# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Sextant pre-set for taking sun-moon lunars
From: Frank Reed
Date: 2016 Aug 28, 11:05 -0700

Ed,

I told you by email that I felt I wouldn'thave time for a reply, but I have found a little...

We can assume you're shooting lunars with both bodies above 15° altitude so that refraction is not important (can always extend to lower altitudes later). The only element you're missing then is the Moon's parallax in altitude and its effect on the distance. In order to estimate the parallax in altitude, you would have to estimate the Moon's altitude, but even if you do this only to the nearest ten degrees you'll make a big improvement in the resulting pre-computed distance. The correction in altitude is HP·cos(alt). Note that you don't need to enter an independent value for the HP since you already input the Moon's SD from the almanac. The two are related by SD = 0.2724·HP. Then you need an eyeball estimate of the corner angle --the angle ZMS (zenith-Moon-Sun). When the other body is the Sun or any object close to the ecliptic, you could get this from the orientation of the Moon's horns.

By the way, it may help others interested in replying to you, Ed, if you would explain that you absolutely love programmable calculators. It seems to me that the goal in this project is thus two-fold: to create a tool for navigation, and to work an interesting project with a programmable calculator. You know the math of calculating the geocentric distance between the bodies inside and out, I would say. It's the challenge of creating a solution on a "legacy" device that you find intriguing.

Frank Reed

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