# NavList:

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

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Re: Lunar Distances with Alex's SNO-T
From: Frank Reed CT
Date: 2006 Nov 3, 23:26 EST
Bill you wrote:
"Given the above paragraph and the rapid change of the moon's SD, it seems
foolish to apply sun standards to the moon.  It is not in the moon's nature
to obey the sun's SD rules.  If I recall, the moon's travels were the last
to be nailed down--a wild child compared to other bodies."

That was an issue over a century ago. I gotta say: this is not rocket science! We're just talking about the apparent diameter of the Moon. We know how big the Moon is in miles, confirmed by planting boots on it and all that, over a third of a century ago <g>, and we know how far away it is. So there's no problem calculating its exact angular size in the sky. BUT you can't use the value in the Nautical Almanac. Is that sacrilege? Isn't the Nautical Almanac nearly Holy Scripture for celestial navigation? Some people treat it that way sometimes, but in fact, the modern Nautical Almanac is no more than a well-honed tool for a particular class of celestial navigation observations, namely, ordinary altitude observations. And here and there in the tables of the Nautical Almanac, you will find that some quantities are inaccurate by as much as three-tenths of a minute of arc, because that level of error is not critical for ordinary altitude observations.

To get the correct semi-diameter of the Moon, just take out the Moon's HP from the almanac for the correct hour of observation. Multiply that by 27.27%. Then apply the augmentation. You can calculate this or just use a short lookup table as follows: if the Moon's altitude is between 10 and 30 degrees, add 0.1 minutes of arc, between 30 and 60 add 0.2', above 60 add 0.3' (see "Easy Lunars" on my web site). That's close enough in most cases. Note that if the Moon is lower than 15 degrees or so, you have to take refractional flattening into account, but you could just as well wait until the Moon's higher than that.

"Side error can change the tangency of the horizon and a body if
the sextant is not plumb, or lunars"

Side "error" is no error at all EXCEPT at very small angles --in other words, the ONLY observation it affects in any significant way is this test for index correction using the limbs of the Sun and Moon.

"but does 4SD mean anything other than a
sanity check when doing IE/IC checks with a sphere?"

Right. That's all it is. Just a sanity check. And I never bother with it except with a sextant I'm trying for the first time --as a sanity check.

-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

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