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    Re: Time sight practice: Joshua Slocum's lunar
    From: George Brandenburg
    Date: 2011 Mar 27, 14:49 -0700

    Having taken Frank's 19th Century course last year I had made a little spreadsheet (that fits in my iPhone) for reducing noontime sun sights and/or morning/afternoon time sights. It includes code from a NOAA spreadsheet to calculate the declination and EOT. It has proved quite useful for practicing and demonstrating sights at the Salem Maritime Site where I'm a volunteer.

    So when I saw Frank's practice problem I decided to punch the numbers into my iPhone. Well most of the boxes in the spreadsheet filled with #'s when I entered the 1896 date. My first guess was that the NOAA calculation wasn't good beyond the present epoch. But then I discovered that Excel marks the beginning of time as 1/1/1900, so entering 1896 had produced garbage.

    I noticed that the first thing the NOAA code does is calculate the Julian date by adding 2415018.5 to the computer date. I modified this by subtracting the number of days in two centuries (73050). This meant if I entered the date 6/17/2096, the NOAA code would calculate the Julian date for two centuries earlier, namely 6/17/1896.

    So I plugged in the measured altitudes after correcting for the -2' index error and an assumed -3' dip correction, and assumed the time zone was GMT-9. This gave Lat = 11//6 S and Long = 138 // 55 W.

    Comparing this to Lars Bergman's Mar 24 result, our latitudes agree, but the longitudes differ by 4'. I checked and found that the NOAA code gives an OK value for the 1896 declination, but misses on the EOT by about 15'' of time, which is just what is needed to explain the difference in the longitude results.

    So all in all not a bad historical result for something intended for contemporary use.

    George B

    p.s. In case anyone is interested, I've attached a copy of the modified spreadsheet expanded to show the NOAA calculation, as well as the original spreadsheet.


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    File: 116028.solar_sight_calculator_1800.xls
    File: 116028.solar_sight_calc.xls
       
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