Finding True South (and Why Your Compass Lies)

Winter sun in the northern hemisphere crosses the southern sky. Collectors, glazing, and the windows you hope will heat a room want to face true south, which is a direction on the earth, not the direction a compass needle prefers. The needle points at magnetic north. The angle between magnetic north and true north is magnetic declination. It depends on where you stand, and it drifts over the years. A correction copied from an old book, or from a town a few hundred miles away, can turn a south wall into a southeast wall.

A stick at solar noon finds true south without depending on a sign convention. Use it first, and use a compass afterward only with care. The site survey is where the line you mark gets drawn onto the house.

This guide assumes the northern hemisphere. In the southern hemisphere the noon shadow points true south, and the winter sun is in the northern sky. Swap the words and keep the stick.

Difficulty

Level

Beginner

Time

A few minutes at solar noon, or a half hour with a compass and a lookup

Skills

Telling time and marking a line you can still find tomorrow

Cost estimate

Free

A stick and a clock are enough. A hardware compass is optional and is wrong until you correct it.

Materials

Item	Qty	Notes
A straight stick	1	Set on ground that is level enough that the stick is vertical.
The solar noon time for your town and date	1 lookup	Clock noon is not solar noon. Time zones and daylight saving time move it.
A current declination for your town	1 lookup	Only if you insist on using a compass. Read the east or west note that comes with the number.

The stick

Plant the stick upright. Solar noon is the moment the sun is highest, which is also the moment the shadow is shortest. At that moment the shadow points true north. The opposite direction is true south.

Look up solar noon for your location and your date. It can sit well away from 12:00. If you can watch the shadow, mark it every few minutes around that hour and keep the shortest one. Drive a nail or set a stone at the tip so the line survives the afternoon.

Transfer the line to the wall or the roof you care about by sighting along it, not by memory. A string pulled tight on the ground is easier to trust than a glance.

Why the compass is offset

The earth's magnetic field is not aligned with the poles used on a map. Declination is the angle between them. Agencies publish it as east or west for a place and a date. A rule you will see on many maps: add an east declination to a magnetic bearing to get a true bearing, and subtract a west declination. The notes on the calculator you used win if they disagree with that sentence. Reverse the sign and the collector ends up aimed at the wrong half of the morning.

Phone compasses add their own errors. Steel beams, cars, speakers, and the reinforced slab under you can pull the reading by tens of degrees. Calibrate the phone outdoors, away from the house, and then still check it against the stick once. If they disagree, keep the stick.

Do not store a declination in your head for next year in a different town. The correction belongs to a place and a date, so look it up again.

How close to south is close enough

A surface within about 20 degrees of true south still does most of a winter heating job and most of a photovoltaic job. The loss grows as you turn further. Around 45 degrees you are on a southeast or southwest face: useful, but a different machine, with more morning or afternoon output and less midwinter noon. A due east or due west wall is a cooling problem in summer and a weak winter heater.

You do not need a surveyor's transit for a window heater. You do need to know whether the wall you like is actually south, or whether it only feels south because the street is.

1. Look up solar noon

   Use your town and today's date. Write the clock time in the notebook before you go outside.

2. Mark the shortest shadow

   The shadow points true north. Mark the opposite ray as true south, with something you can find later.

3. Only then correct a compass

   Look up the current declination, read whether it is east or west, and apply the rule printed with that lookup. Check the result against the stick.

4. Write the bearing of each candidate surface

   A south wall, a roof plane, a patch of yard. The site survey and the shade map use those bearings.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

• https://www.builditsolar.com/SiteSurvey/site_survey.htm
• https://www.builditsolar.com/SiteSurvey/FindingSouth.htm
• https://www.builditsolar.com/SiteSurvey/MagDecl.htm
• https://www.builditsolar.com/SiteSurvey/StepByStep.htm
• https://www.builditsolar.com/SiteSurvey/challenges.htm
• https://www.builditsolar.com/GettingStarted/GettingStarted.htm
• https://www.builditsolar.com/GettingStarted/ChoosingProjects.htm
• https://www.builditsolar.com/References/SunChartRS.htm