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    Observational Twilight
    From: Robert H. van Gent
    Date: 2021 Jul 5, 10:23 +0000

    On an average moonless night, how long will the horizon remain visible before it becomes too indistinct for taking star altitudes?


    While it is commonly claimed that this happens at a solar depression of around -12° (nautical twilight), I have seen references to somewhat smaller angles.


    Benjamin Dutton's Navigation and Nautical Astronomy - editions of 1948 and 1952, probably also in later editions to which I have no easy access) defines an "observational twilight" at a solar depression of -10° but cites no sources for this value.


    I also found a 1951 paper by Charles H. Smiley in Popular Astronomy




    which contains the statement


      "Research by the Germans during the last war indicated the horizon

        could be seen until the sun was 8° 51' below the horizon."


    but no source was cited.


    Is anyone on NavList familiar with this German study or to other published studies on the solar depression when the visible horizon becomes too indistinct for useful nautical observations?


    Rob van Gent

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