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    Sudden shift in sextant sights
    From: Zvi Doron
    Date: 2012 Mar 28, 20:34 +0100
    I went down today to Brighton Marina to take some pre-computed sights. Basically I pre-compute sun altitudes for a known GPS position just outside the marina for every 20 minutes, and also note the change in altitude per minute for each pre-computation (for example – at 11:20 UTC, calculated altitude for that position was 41 29.0 and it was increasing by 2.4’ per minute at that time). I then sit on the beach and take 3 sights every 20 minutes as close as possible to the target time, correct them according to their time difference from the target time with a table I prepared, and compare them to the pre-computed calculated altitude for that time (for example, I took a sight at 14:18:38 which was 41 25.0 and added to it 3.3’ to ‘bring’ it to 11:20, obtaining an intercept of 0.7’ away).
    The first 5 sets of sights were all ‘away’ and averaged 0.5’ but then something strange happened – from about an hour and a half into this, the next 5 sets were all ‘towards’ and averaged 1.2’ . A sudden shift of about 1.7’ . I have not changed anything, and kept moving up the shingle beach with the rising tide to maintain a constant height of eye of two metres. The day was very warm and sunny with clear sky, high pressure and noticeable haze. The sextant, a Cassens & Plath, was exposed to the sun all the time.
    Puzzled by this, I dug out another similar set I did in 2003 in very similar conditions with a Freiberger yacht sextant and to my amazement these too displayed this same sudden shift – the first few sets were all away and averaged 1.2’, and after that the next 6 sets were all ‘towards’ and averaged 0.7’ . A sudden shift of about 1.9’ .
    I thought about this and excluded all possible explanations I could come up with, including alien activity, tectonic plate shift and osteoporosis. Any ideas?
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