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    Re: It's Moon-landing Monday
    From: Frank Reed
    Date: 2009 Jul 24, 00:17 -0700

    I wrote previously:
    "What could you do while approaching the Moon if you could see tenth
    magnitude stars on it's limb?"
    
    And Greg you wrote:
    "I like observing orbiting satellites better."
    
    So do I. But let me elaborate a little on the dramatic effect of seeing so 
    many faint stars. Take the 7x telescope, e.g., from the sextant case and aim 
    it at the dark limb of the Moon. You will see hundreds of occultation events 
    in just a few minutes. With fairly simple software, these would give you an 
    accurate fix almost immediately. The small number of celestial objects 
    visible when the horizon is also visible down here on Earth actually forces 
    us to measure angles. If we would see tens of thousands of stars, then 
    "coincidence events" like occultations and close appulses/conjunctions 
    between objects would be much more useful. Similarly, if there are satellites 
    in orbit around the Moon, typical ranges to them might be on the order of one 
    or two thousand miles. If we have accurate orbital data on them, and if we 
    can observe tow or more of their positions as they pass near known stars at a 
    known instant of time to +/-1 or 2 minutes of arc, then we can get a position 
    fix to within +/-0.3 miles at the low end up +/-1.2 miles at the high end. So 
    toss the sextant, and keep the telescope. 
    
    By the way, the satellite approach also works on Earth. I've described this 
    before, but it doesn't hurt to repeat it --I hope. Suppose we have good 
    orbital data on some satellites with typical ranges above us on the order of 
    500 miles. If we know their orbital data, and if we observe their positions 
    with binoculars to +/-0.1 degrees (not difficult), then we can get a position 
    fix to +/-1 mile. The usual uncertainty in orbital data is the mean longitude 
    on the orbit which is nearly equivalent to an error in the time of the sight. 
    That is, a satellite might be five seconds early but it's unlikely that the 
    path will be much in error, just offset in time. Because of this possibility, 
    it's better to plot LOPs from two satellites under an assumption of an error 
    in the observation times.
    
    -FER
    
    
    
    
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