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    Re: Linear regression and other tools
    From: Antoine Couëtte
    Date: 2018 Oct 12, 08:13 -0700

    RE : Linear-regression-other-tools-TonyOz-oct-2018-g43049

    Dear Tony,

    Ok, I now better understand what you mean.

    In theory, your idea seems good.

    Still, I keep expressing some doubts as to its actual superiority.

    If you interleave the signts, I strongly doubt you that can shoot the same number number of sights as expeditiously as if you were fully observing Bodies in turn : e.g. 5 sights on Venus, then 3 sights on Mars, then 5 sights on a star.

    When observing successive shots on one same body, I generally bag sights every 40 to 50 seconds. Observing "faster" is not necessarily better since you are requiring efforts and concentration from your observing eye. However with a bit of training it is quite comfortable to get and record 5 shots within a 4 minute time span with minimum recording errors.

    When observing bodies in turn - i.e. interleaving the sights - you now need time to preset and/or adjust your sextant height before each observation. Sometimes, stars can be very faint, you may need to first shoot them with your sextant upside down to capture them into the mirror through 'raising' the horizon before shooting them the normal way (remember, it is better to rock the sextant around the eye-to-body axis rather than around an horizontal axis). It seems quite improbable that you will do better than 1 observation per minute of time here. Hence if you observe 3 bodies in turn with 5 observations on each, your observations on any single body will cover a time span of 13 minutes of time (vs. 4) : way too long since the other method can avoid it ! And your horizon or your body might have disappeared meanwhile because of too much darkness for the horizon or too much light for a faint star. Hence whether at dawn or dusk, always observe to the East first and then to the West if feasible.

    When observing a faint star - as faint stars may happen to be your only meal now and then - it is generally quite practicable to observe the same one in a row, and you cling to it until you have successfully secured and recorded a few successive shots.

    To recap, my personal view here is that interleaving sights is probably not a very performing practical option, because:

    - it definitely increases the risk of sooner loosing one "precious" faint star, for unability to reacquire it after a few (3 or 4) minutes of elapsed time devoted to other bodies, and:

    - it is [very] likely to lenghten the total time interval to get the same overall number of individual shots successive heights because of the changes in the sextant heights, and:

    - It certainly requires more overall concentration from the observer hence more toil, which might be prone to increasing the risk of errors, and:

    - it does not improve at all the linear regressions, since instead of being squeezed into a time span of up to 4 minutes, any same body now becomes observed over a much longer time span, which is starting to make linear regression a [much] less performing method because of the sometimes fast emerging and rising effects of the neglected higher order terms.

    You kindly requested my comments. Here they are above :-)

    However, this is only a "best guess" as I cannot not recall having observed interleaved sights. Hence I have no experience with your suggested interleaving method.

    I would read with much interest the feedback of other salty CelNavigators on this subject : Jeremy, Greg, Andrés, Peter and others.

    And if there are no definite comments on this subject, try your proposed method yourself.

    Next time at sea I will also try it on my side.

    Best Friendly Regards

    Antoine

       
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