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    Re: Navy MK 5 Octant Using Natural Horizon
    From: Gary LaPook
    Date: 2012 May 2, 14:35 -0700
    The advantage of using Polaris is that the altitude doesn't change so you can shoot as many shots as you like without doing any additional computations. Below is one of my prior posts about this.

    gl


    --- On Fri, 5/27/11, Gary LaPook <glapook---net> wrote:

    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.

    gl

    Attached File: 116416.q table.pdf (no thumbnail available)


    http://fer3.com/arc/m2.aspx/Fw-Shooting-Polaris-with-bubble-sextant-LaPook-may-2011-g16416



    --- On Wed, 5/2/12, Alexandre E Eremenko <eremenko---.edu> wrote:

    From: Alexandre E Eremenko <eremenko---.edu>
    Subject: [NavList] Re: Navy MK 5 Octant Using Natural Horizon
    To: NavList@fer3.com
    Date: Wednesday, May 2, 2012, 2:21 PM


    Greg,
    Thanks.
    As I understand, the only advantage of Polaris, in comparion
    with some other star will be that the hour angle does not change much,
    so you do not really need to time accurately and even
    reduce your
    observation.
    The same can be achieved with the Sun, but you need to time and reduce
    accurately.

    Alex.

    On Wed, 2 May 2012, Greg Rudzinski wrote:

    >
    > The best way to figure index error for a bubble octant is to use Gary LaPook's Polaris method (see archive).  Observe Polaris at zero LHA or 180 LHA. You can also take 30 observations in a row then average and apply an I.C. that gives a 0.0' intercept that compensates for the average intercept.
    >
    > The natural horizon prism and bubble horizon are separate and require separate trials for determining I.C.
    >
    > Greg Rudzinski
    >
    > [NavList] Re: Navy MK 5 Octant Using Natural Horizon
    > From: Alexandre Eremenko
    > Date: 2 May 2012 14:59
    > Look like very good resuts (for a bubble) to me.
    > The sky just cleared in Indiana, let me try some bubble observations
    > with my MkIX A.
    > I conclude from your messages that your IC is different for the horizon
    > and for the bubble.
    >
    > I never measured my octant bubble IC. How do you do this?
    > Neither I measured it with a horizon, because a horizon is something
    > I see very rarely in my life:-(
    >
    > On the other hand I never detected any systematic error.
    >
    >
    > Alex.
    >
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