Info-to-Build-On Finding True South Without a Compass. by Raoul Hennin April 25, 2003

Summary:
1) Calculate Solar Noon - NOAA has a nifty web site that calculates this for you:

2) Hang a plumb bob in the sun (a little before the time of solar noon).

3) Precisely at solar noon, trace the shadow the plumb bob string casts on a horizontal surface - this is the line of true north/south!

Why not a compass?
A compass does indicate magnetic north/south, however the needle is awfully small and it can be tricky to get accurate readings. Even after steadying the compass and determining what it is trying to say, you still have to convert "magnetic" north to "true" north. This conversion is no simple matter because the lines of magnetic flux on the surface of the earth vary significantly depending on your region. In Maine we add15-18° to magnetic north to arrive at true north. In some parts of California, we would subtract a 15-20° from magnetic north to arrive at true north.

If not with compass, then how?
Every day the sun rises in the east and sets in the west. At some point during the day it crosses over the midpoint: solar noon. At the exact moment of solar noon, a vertical pole will cast a shadow that is precisely oriented along the north/south line. This line is accurate and easy to create using the string of a plumb bob.

When is Solar Noon?
Solar Noon is a simple concept (when the sun is half way across the sky in any given day). The problem is that when our watch says 12:00, the sun is rarely half way across the sky - there are three reasons for this:

1) Time zones artificially place large geographic areas on the same clock time (hint: when solar noon occurs in Calais, Maine, the sun still must travel several hundred miles before reaching solar noon over Buffalo, New York - yet these two metropoli are in the same time zone.) Basically, every degree of Longitude away from the Standard Time Meridien for your time zone counts for 4 minutes of clock time.

2) Equation of Time: as it turns out, days on planet earth are not of constant length. As the earth orbits the sun, our speed of rotation on our own axis actually varies slightly each day. This means that we must take this variation into account when calculating solar noon: in February the sun is more than 14 minutes behind schedule while in October it is more than 16 minutes ahead. You can see a graph of the equation of time at: http://www.srrb.noaa.gov/highlights/sunrise/eqntime2.gif

3) Daylight Saving Time - don't forget to adjust the calculation for the appropriate method of time keeping. Daylight Saving Time starts the first Sunday of April and ends the last Sunday of October.

Ready to Hang your Plumb Bob?
Go to the NOAA web site (http://www.srrb.noaa.gov/highlights/sunrise/sunrise.html) to calculate "Solar Noon". This page also calculates the Equation of Time, Apparent Sunrise and Sunset. Make sure you click the Daylight Saving Time checkbox after the first Sunday in April (and before the last Sunday in October). Once you enter your latitude and longitude (or a city near you), click the "calculate sunrise/sunset" button and the time of solar noon will appear in its box below.

Now you are ready to hang your plumb bob in the sun and mark true south at solar noon on the (horizontal) ground! (Make sure you have the right time on your watch.)

Building Context
Finding true south is only the beginning of orienting a structure on-site. In the Shelter Institute Climate Class, we build on a series of similar steps to capture maximum heating from the low winter sun and optimal shading from the high summer sun. Many other site-specific variables come into play in on-site orientation of a new home.

If you are interested in pursuing this topic, visit the Shelter Institute Campus and sit in on an upcoming Climate Class - see our schedules at:

We also recommend the following book:

Sundials, Their Theory and Construction by Albert E. Waugh, Dover, 1973. This is the original and still definitive work on sundials and time-keeping relative to the sun.