Here is an amsat-bb post from Tom Clark, K3IO, formerly W3IWI, on this subject. Wonderfully detailed. Thank you Tom. I have removed the From line since the original address from 12 years ago may not be accurate. ron w8gus ==== Date: Tue, 17 Jan 95 20:12:13 UTC From: Tom Clark To: [email protected] Subject: North/South vs East/West
The recent thread of Magnetic vs. Geodetic north-south for the calibration of antenna azimuths has overlooked an even easier way to do the deed. The idea is not new -- I believe it is the way the ancient Egyptians had their Nubian slaves establish the orientation of the great pyramids to accuracies of about a half-degree. Instead of trying to establish a North/South line, determine East/West instead!
Here is the recipe (with some footnotes):
1. Wait for the morning of a clear, sunny day. 2. Find a level, flat area. 3. Poke a 3-6 foot (1-2 meter) stick in the ground vertically (south of the flat area in the northern hemisphere or north in the southern hemisphere). 4. Several times during the day, mark the location of the tip of the shadow of the stick. 5. The points you just marked will be on a straight line which is nominally East/West. Drive stakes at the ends of the line (or if your shadow is on concrete, paint the line).
Footnotes:
a. The accuracy of the east/west line is best at the solstices (Dec.21/Jun.21) and worst at the equinoxes (Mar.21/Sep.21). The sun's declination is a cosine curve with extrema of +/- 22.5 degrees reached at he solstices. At the equinoxes, the declination changes about 1/2 degree/day so if you are observing at the equinoxes over an 8-hour period, the line will be in error by about 10 arc minutes.
b. The diameter of the sun's disk is about 30 arc minutes, so the shadow is "fuzzy" at this level. You need to be consistent in picking the point corresponding to the tip's shadow to minimize the penumbral error. The diameter of the stick should be big enough to give a solid shadow -- i.e. the stick should subtend an angle > 30 arc minutes when viewed from the level, flat area. I recommend that the stick have a length/diameter ratio of 20-30. A typical tall antenna tower will not cast a sharp enough shadow to use.
c. The level area need only be level in the east/west direction. A north/south slope will not affect the accuracy. If you don't have a level area, get a 4'x8' sheet of plywood and prop it with short stakes to make it level and cast the shadow onto the plywood. You should strive for east/west "levelness" of one degree or better if you want results good to about one degree.
Hope this idea helps -- 73 de Tom.