# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Data reduction request
Date: 2013 Nov 18, 20:04 -0500

Hi Frank

I read those as 41 degrees 50.285 minutes.  I believe the third group of numbers to be decimal minutes.  But hey, that's just me

On Nov 18, 2013 7:40 PM, "Frank Reed" <FrankReed@historicalatlas.com> wrote:

Richard Gaarden, you wrote:
"I recorded three solar altitudes today. They are:
66 41 53 Lower Limb {at} 15 13 37 GMT
65 49 41 Lower Limb {at} 15 24 18 GMT
64 23 21 Lower Limb {at} 15 39 36 GMT"

Ok, so as I recall, you're in western Pennsylvania and that seems to match the coordinates you've given (but see below). If it's around 15 hours GMT, that makes it about 10:00 in the morning EST. Your "solar altitudes" are larger than 60 degrees and decreasing. These are presumably zenith distances (instead of altitudes), which is, I think, consistent with your statement that these observations were made with a theodolite.

For the first observation, subtracting from 90°, I get 23°18.1' for the altitude of the Sun LL. Then I subtract 2.3' for refraction and add 16.2' for semi-diameter (ignoring solar parallax for now), which should then be comparable with the predicted Hc at the time of the sight. The result is 23°32.0'. But the predicted Hc for your location (from the USNO calculator page here: http://aa.usno.navy.mil/data/docs/celnavtable.php) is 23°35.2'.. For the second observation, the corrected altitude is 24°24.3' while the predicted Hc is 24°30.3'. For the third altitude, the corrected altitude is 25°50.7' and the predicted is 24°40.6'. I could be wrong, and maybe I have mis-interpreted your input numbers, but those look like fairly substantial errors: -3',-6',+10'.

By the way, you gave your latitude as "41 50 285"? I presume that's 41°50'28" What's the third number with three digits? It won't change the outcome much.

-FER

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