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    Re: Azimuth Equation
    From: Herbert Prinz
    Date: 2005 Oct 19, 04:19 -0400

    Hello Todd,
    1. Do you have a manual from which you can you find out what exactly
    "math error 04" refers to? (If not, it's probably online.)
    2. Most programmable calculators have a single step mode that permits
    you to walk through a program step by step and watch the results.
    3. As an alternative to 2, you can display intermediate results at
    critical points in your calculation. Make this additional output
    dependent on the setting of flags and leave it in for a couple of month
    until you have confidence in your program.
    4. Use defensive coding techniques. For instance, did you follow the
    practice to keep the argument of arc functions within the interval
    -1..1, as shown in step 3 on page 279 in the N.A? You really must do that.
    5. When the program does not crash, does it produce the correct result?
    Herbert Prinz
    T. Shanklin wrote:
    > 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
    > Lat=N38.563
    > Todd Shanklin
    > **********************
    > _________________________________________________________________
    > Don?t just search. Find. Check out the new MSN Search!
    > http://search.msn.click-url.com/go/onm00200636ave/direct/01/

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