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    Re: Basics of computing sunrise/sunset
    From: Frank Reed
    Date: 2009 Jun 19, 17:16 -0700

    Jim Wilson, you wrote:
    "Having viewed the last six total solar eclipses, I can now worry about
    the conditions immediately preceding totality."
    
    First of all, I'm jealous. :-) Six total eclipses... very nice!
    
    As for worrying, no, I wouldn't. The people who are at risk in those last few 
    dozen seconds before or after totality are people who are unaware that they 
    are looking at an eclipse. It's really very strange to me how long it takes 
    for casual obervers to notice that there is an eclipse in progress (unless 
    the Sun is near the horizon). I remember observing a 99% eclipse in Chicago 
    years ago, and no one that I spoke to noticed anything wrong with the Sun 
    though some did sense the "harshness" and "strangeness" of the sunlight on 
    the landscape. And incidentally, the next time you see a 99% eclipse, 
    consider that you're seeing as much sunlight and the same total brightness 
    for the Sun as you would see on a sunny day from the rings of Saturn (10x 
    further from the Sun, 100x fainter). Now as the eclipse passes from 99% to 
    99.9% and then totality, quite suddenly casual observers will notice the Sun 
    is "wrong" and they may start staring at it --it's rather spectacular at that 
    point. The catch is that, while the total brightness of the Sun is greatly 
    reduced and does not induce the normal "blink" reflex, the intensity of the 
    light is quite unchanged and staring at it for a dozen or a few dozen seconds 
    will cause permanent damage. Of course, any observer who has been forewarned 
    will not make this mistake. Unfortunately, the media sometimes over-do these 
    warnings and create the impression that an eclipse is beaming evil death rays 
    at the unwary.
    
    And you concluded:
    "We were always advised to never look at the pre-eclipse sun through
    binoculars, even with shades. But during totality, they're indispensable,
    allowing us to see local sun activity like sunspots and solar flares. I
    find that 10x20 are perfect, where the sun fills the field."
    
    Looking through 10x binoculars at a 1% spot of the Sun is no different from 
    looking at the full Sun with the unaided eye (except that the glass of the 
    lenses --even without shades-- significantly reduces the UV portion of the 
    spectrum). So for example, if you're looking at the very end of totality 
    through binoculars and you suddenly see Bailly's beads (small bits of the 
    Sun's disk visible through valleys and depressions along the limb of the 
    Moon), you will not harm your eyes so long as you look away within a second 
    or so --just as you would do if you glanced at the full Sun high in the sky 
    without optical aid. Binoculars or telescopes make the image of the Sun 
    LARGER but do not change its surface brightness, in the sense of either 
    energy per square millimeter on the retina or energy per square arcsecond in 
    the visual field. So if you're looking at a 1% bit of the Sun, then magnified 
    by a factor of ten, its area either in square arcseconds, would be the same 
    as the Sun normally. The total brightness of that 1% bit would be just the 
    same as the full Sun with no optical aid. The magnification alone is 
    responsible for this --there's no additional "enhancement" of the energy.
    
    -FER
    
    
    
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