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    Re: Venus
    From: Frank Reed CT
    Date: 2005 Dec 11, 23:04 EST

    George H, you wrote:
    "I hope Frank will back me in warning sky viewers  about the dangers of
    peering so near to the Sun."
    
    Yep. If you project  the Sun onto a sheet of paper with a typical six-inch
    reflector and hold a piece  of paper --even white paper-- right at the normal
    distance where you would place  your eye, the paper ignites in a fraction of a
    second. I wonder how long it  takes to ignite an eyelid?
    
    And:
    "Ideally, it should have the ability  to be locked to an
    automatic motor-drive, as I imagine was the case with the  instrument that
    Frank used."
    
    No operating clock drive the few times  I've done this, but the axes on most
    equatorial mounts can be clamped, and it's  wise to do that when observing
    close to the Sun.
    
    "As we have  discussed before, the telescope doesn't make a Sun image
    intrinsically  brighter; only bigger, on the retina. Nevertheless, the
    brightness of any Sun  image on the retina is enough to quickly cause
    irreverseable damage. Be  warned!"
    
    I agree with that. Isn't that a bit of a change of heart for  you? I thought
    that you had previously argued that this fact that surface  brightness can't
    be changed by an optical system (which is very true) meant that  the threat of
    eye damage from looking at the Sun through a telescope  (specifically a
    sextant telescope, in previous discussions) was no greater than  the threat of eye
    damage when looking at the Sun for the same period of time  with the naked eye.
    
    There's another approach to observing Venus near  inferior conjunction that
    reduces the risk quite a bit. Set up the telescope  near the edge of the shadow
    of a house or other large object. Aim at the Sun, as  before by projecting
    its image, to get the instrument aligned. Set the setting  circles to the Sun's
    RA and Dec. Wait 20 minutes, or as long as it  takes for the Sun to pass
    behind the house so that the telescope is now  completely shaded. Offset to the RA
    and Dec of Venus adding those 20 minutes, or  however many minutes, you've
    waited (assuming the clock drive hasn't been  running). Of course, modern
    computer driven telescopes can do all this with the  click of a button, but where's
    the challenge in that??!
    
    So that you have  some evidence that other people do this, too, I went
    hunting for examples of  inferior conjunction observations of Venus on the web.
    Here's a nice account  from a guy in Australia:
    "On the morning of August 20th early cloud cleared  at about 11am to a
    sparkling pre-Spring day. Using the shadow line at the edge  of my house, I
    carefully blocked out the Sun (aware of the 3,000 in Britain  reported suffering from
    vision problems after the August 11th solar eclipse).  Using extended fingers
    to mark degrees and natural markers on guttering I was  soon able to explore
    the right hunting area. Five minutes brought no success.  Then I noticed the
    rising gibbous Moon and took a look. My binoculars were out  of focus! Back to
    the search.
    
    Within minutes the silver glowing eyelash  of the crescent Venus jumped out
    at me. Carefully, I moved away the glasses and  after several attempts I just
    glimpsed the planet with the unaided eye. This was  Venus within about 12 hours
    of Inferior Conjunction. Leaving my seat in position  I easily found the
    planet again the following day. "
    
    That "silver glowing  eyelash" is a nice description. But best of all, since
    Venus has a thick  refracting atmosphere, the crescent of Venus extends beyond
    the usual 180  degrees from cusp to cusp when it's very close to inferior
    conjunction. It's a  unique sight.
    
    [complete URL for the above quotation:
    _http://www.asnsw.com/universe/1999/cvas-1099.htm_ (http://www.asnsw.com/universe/1999/cvas-1099.htm)
    the site includes some excellent advice on daylight Venus  hunting]
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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