A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2016 Dec 17, 12:45 -0800
Hmm. Check accuracy? The intercept calculation is intrinsically less accurate than a time sight calculation (by the way, I have been meaning to say: the expression "long by chron" is decidedly archaic and misleading --just call it a time sight calculation or a local time calculation).
I deliberately used the term Long by Chron because it is archaic (-; My use of the word accuracy was wrong. What I meant was that calculating the intercept (with AP = GPS) is an easy way of determining if the sight is to be kept or thrown away.
After you have taken your a.m. and p.m. sights, plot them against UT/GMT as accurately as you can. Then draw the best line you can through the a.m. sights. Then draw the best line you can through the p.m. sights. If they're close enough to noon that you can see the curvature in the plot, then you should draw a nice parabola through the whole bunch.
Thanks for describing a graphic method that does not require the PM sights to be taken at identical altitudes.
The double Sun limb to limb observation in the artificial horizon should have the two Suns shaded to match in brightness. I like the images on the dim side. Check focus each time since putting the eye to the scope can change focus slightly. Pre-calculate the PM observations to match AM observations if not using Frank's graphing method. This will save your arm. Expect 1' precision or better from GPS for each individual observation (not average) with artificial horizon if doing everything right using a metal sextant. The average should be better than 1' precision.
You might also try splitting the difference between an upper and lower limb observation. If you wear glasses then try some observations with scope focused with and without glasses on to see what shows best to the eye.
Thanks for those hints. I wear glasses and sometimes wear them when taking sights. Other times I take them off. My telescopes do not work so usually I use the sight tube. Yesterday I used the binoculars with one tube blanked off. I think it gives me 2X which makes sights a little easier.
Thank you for posting your sights. It is -5 degrees F here in my back yard so even if the sun was shining ( which it is NOT ) I would not be outside with my sextant. You got to love New Zealand in December!
NZ went metric when I was a youngster. F = 9/5C + 32 ....... so your outside temp is -20° C. You live in the Alaska????
In three days time it will all be all downhill again. At least the length of the day will be. I hope to take some sights when the sun is at dec S 23° 26.1'. Because the actual weather lags the seasons there should be plenty of opportunity for sight taking over the next five months or so. May is the most settled month of the year although the temperature is getting cooler. NZ having an island climate weather tends to be variable. Yesterday was blue sky all day, no wind and a pleasant 20°C. Today it is 8/8 cloud with wind and a current temp of 15°C, with a forecast max of 17°C. Therefore plenty of time for me to work out the intercepts for the 19 sights!