# NavList:

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

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Re: Jupiter and the Moon
From: Paul Dolkas
Date: 2013 Jan 22, 21:27 -0800

Sean & Frank-

Appreciate the help! It’s going to take a bit to get through this – I’m going to have a few questions about Frank’s Lunar calculator, and once I’ve gotten that straitened out, I’m going to compare results using his calculator vs. my interpolation method (using the same set of sights).

‘Course if there was an easier way of doing it, I’m sure Mr. Bowditch would have been all over it. Should be interesting.

-Paul

From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Sean C
Sent: Tuesday, January 22, 2013 5:13 AM
To: paul{at}dolkas.net
Subject: [NavList 22063] Re: Jupiter and the Moon

"I’d love to know if I missed something. Then I can stop blaming the Davis…"

"I fed the numbers into a spreadsheet I ginned up, since I’m useless without one..." -Paul

Hey Paul,

Sounds like you're going through what I went through about a year or so ago. I stumbled onto lunars while teaching myself C.N. as a hobby. I was using a Davis Mk.15 and my atomic watch as well! And I too had, well...disappointing results. I found my first lunar to be completely outside of the bracketed times I had calculated. Luckily, Frank and another gentleman (I believe it may have been Rodger Farley) helped me out immensely.

My problem was that I did not fully understand the clearing process. If you prefer making a spreadsheet, I'd recommend Frank's "Easy Lunars" page found here:

It has all of the formulas you'll need. Just remember to use the average of the corrections from the almanac and in the final equation, reverse the signs of 'dh_Moon' and 'dh_Sun'. Once I got a grasp on it, my results improved dramatically.

Oh, and about the Davis: It is capable of very accurate measurements, but it is also a very touchy instrument. When I hold mine to my eye and watch the Sun emerge from the clouds, I can see the two images align themselves as the sextant warms up. I note the index error at least before and after my sights and maybe even a couple times during them. I usually try to adjust the error out, although I hear it's not good to keep fiddling with the adjustment screws as they will become loose over time as the plastic wears.

I hope Frank is right about the difference between plastic and metal sextants. I'm saving up for a metal one now. Probably an Astra. Anyway, feel free to contact me with any questions anytime. I'd be happy to help!

Regards,
Sean C
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