From: spam@merlyn.demon.co.uk   
      
   JRS:  In article <72d32143.0405081423.3115a96f@posting.google.com>, seen   
   in news:sci.space.tech, Christopher M. Jones    
   posted at Sat, 8 May 2004 15:23:32 :   
   >cray74@hotmail.com (Mike Miller) wrote in message news:<5dcb47d   
   .0405061609.78da   
   >1818@posting.google.com>...   
   >> What is the threshold of brightness for seeing an astronomical object   
   >> in the daytime Earth sky?   
   >>   
   >> Specifically, would a star with 70% of Sol's absolute brightness be   
   >> visible at 45 AU?   
   >   
   >Yes.  It would be hundreds of times brighter than a   
   >full moon.   
      
      
   70% is not much different from 100%.   
      
   The Sun, at 1 AU, is magnitude -26.8.  At 45 times the distance, it   
   would be 45^2 ~ 2000 times less bright.  Since 5 magnitudes is a factor   
   of 100, a factor of 2000 must be about 8 magnitudes, giving the star at   
   about Mag -19.  Since the Full Moon is about -12.6, the star would   
   indeed be over a hundred times brighter - though perhaps (remembering   
   that 70%) not quite as much as hundreds.   
      
   This differs from my previous answer (not yet seen in News), in which   
   four zeroes escaped; 10000 should be 100000000.   
      
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