Sun Pointing Symbol

© 2003 Winston Mitchell

Introduction

When making drawings of architectural subjects there is frequently a need to accurately aim a directional light to represent the sun at a particular time of day and a particular time of the year. On the internet, there is lots of information and tools available for dealing with this problem. The purpose of this article is to present and share my solution to the problem with other TurboCAD users. I picked this solution because it is easy and quick to use. Because it is a visual tool, the user can plainly see what he is doing and hopefully understand a little more about what the sun does.

The Symbol

The tool is a symbol. To use it, drag it into your drawing anywhere you like. It does need to be parallel with your drawing's effective World Plane. If necessary, rotate the symbol about the z-axis to align it with your drawing's North direction (normally, positive y). Snap your directional light's target vertex to the plain end of the North indicator and snap the source vertex to a grid location for the desired day and time. After you have aligned your light, you can either put the symbol on a hidden layer or delete it. If you delete it, you can always drag it in again.

The image below shows the symbol. The surface defined by the grid is a portion of a sphere that represents the locus of all sun positions that occur throughout a year at a pariticular latitude. The symbol shown is made for latitude 43.37° North (Boise, Idaho). Some details:

• Green arc: the sun's path at the Summer Solstice (June 21st).
• Purple arc: the sun's path at the Winter Solstice (December 21st).
• Long red arc: the sun's path at the Spring and Fall Equinoxs (March 22nd and September 21st).
• Short red arc: Solar Noon (the sun's highest elevation through the year).
• Blue arcs: location of the sun in one-hour increments before and after solar noon.
• East-West grid lines: the sun's path on a date given by a line's label. The dates between gridlines are the dates you get when you use mid-point snaps between North-South vertices.
• North-South grid lines: same is the blue lines except separated by twenty minutes. Mid-point snapping yields a resolution of ten minutes.
• There is a directional light located at 40 minutes before Solar Noon on September 8th/April 4th.

About the Grid

The grid is composed of form-building edges that appear when this feature is turned on (Drawing Setup/Display) and Degenerative Faceting is turned off (Drawing Setup/ACIS). In the above image the grid resolution is what you get with the Faceter Mode set to "Quality" (Drawing Setup/ACIS). If you need higher reolution, You can choose "Custom" and a Normal Tolerance with a sub-multiple of 5° (the value that "Quality" is set to).

Accuracy

The model in the symbol is accurate enough for its intended purpose. No attempt has been made to accomodate location longitude, time zones, etc. All times are relative to the sun's zenith. Also, no attempt has been made to track the sun's change in declination within a day or to account for the fact that the Earth's orbit is slightly eliptical.

The Price!

The price is that you have to customize the symbol for your own location or any other location you wish to model. Before going to that trouble, you can download the file shown above and evaluate it.

If you want to create a custom version of your own, download this template and follow the instructions below.
Note: steps 6 and 7 are optional. If specific dates are unimportant to you, just delete the date labels and leaders.

1. Rotate the ring about the x-axis by your latitude. If you are in the Southern hemisphere, don't forget the minus sign!
2. Explode the ring.
3. Slice off the portion of the grid below the WorldPlane and delete it.
4. Delete all blue arcs not in contact with the grid.
5. With the Node tool, edit the excessively long arcs by V-snapping their control points to where they cross the boundary of the grid.
6. It is likely that the top five date leaders will no longer be aligned with their repective East-West gridlines. With the Node tool, edit the five label leaders to realign them.
7. Mirror copy the leaders to complete the date labeling.
8. Select all and snap the selection's reference point to the small end of the North indicator (WorldPlane 0,0,0).
9. Group the selection.

10. Save to your Symbol folder.

    A brief version of these instructions can be found in the template file's Summary Info (File/Summary Info...).

Please post comments, suggestions, problems, and questions on the TurboCAD User Forum.

Revisions:

8/24/03

Original post