A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Aug 29, 12:56 -0700
Bruce Pennino, you wrote:
"And I'm hoping to stand on a Cape Cod dune and take sextant sights at Hs angles less than 10 degrees. My height of eye will be over 100 ft. Wondering about accuracy......gotta be a lunatic?"
As with your low altitude Sun sights, you can expect very good accuracy even for altitudes as low as 1.5°. Below that there's always the risk of unusual refraction. Of course if you don't have an accurate value for height of eye, you might want to grab a few star sights, too, and treat dip as an unknown to be solved for from the sights.
For some real fun, you could even solve for GMT as an unknown! Shoot altitudes of Altair, the Moon, and Deneb, timed with a watch that's set to some wrong time. Then work the sights by trial and error for some difference between GMT and watch time until they are all consistent (a simple case of longitude by lunar altitudes).
One potential "gotcha" with the Moon in late twilight: if the "glare" of the Moon on the ocean below the Moon is just right (just wrong!), it can create the impression of a false horizon. That bright "road of light" leading across the water to the Moon will sometimes terminate early and in late twilight it can be easy to assume that the end of the "road" is the actual horizon.
Other than that, as long as you read and use the Moon altitude correction tables correctly, there's no special difficulty.
Conanicut Island USA