A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Oct 11, 10:05 -0700
AlanS you wrote: Don't have a boat, so my efforts at “celestial navigation” are limited to shooting the sun while standing on the beach when we get down to North Carolina. Instrument used: Astra IIIB with 4 x 40mm scope that came with it, and a 7 x 35mm monocular. Shooting index error with the 4 x 40mm scope presents no problem at all. I experience difficulty determining index error with the larger scope. I assume that procedure would be the same with both, that is turning the sextant adjustment knob to achieve a continuous horizon line. Using the smaller telescope, I get a clean line, with a clear break. Cannot seem to achieve this with the monocular. Does an operational error on my part come to mind? Please advise, and thanks.
Index error is caused by the horizon mirror being out of adjustment in the up and down plane, so unless you bash it while doing it, I don’t see how changing eyepieces would affect index error. If you can measure index error satisfactorily with a 4x eyepiece, read it with that, and then swap eyepieces. The eyepieces just magnify whatever’s coming into them. It all comes off or through the horizon mirror. Sometimes I just look through the hole with no eyepiece fitted, and it works for my ‘just for fun (3nm is good, and less1nm is brilliant (or very lucky))’ type of observation. To my mind, it’s the quality of the distant line or object that matters rather than the magnification of the eyepiece. Also, if you’re observing the Sun, or the Moon, you don’t necessarily have to look in that direction to measure index error; with the Sun especially; you might find a better defined horizon elsewhere. If observing a star, set the sextant to zero, look directly at the star, and make the images merge; you can’t find an object more distant than that. On my sextant: a house ridge opposite gives coincidence at 4’, a house ridge in next block gives it at 1’, a star gives it at 0’. DaveP