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    Nautical Twilight on an inland lake
    From: Bob Goethe
    Date: 2015 Aug 20, 11:51 -0700

    On Monday, I went out in the pre-dawn to see to what extent it was possible to take viable star sights on a lake near where I live.  The lake is oriented east-to-west and 8 nm long...which is long enough that although I SEE the trees and hills on the far shore, I have a true horizon to work with if I look down the lake at celestial objects which have an azimuth greater than 74° and less than 109°.

    On Monday, at my location, nautical twilight was to last 51 minutes.  I began taking sights at 4 minutes after the beginning of twilight, when I judged that I could see the horizon.  I was mistaken, however.  Sights taken during the first 25 minutes of nautical twilight had an average error of 190 nautical miles.  It became obvious once I reduced my sights that I had only THOUGHT I could see a clear horizon during this period.

    Sights taken during the remainder of the SECOND HALF of the twilight period (i.e. the final 26 minutes of the period) had an average error of 4.9 nautical miles.  So for the second half of the period between the beginning of nautical twilight and the beginning of civil twilight, the horizon of the lake is well enough defined against its background of trees and hills that sextant sights may be taken.

    This means that while the pre-dawn window for doing celestial navigation is shorter than when you are at sea, it is certainly possible to give people realistic sextant training on an inland lake.  Good thing that this has proven to be so, as I have am instructing a class that is scheduled to begin shooting stars at 5:30 AM from this particular lakeshore, a week from this Saturday, and then again a week after that. 

         Two days hence will be the beginning
         of the class...but week 1 will involve
         instruction in background knowledge
         and the mechanics of sextant use during
         the pre-dawn period.  We will shoot the
         sun once it rises.

    (On Monday last, the azimuth of the rising sun was ~35° north of the stars I was shooting.  These stars became invisible within 60 seconds of the predicted beginning of civil twilight.  Perhaps I could have seen stars a little bit longer had I been looking to the west rather than the east, but the only feasible way to use this lake is from the western shore, looking east.)

    Bob Goethe

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