NavList:

G Becker observed that, "FYI, there is a Roelofs T2 solar filter on ebay."

I appreciate your pointing this out. I know what it is and will buy it if it sells for a reasonable price. Mine is the older T21E so it may not directly fit. Still, with an adapter it would be a great addition to my astronomical efforts.

Frank Reed's observation that it would be difficult to misjudge the lower limb by 10 minutes was classic understatement. It is a blatant error and I expect to discover its source. I didn't even notice this. I only looked at the average of the readings. Perhaps I misread the instrument??

I have looked into the programs I wrote for the HP50. The greatest problem was that when I entered the data from Karl's book I was entering fully corrected data as opposed to my own less well corrected data. His data worked--mine did not. This Celestial Navigation is a fussy business. I do my best work with big concepts and often fail to honor the small details.

None the less, using data calculated by the USNO.NAVY.MIL site (thanks to Frank for pointing this out), my program still produces an Hc which differs from their Hc by varying amounts which so far are less than half a minute. The algorithms I used were presented as approximate. It may be that that just the way is works and good enough for casual use.
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