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    Re: MkIXa bubble sextant performance
    From: Gary LaPook
    Date: 2007 Apr 24, 09:53 -0700

    Gary LaPook wrote:
    A very good way to check your index error and the accuracy of you
    sextant is to shoot Polaris when it is on the meridian. At those times
    the altitude of the star remains constant for a considerable time
    allowing you to take a large number of sights in a short time without
    having to do any computations. Right now Polaris is on the meridian
    about 1:26 am local time (your zone time corrected for your longitude
    difference from the central meridian of the zone) and it gets 4
    minutes earlier every day. By this date next month it will be at a
    more comfortable 11:30 pm local time. For observers in the mid
    latitudes the altitude of Polaris does not change by even one tenth of
    a minute of arc for a 30 minute period centered on meridian passage.
    The change doesn't exceed half a minute for a 68 minute period and
    does not exceed one minute for an hour and a half. Meridian passage
    now is for the lower passage so its altitude is calculated by
    subtracting  the distance from the pole of Polaris (90� minus its
    declination) from your known latitude, map reading or GPS. This gives
    you Hc. Then add the refraction correction to Hc to get Hp which you
    can directly compare with Hs to find intercept. Take the average of
    the many intercepts you get and you have your I.E. and the sigma of
    the observations will show you your overall accuracy.
    On Apr 4, 10:41 pm, jean-philippe planas
    > Hi Alex,
    >   Did you try to identify an Index error or any systematic error?
    >   Did you solve the issue of the bubble getting bigger during the 
    observation after having been reduced when starting the observation session?
    > alex  wrote:
    > After a month of practice, I can find my position within 2-3 miles
    > with this sextant.
    > The use of the averager gives better results than a single
    > observation.
    > But averaging 5 single observations by hand gives the same precision
    > as the mechanical averager does (it averages 60 observations).
    > Here is a typical complete fix with the averager:
    > GMT April 3, 1:28:46 Sirius 28d22' ; 1:51:55 Venus 15d18'.5
    > Using DR 40N, 87W, I obtain
    > Alt of Sirius 28d44'.2, Azimuth 207d,
    > Alt of Venus 15d12'.8, Azymuth 282d.
    > Correcting for refraction and plotting on the Universal Plotting Sheet
    > gives this position 40d20'.5N, 86d54d.5W, as read from the
    > plot with 0'.5 resolution.
    > My true position was 40d27'.2N, 86d55.8W,
    > so the error is slightly more than 1 mile.
    > In this reduction I used the Complete on Board Celestial Navigator,
    > which is not very precise but very handy indeed.
    > Here are two typical series of individual observations (averager off,
    > reduced
    > with the Almanac and computer).
    > Venus, errors: 1'.8, -2'.4, -0'.9, -0'.2.
    > Sirius, errors: 1'.2, 1'.7, 3'.0, 0'.0, 1'.8.
    > Alex.
    > ---------------------------------
    > Food fight? Enjoy some healthy debate
    > in the Yahoo! Answers Food & Drink Q&A.

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