[TowerTalk] north

[Jim Lux]

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.