NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Shadow Catcher for sun sights
From: John A Brown
Date: 2010 Mar 9, 18:43 +1030
From: John A Brown
Date: 2010 Mar 9, 18:43 +1030
Greg Rudzinski wrote
" When looking at the leading edge of the
Sun's shadow through the magnifier it should appear as a faint shadow transiting
to a darker shadow."
This is correct. Depending on the altitude of the
Sun and the gnomon used, the effect of the penumbra can vary from being almost
negligible to quite marked. In using any simple instrument there are
certain skills needed to get the best results and with the Shadow Catcher it is
the ability to pick the right spot where umbra and penumbra merge. After
thousands of sights it has become intuitive and is no longer a
problem.
Greg also wrote
"I recommend that the blocks be glued together
securely and painted white."
I have always enjoyed the fact that the whole
outfit can be fitted into a very small tin. If I joined the block together I
would require 21 blocks instead of the current 6 and would lose that
compactness. The blocks are generally outside for no more than 15 minutes at a
time for a set of sights so I don't think that would be long enough to cause
expansion from heating. I will make a trial of using gnomon made with MDF
and see if it makes a difference.
Greg also wrote
"Also check the plastic ruler using a metal caliper
or micrometer."
This is certainly good advice as I have found
problems with Chinese made plastic rulers in the past. I check the rulers that I
use against a good quality Japanese architect's ruler.
Greg also wrote
"Try measuring your shadow using a pixel count from
a digital image."
I haven't tried the pixel count method but I have
taken photos of the setup after levelling and have included a watch set to UTC
and a piece of paper with the date and lat and long. The length of the shadow is
read from the ruler as displayed on the computer monitor. It works quite
well.
Thank you for your interest.
Regards,
John Brown.
Adelaide, South
Australia.
