For my part, Polaris - Kochab was just a piece from my working past sitting
unused until the discussion started in the direction of greater precision
and higher education. If one is going to use Polaris to find true north,
(not that precise unless the Polaris, 43H Cephei, and Delta Ursa Minoris
method is used) one should do it correctly.
73 & DX,
Gary - AB9M
-----Original Message-----
From: Kelly Taylor
Sent: Thursday, December 25, 2014 11:22 AM
To: Gary - AB9M ; towertalk@contesting.com
Subject: Re: [TowerTalk] north
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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