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    Practice with your artificial horizon and bubble sextant
    From: Gary LaPook
    Date: 2015 Jun 21, 14:57 -0700

    I have posted several times in the past about using Polaris to parctice with an artificial horizon and with bubble sextants and to use Polaris to find the index error of your bubble sextant. Polaris is crossing your meridian about 9:30 pm and am now, the end of June so is a convenient time. Here is a cut and past from some of those posts:



    From: glap...{at}PACBELL.NET
    Date: Sat, 17 Feb 2007 03:10:16 -0800
    Local: Sat, Feb 17 2007 3:10 am
    Subject: Using Polaris
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    An easy way to practice with a bubble sextant and to take a number of
    observations to allow you to determine its accuracy is to use the
    meridian transit of Polaris. When Polaris is crossing the meridian it
    is moving horizontally and its altitude doesn't change. For the period
    of more than 15 minutes both before and after meridian passage the
    altitude of Polaris changes less than one - tenth of a minute (0.1').
    For a period of 34 minutes before and after passage the altitude of
    Polaris changes less than one half minute (0.5'.) For 48 minutes
    before and after its altitude changes less than 1 minute.
    Calculate the time that Polaris is crossing your meridian and get out
    early and start shooting. It is about 4:45 am and pm now but wait a
    couple of months and the time will  become more convienent. Calculate
    the altitude by adding your latitude to the polar distance of Polaris,
    now 43 minutes, and then ADD the refraction correction (yes ADD) which
    will then give Hp (precomputed altitude, this is how it is done by
    flight navigators) because this porceedure allows you to compare your
    Hs directly with Hp to determine the intercept immediately, or the
    error in the observation if taken from a known point. If shooting a
    lower transit subtract the polar distance from your latitude. You only
    have to update this Hp from time as the declination of Polaris changes
    Gary LaPook


    From: Gary LaPook <glapook---.net>
    Subject: Shooting Polaris with a bubble sextant
    To: "Navlist" <Navlist@fer3.com>
    Date: Friday, May 27, 2011, 12:26 AM

    And for everybody with a bubble sextant, this is a good time of year to test the accuracy of your sextant. When Polaris is crossing your meridian its altitude doesn't change by even 0.1' for a half hour because it is traveling horizontally as it crosses your meridian. You can take many sights without having to worry about recording the exact time or working out the Hc for each shot. The altitude doesn't vary more that half a minute of arc for a period 68 minutes and doesn't vary more than1.0 minutes for 96 minutes. 

    Right now (May 26th)  it is crossing your meridian at about 11:30 pm and gets earlier by four minutes every day. 

    To check the accuracy, calculate what you should measure with the sextant from your known location. To do this simply start with your latitude and subtract the polar distance of Polaris (which is 41.3' since its declination now is 89° 18.7' north) to compute Hc at your location as it crosses your meridian. Then to make this directly comparable to your sextant altitude add the refraction correction to the Hc to determine what flight navigators call Hp (precomputed) and this should be the altitude measured by your bubble sextant. By applying the refraction to the Hc with the sign reversed you make it directly comparable to the Hs. 

    If you look at the "Q" correction table for Polaris found in the Air Almanac you will see that the correction is -41' while the LHA of Aries in in the range of 31° 49' to 51° 18' and + 41' while LHA Aries is 211° 42' to 231° 25'. Simply reverse the signs from the Q table and apply to your latitude to compute Hc. Calculate when the LHA of Aries is within these ranges and go out and shoot Polaris.


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