NavList:

Mark Coady, you wrote:
" Was very pleased as 6 came out under one minute of angle (.7, .8. etc...and one within .2 (pure chance as it exceeds theroetical accuracy)."

Hmmm. Maybe I'm misunderstanding you here. What do you mean when you say 0.2 minutes exceeds theoretical accuracy? How is that more "pure chance" than the 0.7' difference? It's quite possible that you will find, after some practice, that you can reduce the mean error of your observations to near zero and the standard deviation of your lunars observations to one-half or even one-quarter of a minute of arc, under good observing conditions using that 7x scope. If you can get the mean error to near zero (do-able with a lot of fiddling) and the s.d. down to a quarter of a minute or arc (depends on the individual, but repeatable with enough practice), then you could expect that roughly half of your lunars will have absolute errors of 0.2' or less.

You also wrote:
"One its awkward to hold the sextant (especially after a very long swim) so i tended to rest the rubber eyecup on my eye or sextant parts onto my hand. My perception was this produced very minor deflection and some loss of accuracy. It was also hard to really keep the sextant scope centered on my eye because of the awkward hold. I was using the moon as the viewed, sun as reflected body. This meant the sextant handle is on top. and the sextant hanging under the hand."

Yes, this is a very uncomfortable pose, and it can significantly degrade the quality of your observations. It's important to remember with lunars in a historical context that they could be taken when convenient. These were not mandatory, time-dependent sights like Noon Sun or a 3pm time sight which were done with a certain ritual on nineteenth century vessels. Lunars were taken when convenient in the period when they were actively used (ending around 1850). If you had to wait a day or three, it didn't matter. If you find your arm aching, and you're standing in an awkward position, you should remind yourself of the old joke: "Doctor, doctor, it hurts when I do this!" ...to which the doctor replies, "Vell, then don't do that!!"

As far as the difficulty of finding the Moon, especially a thin crescent in daylight, I agree that the best approach is to view the Moon direct, through the horizon glass, and then rotate to pick up the Sun. Of course you should preset the sextant to the expected lunar distance. Many modern celestial navigation students become overly obsessed with the idea that they cannot "cheat" by setting a sextant to an expected angle. But let's not forget that historical navigators necessarily knew the lunar distance at the time of their sights within a degree since they had tables of predicted lunar distances and since their knowledge of GMT was never more than an hour in error and usually much less (equivalently their dead reckoning or other estimated longitude was never in error by as much as 15° except in very rare circumstances). After setting the sextant to the expected lunar distance and sighting the Moon, you can easily pick up the Sun by rotating the sextant until the Moon's horns are "horizontal" in the sextant's field of view (horizontal in the normal sense when looking through a sextant, but really this means that the line of the Moon's horns is perpendicular to the sextant frame. This trick of aligning the horns also works with planets and those bright stars which happen lie close to the ecliptic.

Another trick: don't adjust live. We're accustomed to Sun sights where the altitude changes so rapidly that we need our fingers on the micrometer continuously. This isn't necessary with lunars. Hold the sextant with two hands on the frame for greater stability and less fatigue. Swing the Sun and Moon past each other and make a ballpark estimate of the gap or overlap. Then lower the sextant from your eye, adjust the micrometer, and repeat. Zero in progressively until you're tweaking a tenth of a minute at a time. It takes about twelve seconds for the lunar distance arc to change by a tenth of a minute so you have plenty of time to do this without live adjustment. 

By the way, you should have joined us this weekend! A few of us were shooting lunars at Stonington Point Sunday afternoon and then at Avery Point in Groton on Monday afternoon. 

Frank Reed
ReedNavigation.com
Conanicut Island USA

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