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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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Sun SD, Sextant precision, SNO-T
From: Alexandre Eremenko
Date: 2004 Oct 10, 16:12 -0500

```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.

```
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