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    Operation of A-7 bubble sextant
    From: Gary LaPook
    Date: 2011 May 4, 13:12 -0700

    I have attached 14 photos showing the operation of the averager on the A-7 bubble sextant. The first photo, "octant," is of the Pioneer octant which is the type used by Noonan on the Earhart flight.

    IMG 5845 and 5846 show the A-7, the only differences are that the altitude adjustment knob is taller to allow for making the averager marks, the pencil marking arm, its lever and the shot counter.

    IMG 7869 shows the sextant set up to take an observation. I have set the altitude to 20° 00'. The "2" in the window indicates 20 degrees and the scale on the altitude adjustment knob is on zero degrees and zero minutes. Each division is 2'. One rotation of the knob changes the altitude by 5 degrees and the altitude is read by combining the reading from the window with the reading on the knob. You can see that the shot counter has been reset to zero.

    IMG 70 shows the pencil just touching the knob for the first shot and the altitude is still at 20° 00'.

    IMG 71 shows the first mark and IMG 72 shows that the shot counter has advanced to "1".

    IMG 73 shows the result of ten shots and IMG 74 shows he counter advanced to "10".

    IMG 75 shows the lowest reading of the sextant, 19° 14'. In the window the index is just below the "2" so we know the altitude is just less than 20 degrees and is above 15 degrees so we add 15 degrees to the reading on the knob, 4 degrees 14 minutes to find the altitude of 19° 14'.

    IMG 76 shows the pencil again lined up with the first mark made as shown in IMG 71 which was made at the highest setting, 20° 00'.

    IMG 77 shows that lining up the pencil with the first mark brought the altitude back to 20° 00'.

    IMG 78 shows the pencil lined up with the center of the ten marks, between mark number five from the right and mark number number five from the left.
    This is the average of all the shots.

    IMG 79 shows the reading of the altitude scales with the pencil positioned at the average of the pencil marks. The average reading for the ten shots is 19°34'.

    gl


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