True South Marker Using the Sun

A true south marker on the ground about 15 feet from the antenna is very useful when setting up a polar mount satellite dish.   The most accurate way to set a true south ground marker is to use the sun.  Here is how you can do that.  This sounds way more complicated than it really is.  You can do all this in just a few minutes once you learn the process.

In the early morning or late afternoon, lay out a shadow line that crosses a point about 15 feet south of your antenna mast.  Mark the shadow with two little metal rods.  Note the exact time when you mark the shadow.  To make a shadow you could stand up a 2x4 piece of lumber and hold it plumb/vertical. Use a carpenters level.

Make a little drawing like the one above.  Measure the distance from the center of the mast to each of the markers and the distance between the two shadow markers.  Write the measurements on your drawing.

Now go inside and use a computer and the internet to find the azimuth of the sun and calculate where to set the true south marker.

If you don't already know your Latitude and Longitude, find it here:

http://itouchmap.com/latlong.html
 

Find your location on the map and write down your Degrees, Minutes, and Seconds latitude and longitude.
 

Go here for sun azimuth information:

http://aa.usno.navy.mil/data/docs/AltAz.php

Enter your city and state in form A or for more exact information Use form B and enter the latitude and longitude you wrote down earlier. For the United States, longitude is West and latitude is North.
 

Set tabular interval at 1 minute.
 

Time zone for USA:
Eastern time zone is 5 hours west of Greenwich
Central time zone is 6 hours west of Greenwich
Mountain time zone is 7 hours west of Greenwich
Pacific time zone is 8 hours west of Greenwich

Click the Compute Table button.
 

The column on the left shows the time in hours and minutes. Daylight time is not implemented in this program.  Daylight time is one hour ahead of the time in the table.  If you marked a shadow at 9:02 am daylight saving time, look in the table for 8:02.  If you mark a shadow at 5:02 pm daylight saving time, look in the table for 16:02.  Find the time of your shadow and write down the azimuth of the sun.

Look at the drawing below.  If the sun azimuth is 110, the other end of the azimuth is 110 plus180 degrees which is 290.  Angle b of 70 degrees is 360 minus 290.

Now go to a triangle calculator and enter the three distances that you measured as the sides of a triangle. The distances were from the center of the mast to each of the shadow marker rods and the distance between the two shadow marker rods.  The triangle calculator will give you the angles of the three corners. (Not angle b)

You only need Angle a and Angle b which you calculated earlier.  And you need the side length labeled above as Distance a.  Now use the triangle calculator and enter Angle a, Angle b, and Distance a.

The triangle calculator will give you Distance b and Distance c which will allow you to go outside and set in the Marker Rod that will be exactly true south from the mast.  It's really quite simple.