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    Azimuth Equation
    From: T. Shanklin
    Date: 2005 Oct 18, 23:12 -0700

    I've been recently programming a TI-81 Calculator to do my sight reductions.
      Everything has been going swimmingly, getting closer and closer with my
    sights and shooting more stars- until the past week or so when the LHA of
    Altair at twilight became closer to 360 degrees.  With all my fiddling, I've
    narrowed the problem and any accompanying possibly useful troubleshooting
    information down to this:
    The calculator will find Hc, but when it tries to calculate the azimuth, it
    sends back the error message "Math 04 Error".  When I enter the equation and
    values manually as one equation I get the same message after pressing enter.
    Strangely it seems, I have found that when the Declination is South (that
    is- the dec value is negative) the error message does not occur and
    calculator works just fine.
    The error message only shows up when the LHA is 345 degrees or greater.
    I'm using the azimuth equation out of the Nautical Almanac:
    cos-1[(sinDec cosLat)-(cosDec cosLHA sinLat)/cosHc]
    To find Hc: sin-1[(sinDec sinLat)+(cosDec cosLHA cos Lat)]
    In entering the equation, if the Declination or latitude is South, or the
    Longitude is West, the value is negative.
    I am unable to figure this out; I've reached the end of my mathematical
    tether at this point.  When I work it out manually one step at a time
    everything works out fine (e.g. calculate the sin Dec*cosLat, save that
    value, calc. cos Dec*cosLHA...)  Does anyone have any clue as to why this is
    happening?  Like I said, I had no problems whatsoever until the LHA began to
    approach >345 degrees.
    One example from my recent sights during which the problem occured:
    Altair-10/13/05- 0157:28 GMT
    LHA= 349.9983333
    Dec= N8.885
    Todd Shanklin
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