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    Re: Basics of computing sunrise/sunset
    From: Douglas Denny
    Date: 2009 Jun 18, 14:33 -0000
    NEVER never NEVER look at the sun at any stage of its being above the horizon with any from of magnification:  binoculars, telescopes - anything.
    I have seen 'holes' in the retina with consequent poor central vision due to this in practice.
    Outside of visible spectrum:  especially  infra-red emissions are still there and can damage the fovea - permenantly in seconds if there is any intensity in the sun's light at all.
    Even some so-called "dark glass" or "smoked glass" filters, especially on early sextants are suspect for not filtering out the infra-red sufficiently. They were not aware of the poor quality of filtering with some 'coloured glass filters' outside of visible spectrum in the ninteenth century for example, as the physicists like Tyndal and Brewster were only becoming aware of the physics.
    Make sure you use correct filters whenever using a telescope for viewing the sun.
    Douglas Denny.  Optometrist.
    Chichester. England.
    ----- Original Message -----
    Sent: Thursday, June 18, 2009 12:53 PM
    Subject: [NavList 8694] Re: Basics of computing sunrise/sunset

    >> "I recall .... using.... binoculars to spot the moment of sunrise/sunset"

    >>> Readers be warned never to do this if you value your eyesight.
    > As I see it the sun is an orange (elongated horizontally, actually squashed
    > vertically) ball as it is rising or setting, and does not have the magnitude
    > it does higher in the sky thanks to the added atmosphere.  Of course lenses
    > may magnify its "brightness."  At any rate, what we want to look at is when
    > the upper limb is just kissing the horizon (rising or setting) and I do not
    > perceive this as a great danger because of atmosphere, refraction, and the
    > sliver we can see when the body is physically below the horizon. I am open
    > to correction, which is one of the uses of a group like this.

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