Find true south with a vertical stick: mark its shadow tip in the morning, draw a circle of that radius, and mark where the afternoon shadow tip touches it again; the line between the two marks runs east-west, and the perpendicular to it is the true north-south line. A magnetic compass points to magnetic north, which can differ from true north by 5-20 degrees or more.
Part of Passive Solar Building Design
Methods for setting out true north-south from the sun, and for finding how far a compass is wrong at your site.
True South Is Where the Sun Is at Its Highest
The sun is at its highest, and its shadow shortest, at solar noon. At that moment the shadow lies along the true north-south line. The carrier states the noon-shadow rule: the shadow points true north in the Northern Hemisphere (so the sun is in the south); in the Southern Hemisphere the shadow points true south and the sun is in the north. Solar noon is not 12:00 on a clock: it depends on your longitude within the time zone and on the time of year.
What You Need
- A straight stick — about 1 m long, pushed in vertically. Check with a plumb line or a weighted string.
- Level ground — packed and flat, with no slope.
- A string and stakes or pegs — to draw the circle and mark points.
- A watch, if you want the solar noon method.
Method 1: The Shadow-Tip Circle (No Clock Needed)
This is the carrier’s shadow stick method.
Step 1: Mark the first shadow In the morning, say two or three hours before noon, mark the tip of the stick’s shadow with a peg.
Step 2: Draw the circle Tie a string to the stick at ground level, stretch it to the peg and sweep a circle on the ground. Its radius is the length of that shadow.
Step 3: Catch the afternoon shadow As the day goes on the shadow shortens, then lengthens. When the tip touches the circle again, mark it with a second peg.
Step 4: Read the line A line between the two pegs runs east-west. Its perpendicular through the stick is the true north-south line. Both shadow tips lie on the pole side of the stick, so the direction from the stick to the midpoint between the pegs is true north in the Northern Hemisphere (true south in the Southern Hemisphere), at places where the noon shadow points north. Confirm by watching the shadow tip pass through that line at its shortest.
Method 2: Solar Noon by the Clock
Clock time of solar noon is 12:00, plus four minutes for every degree your longitude is west of the standard meridian of your time zone (minus four for every degree east of it), minus the equation of time.
The equation of time ranges from about -14 minutes in mid-February to about +16 minutes in early November (the sundial is ahead of the clock when positive). Add one hour if a summer clock change is in force.
Worked Example
A site at 100 degrees west in a zone whose standard meridian is 105 degrees west, on 3 November, with no clock change in force.
- The site is 5 degrees east of the meridian: 5 x 4 = 20 minutes earlier.
- The equation of time on 3 November is about +16 minutes (sundial ahead), so solar noon is 16 minutes earlier still.
- Solar noon = 12:00 - 20 - 16 = 11:24.
At that moment a vertical stick’s shadow lies along the north-south line. Mark the shadow tip and the stick’s base and join them. This method needs a watch kept to the minute: four minutes of clock error is one degree of hour angle, and the shadow direction moves by about a degree in winter and several degrees in summer, when the sun is high.
Checking the Compass
Lay a compass beside the true north-south line. The angle between them is the local magnetic declination for your site. Write it down; later layout can use the compass if you add or subtract the declination. The carrier warns that the difference varies by location and can be 5-20 degrees or more.
Common Mistakes
- Stick not vertical — a leaning stick moves the shadow. Check with a plumb line.
- Uneven ground — shadow length is then unreliable.
- Using 12:00 on the clock as noon — it can be wrong by half an hour or more.
- Wrong hemisphere — in the Southern Hemisphere the noon shadow points south.
Safety
Do not look at the sun to find it
Watch the shadow, not the sun. Looking directly at the sun damages eyes within seconds and permanently.
What’s Next
- Learn next: Seasonal Sun Angle Calculation — how high the sun is on any day
- Combine with: Sun Methods and North Star Finding: Big Dipper and Cassiopeia — other ways to find direction