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    Re: Noon Longitude by Equal Altitudes
    From: David Pike
    Date: 2021 Nov 25, 08:13 -0800


    Further to my message this morning, it looked like being another nice day, so I thought I’d have another crack at the Sun with two modifications.
    1.  I reworked my index error for 10 + 6 degrees on the Mk2C using my Mer Pass observation on 23rd and a more accurate refraction value to give me a revised I.E. of -3.0’.  That’s only 1.5’ off Fenn’s of Farnborough’s 1997 factory calibration. 

    2.  I set 1’ less on the index scale for my second equal altitude observation two hours after the first. 

    I was quite pleased with the result.  To be honest, I was a bit late getting to the sextant; moving it round fifteen degrees azimuth; and repacking my playing card shims on the window ledge to get it level.  By the time I’d done this, the Sun was sitting on the line.  Then my finger missed ‘Pause’ on GPS Anti-Spoof, which I was using as a chronometer, so I had to press it again.  That might explain some of the possible 8 seconds time overread. My working is attached.

    Another future exercise is to work out how I’d have handled observations taken from a moving platform.

    I’m beginning to like this lower limb observing with an aircraft sextant.  The next thing I want to experiment with is sitting the Sun on the top or bottom of my giant A12 bubble.  That’s no good for Mer Pass latitude of course unless you’ve also calibrated the giant bubble size, but it might work for equal altitude longitude. DaveP


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