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    Re: Working lunars from calculated altitudes.
    From: Bruce Stark
    Date: 2002 Mar 31, 01:08 EST

    In a message dated 3/29/02 4:12:37 PM, george{at}HUXTABLE.U-NET.COM writes:
    
    << It's clear that this long-winded reiteration process does not lend itself
    to hand-calculation, though a computer would do it in a twinkle.
    
    If anyone (and here I am thinking particularly of Bruce Stark) can suggest
    a way around this problem, I would be pleased to hear about it. >>
    
    Bruce responds: Well, I had my first go at it this evening. Calculated the
    altitudes for that first set of Pollux distances taken Monday night, using
    GMT 04:00:00. That's three-quarters of an hour early of the truth. Pollux's
    altitude came out O.'4 different from the that gotten with the correct GMT.
    Moon's altitude came out 11.'6 different. GMT found by lunar observation came
    out 19 seconds different from that found with altitudes calculated with the
    correct longitude and GMT.
    
    The first step in finding the hour angles for calculating the altitudes was
    to find SHA meridian, the north-south line directly above me at the moment of
    the observation. I found the sun's SHA by subtracting GHA Aries from GHA sun
    (had to add 360� to GHA sun in this case).
    
    I'd taken a time sight of the sun the afternoon before the lunar observation,
    so knew the error of the watch on local apparent time. By subtracting local
    apparent time SINCE NOON (converted to arc) from SHA sun I got SHA meridian.
    That nailed my east-west position in the celestial sphere at the moment of
    observation.
    
    The difference between SHA meridian and SHA of the body was the local hour
    angle needed to calculate the altitude. Pollux's SHA came directly from the
    Almanac, of course. The moon's SHA had to be found the same way as the sun's.
    
    Perhaps a study of the "equatorial stars" part of the star chart on pages 266
    and 267 of the Almanac will make this understandable.
    
    The purpose of the drill was to get GHA, with its huge change per hour, out
    of the picture.
    
    I hope to do more work on this tomorrow.
    
    Bruce
    
    
    

       
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