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Douglas -

This is interesting. I have never tried lunars before, but the recent
heads-up about Frank's Lunars Calculator gave me the impetus to try,
because I figured it would let me see how I was doing right away.
Otherwise, I'd be left to slog through the math and then, if I got
screwy results, I'd wonder if I'd blundered in the algorithm or if it
was my sextant work.

My sextant is a Davis Mark 15 with a 2.5X scope from an Ebbco. A
friend found two Davis sextants in the bilge of a boat he'd recently
bought and gave me the moldiest one, which I was able to spruce up
with fresh water and new mirrors ordered online from Davis. It's
worked just fine for the normal celestial stuff.

To make a long story short (too late, I know, I know) I made my first
try at Junprer-Moon Wednesday night, which turned out perfectly
miserable - off by 2�. Thursday got it down to 1�.
Last night I made my third try and came in on Frank's calculator with:

Error in Lunar: 0.9'
 Approximate Error in Longitude 28.3'.

This is interesting to me on two counts. First my results are in line
with yours, which is heartening. Second, my progress of halving the
error on successive tries agrees with my experience in teaching. I've
found students make the same progression in learning to use a sextant
from scratch.

In case anyone is interested in the actual data, here it is as it went
into Frank's Lunars Calculator - a truly awesone learning tool, IMHO.

Lat     18�19.9 N    (Lat & Lon per GPS)
Lon    64�42.5 W
IC       -1.5'
Temp  82�F
Press  29.90 in
HE      230.2 feet (per GPS. I live part way up the side of a mountain).
Date   October 22 2010
Time   23-31-21 GMT
Dist    35�10.8'  Far

-Hewitt


On 10/24/10, Douglas Denny  wrote:
> Thank you for your suggestion, but sorry Frank, but I am now fully happy
> with the assessments I have made and conclude it is near impossible to
> obtain better sights than around one minute of arc (plus or minus 1/2 moa)
> generally and easily for Lunar distance measurements - which resolves to a
> time indication of no better than two minutes in time.
>
> ...and that for Moon Jupiter measurements I have been testing, where Jupiter
> is like a lighthouse beacon in the sky, and the relative movement of
> Moon/Jupiter is over 31 minutes of arc per hour - as it is just now.
>
> I accept it might be possible with care and experience to better this
> perhaps by a factor of two, but it is certainly not easy; and I can't do
> it.
>
> In fact I have just gone out right now and obtained a sight with a 2min-29"
> error (1 and 1/4 moa error) for Moon/Jupiter at 20Hr-51min GMT on Sunday
> 24th Oct 2010.
> The variation I obtain can be as much as + or - six minutes of time out -
> three minutes of arc.
>
> Nearly all of the tests I have done whether with my 'best' sextant or even
> as I have included - a box sextant,  are no better (mostly) than within +
> or- four minutes of time, equivalent to two minute of arc. If I am near two
> minutes of time - one minute of arc I feel I am doing well.
> It seems a standard deviation of around one and a half minute is the best
> one can expect in normal practice.
> Your suggestions of measurements better than this to consistently sub-minute
> accuracy I do not obtain and (forgive the pun) frankly, doubt it possible.
>
> We shall have to agree to disagree.
>
> Douglas Denny
> Chichester. England.
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