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    Re: Accuracy of sextant observations at sea
    From: Hewitt Schlereth
    Date: 2010 Nov 30, 22:10 -0400

    Alan -
    
    I know what you mean by just diving in and thrashing away. That's my
    preferred technique, too.
    
    Do try averaging a run of five sun shots and compare the intercept to
    the GPS. People I've taught on the beach here in St John (USVI) using
    a Davis Mark 15 pretty routinely get .8' to .6' intercepts.
    
    Hewitt
    
    On 11/30/10, Alan  wrote:
    > Hewitt:
    >
    > Iam still "here", likely more than somewhat overwhelmed by the input from
    > helpful professionals such as  Frank Reed, Garry, Anabasis and others..
    >
    > Re sights, I haven't tried averaging, something I likely should as when at
    > the beach, spring and fall, weather allowing, I usually do a run of shots, 5
    > upper limb of sun, 5 lower limb of sun. I try o allow 5 or 6 hours between
    > shot runs, to get a better spread between LOP's.
    >
    > I also, when both are available, shoot sun and moon, usually a string of 5
    > shots each upper and lower limbs, when possible, so I would have sufficient
    > data to average, I believe. As to your observation about "From your posts I
    > have the impression you've started right in on multi-body fixes, and
    > difficult ones at that." I've tended to doing things the hard way for years
    > and years. Fools rush in and so forth. At home, in Pittsburgh, it's a case
    > of shooting across the Monongahela River down town, the distance is 500 or
    > so yards, requiring the dip short correction, and then there are boats going
    > by, or standing in the parking lot at our apartment complex,  using an
    > artificial horizon, shooting the sun in AM and again in the PM 5 hours or so
    > later. Using the AH, once I opted to actually read the instructions, my
    > calculated positions are usually quite close to GPS coordinates, within a
    > couple of miles, sometimes less. Shooting sun, as above described, at the
    > beach, I usually end up within a couple of miles (2 to 3) of GPS
    > coordinates, and have come as close as 1 NM. Of course, there were some
    > shots about which the less said, the better. The three body fixes I've done,
    > at the beach were shot at evening civil twilight or thereabouts.
    >
    > Greg:
    >
    > On your comments regarding the three body fix I posted, Altair, Fomalhaut
    > and Jupiter, 23 October, the data I posted are, as I recall, what Frank had
    > requested. As for Height of eye, and IC that you questioned, the H/E was 6',
    > IC was 0.1' Off Arc, or so it appeared.  My a distances were all Toward, and
    > my "fix" was inside the "cocked hat" or triangle. I determined it's location
    > via an exercise in geometry, didn't think I remembered any, bisecting each
    > side of the triangle, erecting a perpendicular, and extending this line for
    > each side. Where the lines crossed, is the location of my fix, or so I
    > assumed. The time between shot of Fomalhaut and the others, being longish,
    > could have thrown the whole thing off, but I ended up, measuring with
    > dividers, with my fix being less than 2 NM fro GPS coordinates, as mentioned
    > previously. Maybe just dumb luck, I cannot say.
    >
    > Even with weather permitting, here in Pittsburgh, doing star shots "downtown
    > by the river" is, due to light pollution, an exercise in futility, and
    > beside that, it's often cloudy, not to mention cold this time of the year.
    > I'm limited to using the AH in the parking lot during the daylight hours,
    > weather permitting or delving through a couple of years worth of data
    > derived from standing on the beach in North Carolina.
    >
    > As I mentioned to Anabasis, being at sea, I never have been so, where one
    > has a 360 degree horizon must be and entirely different situation, for
    > shooting in North Carolina, from my location there, I'm limited to shooting
    > with an arc running from roughly 110 Degrees to 250 Degrees. I plot stars
    > with the 2102-d, but realize that I'm missing one hell of a lot of sky.
    > Shooting to the North is out, as I'd be over the land. Problems,
    > everywhere..
    >
    > All:
    >
    > Lest I forget, thanks  for your patience, time, attention and efforts.
    >
    >
    >
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