A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Jan 9, 17:32 -0800
Jim Rives, you wrote:
"I am trying to clear a series of 6 sun-moon lunars taken on 2 January. I have a best fit straight line throught the data and came up with what I think is a pretty good regression (but really don't know how good is good enough). Average LD 56.05, R^2 = .987, StdDev 1.7."
Regression is overkill. You could just average. Regression won't hurt you, but it doesn't help much --and it sure isn't necessary!
"In any case, I decided to pick a LD on that line coincident with a convenient time... exactly 1800z."
Yes, that's a good idea. Without a regressed line, you could accomplish the same by plotting your observed distances on common graph paper, and then running a line through them just "by eyeball". That, too, would allow you to pick off a value at some nice clean instant of time, like 18:00:00.
Next you wrote:
"And, I went into Stellarium to get the altitudes I needed for that time of the sun and moon. I read in a post on this subject back in June of last year that Stellarium builds in the refraction and weather related anomolies and those can be eliminated by switching the atmosphere off. I couldn't find out if the altitude that stellarium provides is geocentric or topographic (or from the surface of the earth as I understand the term)."
Don't turn off the refraction. There's no reason to here. But bear in mind that Stellarium has no provision for height of eye, so when you do the "pre-clearing", you should assume your height of eye is zero. Stellarium also gives altitudes of centers (not UL and LL). But for lunars this could be counted as an advantage since it means that the altitudes have effectively been "pre-cleared" (no +12/-20 to worry about). These altitudes only have to be close enough within about 5 minutes of arc anyway, so don't fuss!
And you added:
"What I believe I need is to get the altitude with HP removed as well, as if at the center of the earth. "
Nope. You want the altitudes that would have been observed with a sextant. So either observe them, or set up Stellarium to simulate them for you. Again, Stellarium gives you the altitude of the Moon's center for an eyeball right on the water's surface. Ah, but that's OK since that just means that, in effect, Stellarium has already done the "pre-clearing" on the altitudes for you.
Before you dive into all of this nuisance with the altitudes, just clear your lunar using my web app!! Remember what I said about the web app? It does all that annoying work for you. You can either let it calculate the altitudes, or you can enter observed altitudes. Later, if you want, you can try out cases where you observe the altitudes. Or if you clear your lunars by hand, you might want the altitudes from Stellarium.
"Then, so armed, I'll be ready to see how I did determining the local time on Cape Ann on Jan 2! "
Ahem. I'll count that a "typo". A lunar does not give local time. It gives absolute time (UT or GMT).