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    Re: A recent movie's depiction of sextant usage
    From: Frank Reed
    Date: 2022 Sep 29, 12:55 -0700

    Joe,

    Thanks for the notice on this. I saw some "nautical" elements in previews and considered watching just to chase down the mere chance of a sextant or other nautical astronomical reference. If anyone else wants to hunt for the scene, it's around time stamp 1:04:40, and it's very short, about six seconds. I have not yet watched the movie. I scanned back and forth to find this scene. Took me three passes!

    You wrote:
    "There's a scene in which the protagonist Jacob uses the sextant to shoot the sun while riding on the back of the Red Bluster.The movie then switches into a view through the sextant telescope, possibly the very first depiction of an actural sextant sight in any movies. By looking at the motion of the sextant sight,it is obviously that the character tries to bring the sun down to a position that is a little under the horizon,and then he let the sun to rise on its own until the lower limb kisses the horizon, a method appreciated by experienced navigators.(The movie sadly did not depict a 'swinging sun', a common practice in the end to do tangency checks) The image of the sextant view itself is errorneous, although the director or whoever rendered this scene is quite commendable for having an accurate idea on sextant sights being vertically split. Just not in the way they presumed to be: a sharp line cuts the sight in half,with only half the sun and half the horizon visible,the other halves being completely hidden. We know in actual practice, that is not the case,there is no sharp line in the middle if you are observing through the telescope, refractions in the telescope optics would enable us to see a more or less whole yet translucent sun, superimposed over the horizon image. Images depicted in the movie can only be observable if you remove the telescope on the sextant and look directly through the mirrors(or using any antique octant which usually doesnot have a telescope fitted)"

    I agree that the sharp split between the two sides of the field of view is too clean. Two other issues bother me as much, if not more. First, the field of view appears to be about 30° minimum. Second, our poor hero would go blind! The right side of the field of view should be nearly black and the Sun should be a somber disk. Instead, we can see by the view through the instrument, and also by the direct view of the instrument, that this sextant has no shades. In the interest of an "in-universe" solution to this apparent flaw, I can stretch things and notice something else peculiar about this sextant. The index mirror is not mirrored at all. We can clearly see through it as he lowers it. So perhaps the animation depicts that rarest of "optical beasts", a sextant with a fully un-silvered index mirror. As with a Bris sextant, one could look at the Sun directly in some positions. Unfortunately, the Sun is depicted almost equally bright on both sides of the field of view, so I'm afraid this expository escape route is cut off. 

    As for your suggestion that our hero "obviously" brings the Sun below the horizon and then lets it rise on its own, I call shenanigans! :) The rate of motion down past the horizon is depicted as basically the same as the rate coming back up through it, and even being generous to the animators' intentions, it looks like nothing more than a depiction of an overshoot and a reversing adjustment to bring the Sun back up.

    Immediately after the Sun sight, with no calculation or even a moment's thought, our hero (who we now know is also a mathematical genius!) places his finger on the chart and says "we'll be there in three days". I found the depiction of the "Dregmorr Sea" on the chart hopelessly inaccurate, and I am forced to speculate that "Peril Island" and "Rum Pepper Island" are entirely made-up.

    Frank Reed

       
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