NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Basics of computing sunrise/sunset
From: Douglas Denny
Date: 2009 Jun 18, 14:33 -0000
>
>> "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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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 -----
From: "Bill" <billyrem42@earthlink.net>
To: <NavList@fer3.com>
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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Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
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