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Re: [TowerTalk] north

To: towertalk@contesting.com
Subject: Re: [TowerTalk] north
From: Patrick Greenlee <patrick_g@windstream.net>
Date: Thu, 25 Dec 2014 09:55:10 -0600
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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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