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On 27 Sep 2006, at 6:05 PM, FrankReedCT@aol.com wrote:

> Last night at 7:07:00 Central Daylight Time, I  was standing next
> to a street
> light here in Chicago. Looking up the pole, I  noticed Vega very
> close to he
> zenith. So I pulled out an index card ruled with  lines spaced 0.25
> inches
> apart and measured the distance between Vega and the  zenith. I
> judged the
> position of the zenith by visually extending the sides of  the pole
> to the point
> where they met. I knew the distance to my hand at arm's  length
> looking up was 16
> inches so it was a simple matter to get the distance in  degrees. I
> found
> that Vega was 3.3 degrees south of the zenith and 0.8 degrees  west
> of the
> zenith.
>
> Almanac data: Vega's Dec: 38d 48'. Vega's GHA at  1900 CDT (0h
> GMT): 86d 21'.
> So at 1907 CDT, the GHA would be 88d 07'.
>
> I  can calculate my latitude right away. It's 38.8+3.3 or 42.1
> North. Next I
> have  to convert the angle west of the zenith to Local Hour Angle.
> For small
> angles,  that's easy, I think. I divide the measured angle, 0.8
> degrees, by the
> cosine of  my latitude. That gives 1.1 degrees. So my longitude
> must be
> 88.1-1.1 or 87.0  West.
>
> My actual position at the time of this observation was 41d 56'
> 50"N, 87d 39'
> 47"W so the error is about 0.15 degrees in latitude and 0.65
> degrees in
> longitude. I'm fairly confident that I was able to estimate the
> angles accurate
> to +/-0.2 degrees so I attribute the error in longitude to a
> tilted lamp post.
> :-)
>
> -FER
> 42.0N 87.7W, or 41.4N  72.1W.

Frank does this so well!  These kind of posts get me more excited
about navigation than almost any other, because we do not always have
sophisticated nav gear with us, yet he shows us that if we just THINK
a bit more, we can deduce our position in this "back-of-the-envelope"
manner with great satisfaction.

Thanks Frank!

I've got to go find my index cards...

Dan


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