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I gave the spotting scope method a try using the top of the Ritz-Carlton in White Plains, New York, visible at 3.5 n.m. distance and 20x80mm binoculars. I left a bit of side error to be able to distinguish the two images. Correct alignment is discernible to within a very small fractional turn off the index screw.

When I now go back and use the solar SD method to check the index error it doesn’t come out to zero. In some ways that’s not that surprising giving the level of accuracy being sought. Accuracy of 0.1’ requires that adjustments be made to within 1/300 th of the solar diameter in the sextant scope which seems quite a daunting challenge.

How reproducible do people find the results of solar SD method for determining the index error?

Typically when I do rounds of sights after all the usual adjustments and corrections have been taken into account there remains some systematic bias (see for example the observations in http://fer3.com/arc/m2.aspx/Noon-Sun-sights-Jamestown-RI-this-Sunday-Aug-25-Stuart-aug-2013-g25027 ). This could be a misestimate of the height of the eye or similar but may have a component of personal error in it. I have begun to think that the best approach might be do a round of sights from the vessel in a known location to determine a personal correction which would also account for residual misadjustments. Thoughts?

Robin Stuart