Howard Sun Compass Disk

1. Go to the website of the U.S. Naval Observatory:

USNO data services

2. Print out the Altitude/Azimuth table for your latitude, longitude, timezone, and date at 1 minute time intervals.

3. Find the time of local noon, i. e.
the time when the azimuth angle is next to 180° (northern latitude), or next to 0° (southern latitude).

local noon

4. Place the disk on the Sun Compass, the index line pointing to 0° of the "DEGREES" scale, and fix it.

5. Read the azimuth angle for local noon time minus 1 hour.

6. Calculate the positive difference of azimuth from 180° (northern latitude), or from 0° (southern latitude).

180° - 164.6° = 15.4°

7. Mark the corresponding shadow angle on the "DEGREES" scale, using the ruler.

8. Repeat with local noon minus 2 hours , minus 3 hours, and so on.

180° - 149.9° = 30.1°

9. Using Local Apparent Time the shadow angle is the same for equal time distances before and after noon but on opposite sides of the index line.

The result using my applet:


Updated: 2009, Feb 03