Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Basics of computing sunrise/sunset
    From: Douglas Denny
    Date: 2009 Jun 18, 07:38 -0700

    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.
    ====================
    
    
    >
    >> "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.
    >> 
    --~--~---------~--~----~------------~-------~--~----~
    
    
    --~--~---------~--~----~------------~-------~--~----~
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site