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    Re: Side error is not a real error
    From: Antoine Couëtte
    Date: 2021 Sep 28, 11:48 -0700

    Dear Herman,

    In reply to your query, I prefer a LOW Altitude star because if I can see directly both the star and the horizon in the ocular (remember, the sextant height index is very close to zero), it is easy to set the sextant "quite vertical" because you can then see "everybody" in your ocular :

    (1) - The star directly,

    (2) - The horizon directly, and

    (3) - The star via dual mirror reflections. And also

    (4) - The horizon via dual mirror reflections, but this one is harder to see and anyway no need for it.

    Maintaining your sextant exactly vertical is paramount here, otherwise you are ruining your IE observation.

    Starting with both images side by side and sextant approximately vertical, just swing your sextant and see how "fast" both star images start showing up at appreciably different altitudes. If both images are 1 minute apart, if you are off vertical by 1° - with the horizon in sight and some practice you can easily keep your sextant vertical by 2° or better - your height error is 1 arc Minute x sin 1° which is quite negligible (0.02'). But with no Horizon direct view - i.e. observing a "high" star - if your side error is 5' and you are off vertical by 5° (which indeed may easily happen: in case of ship's movement, your local accelerations especially on big ships will skew your inner ear system), then your error exceeds 0.4' which is starting to become just average for an IE calibration.

    And with little practice you will feel very confident about your IE calibrations off Low Alt Stars. You can consistently get quite solid and reliable IE's from Low alt stars even with appreciable side errors. As regards the Sun, and in the case of appreciable side errors, your IE calibrations quickly become rather unreliable because both Sun's do not hit "head on" but "sideways" instead. Sun observations may also be degraded by the use of the shades, which is not the case for the stars since you do not need the shades then.

    Also, instead of Jupiter, use Saturn (as soon as it shows up: preset your sextant altitude) which should be quite manageable now even at N52° because unlike Antares it is further away from the Sun. And I trust - but have not verified - that you should be able to directly see Saturn as well as the direct horizon from where you are.

    Hope it helps,

    Best Navigational Regards,


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