slides on solar geometry

1. UNIVERSITY OF THE WITWATERSRAND
GREEN ARCHITECTURE
ARPL 3022
K STUCKE
M Friedland
21 February 2012
Lecture 2 Solar Geometry
1. Sun & Earth
 The Earth rotates around the sun in one year, and makes one rotation around its own axis in 24
hours. The earth’s axis is inclined 23.5° to the rotation plane.
 22/6: Summer solstice in the northern hemisphere.
 22/3 and 22/9: Equinox.
 22/12: Summer solstice in the southern hemisphere.
2. Solar Diagrams
 The Solar Diagram is a tracing of the sun’s path on the sky vault, projected on a horizontal
plane.
 The sun’s traces are different for different latitudes, and there is one solar diagram for each
latitude.
 Solar diagrams can be used to determine sun shading
 A window facing north in the southern hemisphere is the easiest to handle
When a window is facing a direction other than north designing of shading can be a bit more

challenging

2. Solar Diagrams
The Solar Diagram is a tracing of the sun’s path on the sky vault, projected on a horizontal plane.
The zenith is in the middle, and the horizon is the edge of the ‘pancake’. The azimute (horizontal
angle) is indicated on the horizon, and the altitude is described by the concentric circles.
The sun’s traces are different for different latitudes, and there is one solar diagram for each latitude.
The example below is for 32°N (or 32°S if turned upside down)
Example 1:
In mid-October the sun’s position at 09:00 is… ?
Answer: azimute 125° and altitude 30°.
Example 2:
During which hours in December is the sun higher than 20° above the horizon?
Answer: Between 09:00 and 15:00

3. Sun Shading Design
Method
1. Use the solar diagram that is appropriate to the location
2. Draw a line across the center of the sun diagram , representing the plan of the wall in
consideration
 Where the sun is behind the wall the radiation will not reach the wall and thus does
not have to be considered
3. Mark on the sun diagram the period when shading is desirable
4. Place the shadow-angle protractor over the sun diagram
5. Select the shading mask that covers the required shading period
 Or the shading mask that has the closest possible match
6. Use the angles of the shading mask required to determine the shading device that will be
used

4. Data Sheets
This Shadow Angle Protractor needs to be printed on transparency
YOU WILL NEED TO DO THIS FOR SOLAR THIS WEEKS LECTURE

5. Solar Diagram for Johannesburg