Aside from an intellectual curiosity, which amateurs ? aside perhaps from
EME or terrestrial VHF/UHF enthusiasts ? need to know north any more
accurately than something such as an iPhone provides?
It seems the beamwidth of even the longest HF yagi renders resolution better
than a couple of degrees moot, especially given the direction circuits of
most rotators are hardly precision instruments themselves.
I'm not trying to discourage what is a great discussion on geophysics, just
curious about the practical application for the majority of hams.
73, kelly
ve4xt
On 12/25/14 10:50 AM, "Gary - AB9M" <glhuber@msn.com> wrote:
> For greater precision, say about 0.33 degrees use the Polaris - Kochab
> method as explained in US ARMY FM 6-50
> (http://www.globalsecurity.org/military/library/policy/army/fm/6-50/Ch5.htm).
> Many years ago, I used the Polaris - Kochab method for directional control,
> first finding True North, then the Direction of Fire, to align Howitzers,
> for the precision flying (in an arc) of thirty-five pound projectiles.
>
> Once you get the telescope properly aligned by Polaris - Kochab, you only
> have to lower it verticly to the ground. The line from the telescope to the
> sighted spot on the ground is to True North.
>
>
> 73 & DX,
>
> Gary - AB9M
> -----Original Message-----
> From: Patrick Greenlee
> Sent: Thursday, December 25, 2014 9:55 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] north
>
> 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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