# NavList:

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

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Re: My lunar distance tables
From: Frank Reed
Date: 2014 May 27, 21:05 -0700

Francis, you wrote:
"1) I've been interested in your "short clearance method" using vertical circle "collapse". Looking at your correspondence with the sadly late George Huxtable in 2010, I wonder if you or anyone got round to doing the "quick look up table" as a check test to see if in bounds?"

I created some diagrams for this purpose, but there's an easier way. I'll see if I can make a quick lookup for it this week.

"I think your idea is totally brilliant"

Thanks. If only I had a time machine, I would show it to Nathaniel Bowditch and Jose de Mendoza y Rios, too.

You wondered:
"if it can be used reasonably often.(is it only viable near the tropics?"

It's certainly more likely to be useful in and near the tropics, but I am always amazed by the specific cases that I find where it would indeed have worked. Half the cases of actual lunars I've found in logbooks could have been cleared with no trig whatsoever using this 'vertical circle' equivalent solution. All those poor logarithms died for nothing!

You also wrote:
"2)I'm also, totally irrationally, given the multiple quartz watches aboard, interested in " no clock longitude using lunars." (mostly sun/ moon "fix" using GMT from the lunar.) I've been trying to guess the approx , no clock GMT from the lunar Sd, using my home made 3 hourly predicted LDs printed out from your excellent site.(ie a pre 1912 almanac really). Am I correct in thinking that the maximum error of an non cleared LDsd is about 60'? or have I got it completely wrong? (I guess that would still be a huge incorrect longitude?)"

Yes, that's right. But you know, you can actually do a fairly good job roughly estimating the correction without any tables: you can do HP*cos(alt_moon) in your head to within some reasonable error (within 10% ?), and you can estimate the "corner cosine" at the Moon to within maybe 10% so the net error in the correction would be +/- 15% most of the time just by eyeball estimation.

I'll think more about your question regarding traverse table solutions to clearing lunars. That strikes me as a 'stunt solution' --fun that it can be done that way, but who would need to? Who would benefit?

-FER

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