A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2014 Dec 5, 10:38 -0800
Robert Cagle, you asked:
"Can anyone inform me on what the smallest direct reading would have been on a sextant that would have been used in 1818? In other words late 18th and early 19th century sextants?"
Sextants, which were used almost exclusively for lunars in this period, had verniers which typically could be read to 10 seconds of arc. Quadrants (usually called "octants" today) which were used for all other navigational sights, besides lunars, typically could be read to 1 minute of arc. There are no 'laws' on any of this. There was considerable diversity in instruments and usage in this period, so you might also find a rare quadrant capable of 15" accuracy or a metal sextant engraved only to 30" accuracy. But 10" for sextants and 1' for quadrants would be typical.
Conanicut Island USA