NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Jul 13, 13:45 -0700
Igor
I think it would depend upon how you intended to aim. With Hughes sextants (see photo) you swing the sextant from side to side to keep the bubble roughly in the centre of the tram lines and backwards and forwards to keep the bubble roughly halfway up the field of view. Then you move the sextant in azimuth and adjust Hs to place the celestial body in the centre of the bubble. For this method you need a fairly small bubble. 1/3 – ½ the distance between the tramlines is recommended. Any smaller and the bubble is sluggish. Any larger and it’s difficult to judge its centre.
It looks like the Plath is best aimed differently. Tilt the sextant until the circumference of the bubble best fits the graticule circles. Then move the sextant in azimuth until the celestial body is on the vertical graticule line and adjust Hs until the body is in the exact centre of the graticule. I would imagine this would allow more tolerance in bubble size, because you’re just using the bubble to put the bubble perimeter in line with the graticule circles. Dave P
