# NavList:

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

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Re: Celestial Navigation In The Gps Age: Typos?
From: Bill B
Date: 2017 Aug 31, 14:43 -0400

```On 8/31/2017 1:23 PM, Tony Oz wrote:
> Also I wonder why the /Chapter 10, Accuracy/ advises to use the
> different corrections for the lower and upper limbs of the Sun (the
> -13.5' and +18.5' respectively). Yes, their sum is the very familiar 32'
> of the Sun's diameter, but why they are different?

I have the first edition of Karl's book. I'll have to dig it out and put
the numbers you gave in context.

In the Nautical Almanac daily pages you may notice the Sun's semi
diameter (SD) varies throughput the the year. The difference in the SD
(radius) is a because of the elliptical orbit around the sun which
increases and decreases the distance from the sun.

In the NA refraction tables this is split into two time periods for
simplicity. Again referencing the NA refraction tables, the upper limb
and lower limb vary with the elevation of the sun above the horizon,
just as a star correction will vary. Also note no mater what the
elevation, the corrections add up to a consistant diameter for for the
time period. The differences in UL and LL is due to their distance to
the horizon. If you look at 90 degrees, the UL and LL corrections are
almost identical, but vary significantly at 10 degrees.

The tables are generally an over simplification which is adequate for
use at sea. An interesting proof is measuring the vertical diameter of
the sun versus the horizontal diameter at extremely low elevation. Done
carefully (and without frame flex) it will be wider than it is tall.
Thanks to the difference in refraction the upper, lower and widest
points will not even form a perfect diamond. Good old "sun squish."

```
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