# 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: Bill B
Date: 2007 Mar 12, 20:53 -0500

```Gary wrote

> 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.

I have been trying to understand the components of the moon correction
tables, and playing with the computational method preceding the concise
reduction table in the N.A.  Below S = SD = semidiameter

PA = HP cos H    = (S/.2724) cos H
Ho = H - R + PA plus/minus S
=  H - R + ((S/.2724) cos H) plus/minus S/.2724

Gary mentioned refraction, semidiamter, and parallax.  The N.A. also has a
formula to correct for oblateness. One of my texts mentions correction for
augmentation of the moon (the change in apparent diameter due to change in
distance for an observer on the face of the  earth vs. the center as the
moon rises from the horizon to the zenith.

To better understand the components, I have been deriving my own formulas
for parallax and augmentation.  They are not elegant ;-)

Here is where I get confused.  I can determine distance given the SD and
radius of the Moon, I determine SD given if the distance and Moon's radius
are known, etc.

Thinking inside the box, that 0.2724d equates to 0d 16' 20.64", which if it
is an SD, would put the moon 227,057 miles away (on the close side of its
distance range) given a Moon diameter of 2159 miles.  HP is SD/0.2724, and S
is derived by HP * 0.2724. It feels like a dog chasing its own tail.
Without distance I can only derive distance by converting HP to SD which
uses a constant from one set of circumstances.

That said, four questions:

1. Do the tables make any attempt (lacking Lat input) to account for
oblateness?

2. Do the tables or PA plus/minus S compensate for augmentation? Is that
perhaps built into the daily pages figures?

3.  How the heck does one constant, 0.2724 and a simple expression (PA = HP
cos H) get us even close?

Bill

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