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Horizon sights and index corrections

Jan 1, 2003

I enjoyed the recent article on horizon sights ("Horizon sights," Issue No. 63). The piece reminded me of a time about 20 years ago when I was at Lincoln City on the Oregon coast. As I watched the sun set, I had what I thought was a great original idea. Why not time the moment the upper limb of the sun disappeared, call the sextant angle zero and work out an LOP? I did this three days running, took the results home and computed the LOPs. They were very good. When plotted, all of them were closer than two miles.

I could hardly wait for a chance to share my great discovery. Fortunately, I did wait. Several months later I stumbled on a description of the horizon shot by none other than the great practical navigator of the twentieth century: Captain Weems. As a young ensign he had tried a series on shipboard. He did about a dozen, and only one was errant; it was out by 11 miles.Weems had anticipated me by about 40 years.

One thing about the article that seemed wrong was the use of index correction (IC) in the examples. Since one isn't using a sextant when doing a horizon sight, how can one have an index correction?

John Watkins lives in Vashon, Wash., and is the developer of the Celesticomp series of celestial navigation computers.

Editor's note: The examples we published were in error. IC is obviously not applicable to a horizon sight.


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