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From: Frank Reed (no email)
Date: Sun Apr 03 2005 - 11:27:20 EDT
Bill, you wrote:
"Good and bad. First try was Sun and Moon, March 16, 18:54:15 UT. Lat N 40d
27.7', Lon W 86d 55.7'. Removed the scope as per Alex's off-list suggestion,
and got the hang of finding the angle to measure the great circle distance
instead of what "looked" like the correct angle to hold the sextant. Got
them close and popped the scope on. Two minutes later had an Hs that I had
little confidence in (78d, 48.0'), but came in and plugged it into your
calculator anyway. IC -1.3', temp 50F, pressure 30.07. Results, 0.5' error,
15.7' longitude error."
That's good! By the way, did you preset the sextant by checking the
predicted distance first? This is 100% acceptable --it was the recommended procedure
historically right from the beginning. With the angle preset, you aim at the
Moon and then rotate the instrument around the line of sight until the other
object pops into view. This should happen when the "horns" of the Moon are
nearly pependicular to the frame of the instrument. See why??
And you wrote:
"Encouraged, I went back out and tried to obtain an Hs using the tips on your
lunars page. Hold sextant in both hands, tweak, rest, tweak, rest,
tweak.... By now (20:14:06 UT) the moon was higher and did not have much
contrast against the sky and wispy clouds, but managed to get an Hs I had a
high level of confidence in. Plugged the Hs in and results were 2.6' error,
1 d 19.2' error longitude."
Seems unlikely, doesn't it? An error that big is much too big to believe,
assuming you had a scope attached to your sextant. In my early experience, I
found that these large errors usually resulted from the sort of silly mistakes
that people only make when they're trying something new. For example, you
might right down the time incorrectly by five minutes. Focusing so intensely on
getting the angle right, it's much more likely that we'll screw up something
else that should be really simple.
And:
"I can only surmise you use cookies to rig the calculator like a slot
machine. Intermittent reinforcement, the toughest form of behavior to
extinguish. Big payoff first try, a loss the second try--keeps me coming
back. <G>"
LOL. Well, no... no cookies for you. <g>
And:
"Have had little good "seeing" weather here, so have not tried again--yet.
Been playing with star-to-star distances when weather and time permit to
improve my technique."
Today should be nice for Sun-Moon lunars if you're weather is decent.
And:
"This is going to be like a termite doing calculations for landing the rover
on Mars, but I think on it. ;-)"
Cool. Just remember, this ain't rocket science. Lunars aren't anywhere near
as difficult as they're often made out to be. By the way, have you read Arthur
Pearson's 'Backyard Lunars' article on his web site at
_www.lunardistance.com_ (http://www.lunardistance.com) ?
[note-- I will be travelling for the next day or so and away from Internet
most likely...]
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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