A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Jan 5, 18:25 -0800
David C, you wrote:
"Several times I had to set the sextant to zero and bring the moon down again."
Because.... you didn't look at the arc the first time to see the angle?? :)
For lunars, there is no reason not to preset the sextant to within one degree. Even if you're trying for historical accuracy, you have that data available from the almanac. You said you were looking at Venus in the horizon glass. So with the sextant preset within a degree, you just aim the instrument at Venus and rotate slowly around until the Moon comes into view alongside it. No need to "bring it down". If you have any difficulties, or if you have only a rough estimate of the angle, you can always start with the Moon in the horizon glass. Then for nearly any star or planet, you rotate the sextant until the Moon's horns are horizontal in the horizon glass (perpendicular to the frame). At that point the index mirror's view is directed along the ecliptic, and if the planet is not immediately visible, a little up or down adjustment of the angle will bring it right into view. Once it's found, you flip the sextant over, reversing horizon view for index view, if that happens to be more convenient ergonomically.
By the way, are you really saying you were shooting lunars with no scope? Did I mis-read that? This, of course, will radically reduce the potential accuracy of your observations. The limiting resolution of unaided human vision is about 1 minute of arc (maybe 0.75' under really excellent lighting conditions). Telescopes were added to sextants to improve this in direct proportion to the magnification: a 5x or "five power" magnifying scope lets you visually resolve angles as small as 0.2' all other things being equal. You can always do the rough aligment with the scope out, but you'll get good results only if your magnification is 5x or better. Traditionally some sextants included 12x inverting scopes specifically intended for lunars. Even in the 1950s Tamaya sextants included these high-magnification inverting scopes as a relic of their origin as lunars instruments.