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Tonight I shot the angle between Spica and Arcturus six times with my
Astra IIIB (3.5x scope and traditional split horizon glass). The extreme
values were 1.0' apart, so I may have to take back my earlier remark
that "I find star to star separation angles hard to measure consistently
with a sextant." This was based on a few measurements attempted soon
after I got the Astra. My results were so disappointing, I dismissed the
technique as bunk, at least with my eyesight.

Based on tonight's results, I suspect my inexperience was to blame.
Though I'd owned bubble sextants for years, the Astra was my first
marine sextant.

Now I'm more skillful at positioning both bodies in the split image
horizon glass and swinging the arc. That last is important, I've found.
The pair of stars should appear to collide instead of passing each other
as the sextant is rocked. Contrary to what I expected, attempting to
keep the stars in contact makes it harder to judge coincidence.

When viewing a single star to check index error, that last technique is
all you have, since swinging the arc has no effect.

My six observations tonight don't prove anything. It's easy to get lucky
a few times in a row. However, I think the results are pretty good for
someone who has done maybe a dozen of these in his life. With more
practice I may be able to judge star coincidences as well as I can judge
anything else through a sextant.

Conditions tonight were hardly ideal. The wind was enough to make the
stars dance maddeningly most of the time. My arms soon became tired of
holding the sextant for long periods, waiting for the image to settle
down. I have several tripods, but to make the experiment more
challenging I did not use anything to steady myself.

My opinion of this method for checking sextants has improved enough that
I intend to try it some more.

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