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    Re: Moon's crescent when viewed from low latitudes
    From: Michael Bradley
    Date: 2019 Mar 12, 17:34 -0700

    Tony, you wrote

    'Last evening we saw Moon's crescend with horns almost horizontal.' 

    Yes, a great time for those of us who need spectacles ( for me, these days, Bi-Focals ) to get a free test of our long distance vision.

    First of all, we need a no shades target - the moon against a clear pre dusk sky is excellent. Gazing steadily at it, no sextant yet, yaw your head side to side and then pitch it up and down, watching for the limit of useful movement before losing focus on one or other of the sharp points of the crescent. Repeat at another time when the crescent is upright. All good?

    After that there's the question of collimation of the eye, spectacle lens, and sextant lenses. Start by lining up on a common axis of all the lenses in the system: if you like, taking off any rubber eye cup, feel the sextant eyepiece on the front of the spectacles lens with your free (usually) left hand to make sure it's central on the spherical part of the spectacle lens. Next get all the lenses aligned square across their common axis; the eyepiece contact with the spherical part of the spectacle lens should be on the axis of the lens system. You can feel the eyepiece edges top and bottom, left and right, against the spectacle lens to get an indirect feel for this. You may have to crank your head vis-a-vis the sextant eyepiece away from your previous habit to get this right.

    If you seem to lose focus after a few seconds of fixed gaze, blink. The film of tears on the front of your eye acts as a 'bloom' on a lens. The tear film can evaporate away to alter slightly the optics of the eye, so you renew the film/bloom system by blinking. 

    Nothing new in all of this .....  the tests and adjustments I've suggested are rough and ready, but can make a fair sized difference if you've got into a bad habit. And the crescent moon provides a very useful free optical laboratory target image. Fullish moons, with soft edges, are not so good. 

    Good navigating

     Michael Bradley

       
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