plane.  you can get a very precise local noon 
and south. By taking a series of sights prior to, near, and after local 
noon you can deduce the time and direction of local noon to an arbitrary 
accuracy dependent on your observing conditions and skill taking the 
sights.  It is quite easy and you don't need published tables of data or 
an expensive instrument.  A cheap plastic sextant will git 'er done 
better than you need for aiming antennas.

I personally favor a vertical rod driven in the ground and checked 
carefully for verticallity.  I put small pins in the ground where the 
tip of the shadow is at any given time.  The series of stakes mark a 
smooth curve with equal periods of time indicated on either side by 
equal distances and it easy to see the point where the shadow is 
centered (also longest shadow.)  This is true north from the vertical 
rod (gnomon.  The shadow cast by the gnomon has two shades of gray, the 
umbra and penumbra.  This finite width is NOT a problem, just use the 
center of the shadow.  The shadow width is a function of the rod width 
and the fact that the sun is not a point source. In practice neither 
matter much as it is easy to determined the center of the shadow cast by 
the gnomon.

This thread inspires an experiment.  I will use a non conductive 
(plastic) dodad attached to the antenna directly above the mast and plot 
its shadow position with small stakes starting before and running past 
noon, as per clock time to ensure I start before and continue past local 
noon far enough to give an interpretable curve. I can then easily find 
the location of the shadow at local noon and drive a permanent stake.  
Thereafter sighting through the center of the tower to that permanent 
stake will give me a true north reference.

All caveats listed below are given due consideration.

No ground hogs will be inconvenienced by this endeavor.

Patrick    NJ5G




On 12/25/2014 7:31 AM, Jim Lux wrote:
> On 12/24/14 10:39 PM, Spencer wrote:
>> In the northern hemisphere,  the shadow of a vertical object at solar
>> noon will point north.
>>
>
> I've done all these North finding techniques over the years as kind of 
> a hobby activity (and sometimes professionally, too)
>
>
> In mid-latitudes the shadow and stick works fairly well, although it's 
> hard to get sub-degree precision (for your North facing owl?) because 
> of several reasons:
> 1) The stick has to be vertical to within the accuracy of your desired 
> measurement.
> 2) the sun is half a degree wide, so the shadow edges are indistinct
> (you could check this in a few weeks, if you shave the groundhog 
> first, so the fur doesn't cause the indistinct edge)
> 3) the sun moves pretty fast, so it's hard to tell the exact instant 
> of solar noon.  The earth turns a degree in 4 minutes.  Of course, you 
> don't need a precisely aligned antenna to receive WWV, and, knowing 
> your longitude, you can calculate when solar noon is. (don't forget 
> the "equation of time")
> 4) the shadow is pretty short at noon unless you're well north (in the 
> summer).
>
>
> At 34 degrees north, in the summer (prime tower building season), the 
> shadow is pretty short. The sun's only 10 degrees from vertical at the 
> solstice.  At La Paz or Los Cabos, Baja California, on the solstice, 
> the sun is directly overhead at noon (or so close you won't be able to 
> easily tell the difference)
>
> It's MUCH easier to get north if you mark the shadow periodically. 
> You'll get a curve that you can then use to determine north.
>
> What you do is make a smooth curve of shadow position during the day. 
> Then, you take a string from the *top* of the stick and scribe a semi 
> circle across the curve of constant radius.  Draw a line between the 
> two intersections of curve and semicircle.  The perpendicular bisector 
> of that line faces north/south.
>
> (you still have the 1/2 degree width of the sun to contend with, and 
> the ground has to be level within a fraction of a degree, etc.)
>
>
>
> If you're "between the tropics" (of Cancer and Capricorn) the shadow 
> goes both north and south, depending on the time of year. Think of a 
> stick on the equator: from March equinox to September equinox, the sun 
> goes north, so the shadow is south, and from September to March, the 
> sun goes south.
>
> And for hams aligning their rotor in midwinter in the far north? No 
> shadow at all.
>
> By the way, the north star (Polaris) is about 3/4 degree from true 
> north. It moves around the true pole in a circle. You can look up the 
> position at a given time/date, or watch it in a telescope and look for 
> culmination.
>
>
> Moral of the story:  getting true "north" to even one degree accuracy 
> is harder than it seems at first glance.
>
>
>
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