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> From: Bill
> Bonus question.  "Within two miles" seems to be the standard for cel nav.
> Most observations appear to be running fixes from the sun.  What is the
> level of expectation from nearly simultaneous star/planet observations?

Hey Bill; how long is that bit of string? I've had zero intercepts with
sights made on land, meaning that the error was less than the resolving
power of my method - one tenth of a minute of arc; less than 200 metres. But
could these have been flukes, coincidences? Maybe.

On land getting within one nautical mile with sextant should be achievable.
A surveyor using a theodolite, the same celestial bodies, and assistance
from a variety of methods statistical and graphical, plus being very careful
about common sense stuff like the timing of sights, apparently hits the wall
at one second of arc - about 30 metres.

On sea its a different story. Ships have two great advantages; their
elevated and more stable platform. I guess this is how navigators could
pride themselves in fixing positions to within a mile or so of actual. Even
with less than nice calm weather moderate movement could yield to
experience.

But yachts make a terrible platform for astronomical observations. Just
awful. Move in all directions at once while drenching the wretched observer
and his precious instrument, itself at great risk from physical damage. Plus
they are much too low, sometimes almost at sea level. Can be hard to get a
good horizon, then the one you get can be false. So the rule of thumb is
that if you achieve a fix within 10 nm of the real position that's good, and
if within 5 nm that's excellent.

Incidentally, like Alekandre my sextant seems to have a small systematic
error. Sextants aren't perfect either.

   

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