+- Forums (https://thenauticalalmanac.com/Forum)
+-- Forum: Main Forum Area (https://thenauticalalmanac.com/Forum/forumdisplay.php?fid=1)
+--- Forum: General Topics Here (https://thenauticalalmanac.com/Forum/forumdisplay.php?fid=2)
+--- Thread: Bagnold Sun compass- BillOfTheNorth (/showthread.php?tid=185)
Bagnold Sun compass- BillOfTheNorth - Rdutton - 05-24-2019
Bill,
Your video about the Bagnold Sun compass is fascinating! Your desert attire is perfect!
https://www.youtube.com/watch?v=N-YvH_dUW44
Perhaps because it's so simple to understand....I can't figure it out.
This much I can figure out-
1- Calculate or use a magnetic compass (adjust for variation) to determine the azimuth of the Sun.
2- Align the index figure (which is the Sun's azimuth) to the shadow cast by the gnomon.
Here's where I get "lost". Do I then adjust my course to that the shadow is on, let's say, 270° ?
Could you help further explain?
Great video!
Roland
RE: Bagnold Sun compass- BillOfTheNorth - BigBill - 05-25-2019
Hi Roland,
If I interpret your question correctly, I think you've gotten part of the procedure wrong.
I hope this helps, it's hard to explain without a model of the parts.
Here's the basic drill.
The sun compass is affixed to the vehicle.
Determine the sun's azimuth for a given time, we'll say 10:00 am
Set the compass index mark against the appropriate azimuth on the *shadow plate / azimuth scale.
use that azimuth from 9:45 - 10:15 - the errors average out
Set the course setting disc by putting the arrow on the disc over the bearing you wish to follow on the shadow plate ie: 270°
Move the vehicle until the shadow cast by the gnomon / style is aligned with the arrow on the course setting disc. You should now be facing 270°
At this point you can drive, keeping the shadow on that 270° mark of the course setting disc. The sun is always moving, so you will have a few degrees of error to start but at 10 o'clock everything lines up properly, for an instant. After that more error creeps in but on the opposite side so it will 'cancel' out the original error on the other side of your course. At 10:15 you would then reset the compass with the sun's azimuth for 10:30 and repeat the process.
I would suggest if you have made a model, trying it out on different headings. At night use the actual compass headings and a light bulb to simulate different times of day. You'll either be right on or 180° off until you 'get it' and the understanding falls into place.
There is an excellent book on the subject by Kuno Gross called the Bagnold sun-compass history and utilization
www.desertstories.org
https://www.amazon.com/Bagnold-Sun-Compass-Range-Desert-2011-11-10/dp/B01N51ZU04
Paper model: http://www.desertstories.org/index.php?article_id=22
*In order to not get too confusing I simplified the concept for the video and combined the shadow plate and the sun's azimuth scale. On the real ones they are a single part but serve different uses. Any resulting errors are my fault.
I hope this helps
~Bill