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    Re: Sun altitude limits for natural horizon?
    From: Frank Reed
    Date: 2016 Feb 20, 15:38 -0800

    The year won't matter much to assess visibility. If it's possible, do you have the altitudes listed for the stars/planets in this example? Of course, if there is a planet in the mix, we can probably figure out the actual year. More importantly for the question at hand, the altitudes could confirm that the time is correct (and the identification of it as "zone time" rather than GMT). 

    Did the example simply say that it was from an actual logbook, or was it specific on the vessel details and maybe the navigator's name? I ask because it's an old trick to claim that details are "real data" as a means of creating credibility, even drama. My favorite example of this is the movie Fargo by the Coen brothers, released in 1996. It begins with the famous words: "This is a true story. The events depicted in this film took place in Minnesota in 1987. At the request of the survivors, the names have been changed. Out of respect for the dead, the rest has been told exactly as it occurred." However, as they admitted some years later, they threw this in to create a greater sense of drama. It wasn't true! Viewers were more likely to put themselves "in the shoes" of the victims, the perpetrators, and the investigators of the crime when they believed it was real. The same is true in education as in film. If you are looking at real data collected by real navigators, it is more compelling. And if a teacher tells you the data is real when it's not, does it matter? So long as it has been carefully checked and guaraneteed to be consistent, it serves its purpose. So unless we have actual primary source evidence that these are real observations, you have to be a little wary of any example like this.

    As for the general question, I'm sure we all know that the recommended time for sights using a natural horizon is from "sometime" after sunset (specifically, this is highly dependent on details of sky conditions as well as sextant and observer) up until roughly the end of nautical twilight when the Sun is 12° below the horizon as a matter of definition. But these are not laws of nature. One can continue later with a really sharp horizon, maybe a Moon in the sky, and above all really good dark adaptation. So it's certainly possible that sights could have been taken with the Sun 20° below the horizon. But that would be an unusual case.

    Frank Reed

       
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