NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Moon shots less accurate than star shots
From: Paul Hirose
Date: 2006 Aug 20, 01:31 -0500
The Cassens & Plath Horizon Ultra has a Schüler prism which is claimed
to reduce the dazzle from the sun reflecting off the water. I believe it
splits the horizon view so there is a blind zone in the center.
Presumably it would work for the moon as well. Has anyone tried this
device? (http://www.cassens-plath.de/catalog/085e.html)
To compute the correct limb in cases where it's ambiguous, it seems to
me that that you could compute the position angle (in the az/el frame)
of the sun with respect to the moon. If the position angle is in the 90°
to 180° to 270° semicircle, lower limb should be used.
The calculation is analogous to computing the azimuth in a sight
reduction. In this case, the zenith is the "north pole", the moon is the
"observer", and the sun is the "celestial body". The altitudes of the
moon and sun are the "latitude" and "declination", respectively. Their
azimuth difference, measured clockwise from the moon to the sun, becomes
the "LHA". When you solve the triangle, the position angle is "azimuth"
and the phase angle is "Hc" + 90° (i.e., 180° at new moon, 0° at full).
My SOFA/JPL DLL solves for position angle with the vector algorithm of
the SOFA library rather than spherical trig.
http://www.iau-sofa.rl.ac.uk/2005_0901/sofa/pap.for
Robert Eno wrote:
> I am not sure what point you are trying to make here. Likely any browser
> would categorize this thread under the general heading of "moon".
Perhaps a
> lot of disgruntled navigators/lunarians will cue into the *&^%$~@
descriptor
> because it resonates with them.
It resonated with me, but not in the expected way. I thought you were
expressing disgust that the glare of the moon had interfered with some
dim object. Been there myself. It didn't sound like an interesting
topic, so I skipped the postings on that subject without opening them
and trash-canned the lot. That's brutal, but I don't have the patience
to open every message in my inbox.
Later I became curious, opened a few messages, and realized I was
totally wrong about the subject. By that time the original posting was
cleared out of my trash folder, so I had to go to the Google site to
read it.
--
I block messages that contain attachments or HTML.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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From: Paul Hirose
Date: 2006 Aug 20, 01:31 -0500
The Cassens & Plath Horizon Ultra has a Schüler prism which is claimed
to reduce the dazzle from the sun reflecting off the water. I believe it
splits the horizon view so there is a blind zone in the center.
Presumably it would work for the moon as well. Has anyone tried this
device? (http://www.cassens-plath.de/catalog/085e.html)
To compute the correct limb in cases where it's ambiguous, it seems to
me that that you could compute the position angle (in the az/el frame)
of the sun with respect to the moon. If the position angle is in the 90°
to 180° to 270° semicircle, lower limb should be used.
The calculation is analogous to computing the azimuth in a sight
reduction. In this case, the zenith is the "north pole", the moon is the
"observer", and the sun is the "celestial body". The altitudes of the
moon and sun are the "latitude" and "declination", respectively. Their
azimuth difference, measured clockwise from the moon to the sun, becomes
the "LHA". When you solve the triangle, the position angle is "azimuth"
and the phase angle is "Hc" + 90° (i.e., 180° at new moon, 0° at full).
My SOFA/JPL DLL solves for position angle with the vector algorithm of
the SOFA library rather than spherical trig.
http://www.iau-sofa.rl.ac.uk/2005_0901/sofa/pap.for
Robert Eno wrote:
> I am not sure what point you are trying to make here. Likely any browser
> would categorize this thread under the general heading of "moon".
Perhaps a
> lot of disgruntled navigators/lunarians will cue into the *&^%$~@
descriptor
> because it resonates with them.
It resonated with me, but not in the expected way. I thought you were
expressing disgust that the glare of the moon had interfered with some
dim object. Been there myself. It didn't sound like an interesting
topic, so I skipped the postings on that subject without opening them
and trash-canned the lot. That's brutal, but I don't have the patience
to open every message in my inbox.
Later I became curious, opened a few messages, and realized I was
totally wrong about the subject. By that time the original posting was
cleared out of my trash folder, so I had to go to the Google site to
read it.
--
I block messages that contain attachments or HTML.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
