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

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Re: Sun SD, Sextant precision, SNO-T
From: Lisa Fiene
Date: 2004 Oct 11, 07:49 +1000

```Hi Alexandre,

According to the Sun Ephemeris for 2004, the following is indicated:

October 10th - 1922.1" (for diameter - just halve for SD) = 16'0175"

See:  http://sunearth.gsfc.nasa.gov/eclipse/TYPE/sun2.html#su2004
or

Hope this helps,
Lisa

Alexandre Eremenko wrote:
> Can anyone tell me more precise value of
> the Sun radius on October 10?
> (This quantity is called "semidiameter, or SD" in the
> almanach. The almanach gives for October 10
> the value of  16.0', but I noticed that it gives
> 16.1' beginning from October 12, and the refraction table
> implies the average value of 16.15 for October-March.
> The "Astronomical Almanach" or maybe some software could probably
> give it more precisely (I need one more digit).
>
> The reason I am asking: This morning I have fixed all
> (hopefully!) problems with my inverting scope, and tried
> to fing the index correction from Sun observations
> (with a fixed scope and more experience than I had last time:-)
> The results are the following:
> (everything is in angular minutes. "UL" is the sextant reading
> when the reflected image touches from above,
> -LL is when it touches from below. "4SD" is the sum
> of these readings, "-IC" is the difference divided by 2,
> the negative of the Index Correction..
>
> 1. Rotating the drum in the decreasing direction:
> UL  32.5,  32.3, 32.5, 32.4, 32.3,
> LL  32.0,  32.1, 32.0, 32.1, 32.0,
> 4SD 64.5,  64.4, 64.5, 64.5, 64.3, Almanac: 64.0
> -IC 0.25,  0.1   0.25, 0.15, 0.15  Average: -0.2
>
> 2. Rotating the drum in the increasing direction:
> LL  32.0,  32.0, 32.0, 32.1, 31.9,
> UL  32.2,  32.5, 32.6, 32.5, 32.5,
> 4SD 64.2,  64.5, 64.6, 64.6, 64.4 Almanac: 64.0
> -IC 0.1,   0.25,  0.3,  0.2,  0.3 Average: -0.2
>
> These results indicate that my index correction is
> something between -0.2' and -0.3' (Which is consistent
> with my previous Sun observations, but NOT with the star
> observation; star observation give consistently 0.0'.
> Today will repeat star observations
> with my newly fixed inverting scope.
>
> The problem with the Sun observations above
> is that my measurements of
> Sun's SD indicate
> that it is closer to 16.1' rather than 16.0'.
> This explains my question in the beginning. I have to multiply
> the almanac value by 4. This increases the almanac rounding
> error by the factor of 4.
>
> (The Russian manual says: if your measurement of 4SD is
> within 0.4' from the almanac value, your measurement is OK.
> Otherwise it should be repeated.
> My measurement is on the boundary:-)
>
> (My apologies in advance to those list members who consider
> all this activity "splitting of a hair":-)
>
> Alex.
>
>

--
Kind regards
Lisa Fiene
***************************

CopyCare Pacific Pty Ltd
Lizard Tunes
ABN 93 101 046 888
PO Box 314 Ourimbah NSW 2258
Australia
Phone/Fax: (02) 43 627 583
International: 61-2-43 627 583
E-mail: lisa{at}copycarepacific.com
Web: www.copycare.com/content/local/ccpaceng.asp

```
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