Set the building’s long axis east-west and put the largest windows within 15 degrees of true south (true north in the Southern Hemisphere), because the winter sun is low and sits in that direction at midday. True south is not magnetic south: magnetic declination can be 5-20 degrees or more.
Part of Passive Solar Building Design
The direction a building faces and the sun angles that decide how much winter heat and summer shade its windows get.
The Winter Sun Is Low and Stands in the Direction of the Equator
At midday the sun is as far from the horizon as it will get that day, and it stands due south in the Northern Hemisphere (due north in the Southern Hemisphere). In winter it climbs only a little. In summer it climbs high. A long wall that faces the midday sun therefore sees a low, strong winter sun from the front and a high summer sun almost from above, which a small overhang can shade. That single fact is what passive solar design uses, and it is why the carrier’s one-line rule works: long axis east-west, largest windows within 15 degrees of true south.
This wiki’s passive solar pages follow the carrier in using Northern Hemisphere wording, with the sun in the south at midday. In the Southern Hemisphere, replace “south” with “north” and “north” with “south” throughout. Between the tropics (latitudes below about 23.4 degrees, north or south) the midday sun is overhead on some days and moves from one side of overhead to the other through the year, so the rule needs the sun-angle calculation in Seasonal Sun Angle Calculation.
How High Is the Sun at Noon?
The carrier gives the relations: at the equinoxes the noon sun angle equals 90 degrees minus your latitude; at the winter solstice it is 90 minus latitude minus 23.5 degrees; at the summer solstice, 90 minus latitude plus 23.5 degrees (Northern Hemisphere dates: 21 December and 21 June). For the three latitudes below, computed with 23.44 degrees:
| Latitude | Winter noon | Equinox noon | Summer noon |
|---|---|---|---|
| 36 N | 30.6 degrees | 54.0 degrees | 77.4 degrees |
| 40 N | 26.6 degrees | 50.0 degrees | 73.4 degrees |
| 48 N | 18.6 degrees | 42.0 degrees | 65.4 degrees |
At 35 S the same arithmetic gives 31.6 degrees on 21 June (winter there), 55.0 degrees at the equinoxes and 78.4 degrees on 21 December, with the noon sun in the north. The size of the swing between winter and summer is the same everywhere, 46.9 degrees (twice 23.44), but the starting point depends on latitude, which is why window and overhang figures must be worked out for your own latitude and never copied from another place.
Why 15 Degrees Is a Tolerance, Not a Rule of Nature
A wall turned away from due south catches less of the midday beam. For the noon sun the amount falls with the cosine of the angle of turn: cos 15 degrees = 0.97, so a wall 15 degrees off south loses about 3% of the direct noon sun that a due-south wall would receive. At 30 degrees the figure is cos 30 degrees = 0.87, a loss of about 13%. That is why a small error is harmless and why the carrier’s 15 degrees is a sensible limit. A wall turned toward the morning sun catches more of it and loses afternoon sun; which side you favour depends on whether your house needs early warmth or afternoon shade.
What You Need
- True south — see Finding True South.
- Your latitude — from a map, or from the noon shadow ratio.
- A building site where the south side is open — trees and hills to the south cost winter sun. See Sun Exposure & Shadows.
Common Mistakes
- Using magnetic south — the compass can be off by 5-20 degrees or more. Find true south by the sun.
- Long axis north-south — it gives the south side only a short wall, and large east and west walls take low summer sun that overhangs cannot shade.
- Copying another latitude’s numbers — noon sun angle changes with latitude.
What’s Next
- Learn next: Finding True South — setting out the orientation
- Combine with: Window Placement & Sizing — how much glass the south wall should carry