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    Re: sun sights
    From: Gary LaPook
    Date: 2011 Jan 31, 01:11 -0800
    Two obvious errors, you transposed the labels for latitude and longitude and you made a math
    error, your intercept should be 40.2 towards not 10.2 towards (BTW, intercept is in NM not
    minutes.)

    Since you would expect the intercept for the first shot to be about the same as the other shots,
    there must be an error. Since it is easy, when using an artificial horizon, to mistakenly measure
    the upper limb of the sun instead of the lower limb, an easy check to make is to subtract twice the
    semi-diameter from Ho and see if you get a more reasonable result. The S.D. was 16.2' so twice
    that is 32.4' subtracted from the Ho gives a corrected Ho of 33̊ 40.0' with a resulting intercept of
    7.8 NM toward which is in line with the other sights. (An easy way to check that you are taking
    the lower limb is to try it both ways, touching the edges of the sun one way and then the other,
    the method producing the lower altitude is the correct way to measure the lower limb.)


    You can also adjust the Hc calculated from the AP to the coordinates of the DR so that you can
    directly compare your Ho with the Hc at the DR which then shows the accuracy of you sextant
    work. First we calculate the adjustment for the difference in LHA. The LHA at the DR  is 345̊
    45.2', 14.8' less than the LHA you used for your calculation. Now look in H.O. 229 for latitude
    37̊, declination 17̊ contrary and LHA 245̊ which is 34̊ 10.7' , 13.8' less than the tabulated Hc
    that you used. This means that Hc changes 13.8' less for a whole degree change (60') in LHA. So
    doing the proportion, (-13.8  X 14.8 ) / 60 = minus 3.4'.

    Then we make the adjustment for the minutes of latitude, in this case 10' less than the latitude
    you used. This adjustment is made based only on this difference and the azimuth. Unfortunately
    the table to use for computing this adjustment is not included in H.O. 229 though it was in the
    earlier tables, H.O. 214. The table is available here:

    http://fer3.com/arc/img/105707.ho214-delta%20lat.pdf

    Referring to this table for azimuth 164̊ and 10' of latitude shows the adjustment to be plus 9.6'.
    Combining these two adjustments with your Hc makes the adjusted Hc at the DR 33̊ 38.4'.
    Comparing this with the corrected Ho of 33̊ 40.0' makes the intercept 1.6 NM toward, not too
    bad!

    (BTW, to make the degree symbol use NUM LOCK  followed by ALT 248 on the number pad,
    not the top row of keys.)

    Gl



    --- On Sun, 1/30/11, Hewitt Schlereth <hhew36---.com> wrote:

    From: Hewitt Schlereth <hhew36---.com>
    Subject: [NavList] Re: sun sights
    To: NavList@fer3.com
    Date: Sunday, January 30, 2011, 7:16 PM

    Good Evening, Patrick -

    The first thing I did was to run your signts on my StarPilot hand
    calculator using the lat/lon of your deck to get an idea of how the
    sextant readings were going.

    1) In the 1621 sight at first go it looked like you might have taken
    an upper limb sight instead of the lower you indicated. Changing the
    input to upper limb, the SP gave an intercept of 1.7' T.  So nothing
    terribly awry with your sextant work.

    Running the 1621 sight from the assumed position gave and intercept of
    10.3' T. Since your assumed latitude is 10NM north of your deck, this
    makes sense.

    2)  For the 1718 sight the SP calculator gave an intercept from the
    deck of 0.9' T. Real good!

    From the assumed position, SP gave an interdept of 10.7' T. Again,
    sensible given the distance of the AP from your deck.

    3) For the 1842 sight the calculator gave an intercept from the deck
    of 0.2'. Bravo!

    From the AP the calculator gave and intercept of 10.7' T. Again,
    consistent with the fact that you AP is 10 NM north of your shooting
    position.

    All in all, I'd say you've got the hang of things.

    Hewitt



    On 1/30/11, Fred Hebard <mbiew---.net> wrote:
    > Patrick,
    >
    > I'm glad to hear that tectonic forces are not too aggressive in the
    > vicinity of your garden.  I'm at about the same latitude, but 81* 50'
    > W, and found out the same thing about the deck attached to the back
    > of my house, much to relief of my family!
    >
    > I didn't work through all your sheet, but your assumed longitude for
    > the first sight is 76* 7.2' W when the GHA would suggest that 62* 7.2
    > W is correct.
    >
    > I started out with HO 229, but then moved to computerized reduction
    > to focus on my sextant technique.  By the time I had the technique
    > down, I was too lazy to go back to hand calculations!
    >
    > One thing I did find was that accurate timing, to the second, is
    > critical. I used the difference between the observed altitude and the
    > calculated altitude as a metric of accuracy.
    >
    > Fred
    >
    >
    > On Jan 30, 2011, at 7:40 PM, Patrick Goold wrote:
    >
    >> Dear Nav-listers,
    >> This is boring compared to recent lunar observations, but I am
    >> still trying to determine the position of my front garden using sun
    >> sights.  As advised by several of you, I moved past using meridian
    >> passage.  On Saturday I took three sights one approximately an hour
    >> before LAN, one at LAN and one approximately one hour after.  I
    >> then reduced the sights using H.O. 229 (which I discovered is a lot
    >> like doing your taxes) and plotted the results on a universal
    >> plotting sheet.  The plot showed that the altitude intercepts of
    >> the second and third sight intersected at a point very close to my
    >> GPS long/lat.  The first one, however, seemed out.  I ran my data
    >> through the on-line sight reduction calculator at <http://
    >> www.celnav.de/sightred.htm> and it confirmed the problem.  I cannot
    >> find my mistake.  Worksheet attached.  All assistance gratefully
    >> received.
    >>
    >> BTW, the garden is, relatively speaking, stationary.
    >>
    >> Best regards,
    >> Patrick
    >>
    >>
    >> Charles Olson: "Love the World -- and stay inside it."
    >>
    >> <29 Jan sunsights.pdf>
    >
    >
    >
    >
    >



       
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