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Fred wrote, regarding land locked index error checks:

> By the way, for artificial horizons, I like to set the two images
> slightly apart and let them converge, then mark the time.

Paul

Regarding artificial horizons the Sun:

Any method, IMHO, is as much an art as it is a science. And very personal. I
feel there is a learning curve on learning to *see*.  As you progress and
get feedback from your observations, you will learn what does and doesn't
work for you.  Until I got lots off feedback from bringing Sun images
together it was very difficult for me to determine how close constituted
"together."

When using an artificial horizon I tend to like setting the two disks you
see to overlap, and wait until they are separated.  The plus side for me is
that when the disks overlap, there is a bright spot where they overlap.
When that spot is JUST gone--not even a *spot* of brightness, you are there.
No guessing about how big the apparent gap should be or overshooting then
having to back off (moving drum in wrong direction so gear slack comes into
play).  Since we don't worry about irradiation (Sun looking bigger and
horizon looking lower) with an artificial horizon, which limb (upper or
lower) you use should not matter as long as you use the appropriate
refraction correction.

Regarding Index Error/Correction using the Sun

Initially, use Sun elevations above 30d to avoid the Sun appearing like an
egg on its side due to refraction at low elevations. As you develop
technique/your personal style rotate the sextant to the horizontal to
(mostly) avoid refraction distortion.  Another trick I use is to separate
the bodies for the on-the arc reading, then start with the bodies separated
and adjust the sextant in 0.1' or 0.2' increments (in the same direction as
the on-the-arc observation) for off-the-arc observations.  Keep doing that
until an overlap appears, and use the setting before the overlap appeared as
your off-the-arc figure.  Sneaking up on it helps with arm and eyestrain
IMHO.

As to total values being 4X published semi diameter, I have conceptual
problems with that. I am sure the veterans will set me straight ;-)

1. The first min-concern being irradiation (the eye's tendency to make
bright objects against a dark background look larger than they are).  It is
possible the two disks you see will touch (to your eye) while they are still
a wee-bit apart in reality.  Or they may appear to overlap when physically
they are apart.  I am not clear if the published SD is what one would see,
or based on the physical diameter of the Sun and distance from Earth without
the eye's intervention.  Would guess the latter.

2. When you add the off-the-arc and on-the-arc values, you are looking for a
figure, as stated in Fred's post, that is 4X the almanac semi diameter (SD).
Note the SD on the daily pages jumps by .1' from one day to another when it
hits a rounding limit.  Also note it stays at a value for a goodly bit of
time so must have a midpoint as well.   So if the SD is 15.9 one day, and 16
the next, it is most likely 15.95 at that point.  Multiply published SD by
4, and you get 63.6' one day and  64.0' the next. In a case like the above,
I would use 63.8 (15.95 X 4) as my sanity check.  If using your totals as an
indicator of whether it was a successful round of IE checks or not,  0.4'
off is not so good. Although in reality you would be 0.1'  (0.4/4) off the
published SD.  If your total is 63.8 during the above transition period, you
are not 0.2' off, you are spot on.  My suggestion, don't place *too* much
emphasis on your totals vs. 4X SD values.

Personally, my usual ability to *see* at this juncture ranges from 0.1' to
0.2'.  Occasionally better, sometime much worse.  Some days, as you will
find, it all falls into place--nice tight groupings.  Others, you wonder if
you could find a continent with a GPS.  Studies indicate the learning curve
exceeds 3000 observations.

You might ask, what the heck does 0.1' or 0.2' matter at sea with all the
fuzzy variables?  Not much.  To paraphrase one of the list gurus, on an
ocean chart a .5 mm mechanical pencil line may be 2 miles wide.  What does
0.2'  (at worst 1215 feet) matter?  But if you get the bug and start
shooting lunars, every error is magnified by approx. 30.  So a .2' error
becomes a 6' error in longitude.  Given the difficulty of some observations,
12' off is pretty good.  Star to star distances also give our list
perfectionists fits if IC etc. is not spot on.

That's my 2-cents worth based on the path I have taken to date.  Hope you
enjoy the journey as much as I have.

Bill

   

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