# NavList:

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

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Re: Raw data for bubble
From: Gary LaPook
Date: 2007 Mar 12, 00:41 -0700

Gary LaPook wrote:

Yes that is the reason, trying to estimate the center a non full moon
and place it in the center of the bubble. Even if the moon looks full
it may not be exactly so a possible error. You also have trouble
trying to put the limb of the moon in the center of the bubble.

Regarding the computations, you work it like you would a the sun or
planet using H.O 249 vol. 2 or 3. The only extra step is the parallax
in altitude correction but this is simple since there is a table on
each daily page of the Air Almanac listing this correction for the
moon on that day. You would add it to Hs if using marine practice or
subtract it from Hc to compute Hp using aernautical practice.

Another point, for those learning celestial, I think it is usefull to
use the sextant correction tables from the Air Almanac rather than
those in the Nautical Almanac. In the N.A many of the corrections are
combined into one, e.g. refraction, semi-diameter and parallax in
altitude are all combined in the moon correction table. A learner will
not see the pattern or where the various items of the correction comes
from. Using the A. A. you have separate corrections for refraction
(for all bodies), dip (if using the natural horizon), semi-diameter
(if using the moon or the sun), and parallax in altitude (if shooting
the moon.) This can give you a better grasp on what is going on rather
than rote memorization of the process using the N. A. correction
tables.

On Mar 11, 10:27 pm, FrankRee...@aol.com wrote:
> "Why does the moon decrease the accuracy of the fix?"
>
> Apart from the usual reason (extra steps in the calculation leave room  for
> errors in calculation), this has to do with the nature of bubble sextant
> observations, if I'm not mistaken. You center the object in the bubble. Since  the
> Moon is only perfectly round one day a month, that is a somewhat difficult
> task. You have to imagine where the true center of the Moon is and place that at
>  the center of the bubble. Doesn't apply to ordinary sextants!
>
> Still hoping to get back at that darn Moon, eh, Robert?
>
> -FER
> 42.0N  87.7W, or 41.4N  72.1W.www.HistoricalAtlas.com/lunars
>

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