NavList:

Lars B,
You wrote about a book by Roswall, printed in Stockholm 1824. You said that he described "index error" using a term that translates directly to that (in Swedish, yes? is that the original language of this text?).

You added that he recommended measuring "the sun's diameter both ways, the mean of the two readings is the correct zero point, half their difference is the error"

That's interesting. That's not a normal method. It suggests very strongly that this is advice, not for an ordinary navigator, shooting the Sun's altitude, typically, but for a lunarian. Is there any other hint of that in the book? Maskelyne also recommended the Sun-Sun test for highly accurate zeroing (index error) tests when shooting lunars. Of course that method works for any sights, but it is normally not necessary unless accuracy of a small fraction of a minute of arc is required.

A common early name for the error that we now universally call index error was just "error of adjustment". This perhaps reflects the common way of using a wooden octant (or reflecting quadrant, as it was also termed). Like a modern plastic sextant, it was normal to "adjust" the instrument to eliminate any "zero-ing error" immediately before using it. Also in that early period, what we call the index "correction" today might be labeled in actual working as simply "Correction" or "Corr". No 'index' about it. :)

You concluded:
"Above examples show that some authors did make a distinction between error and correction, some did not. But all examples provide instructions on how to handle the issue with non-parallel mirrors. And the use of the word index correction can be traced at least back to 1876."

By the late twentieth century there was a nearly obsessive attention to the IE vs IC definition. When did that get going? We all know, of course, that a phrase as trivial as "index correction" existed long before it became an object of obsession, but that's not the point.

Frank Reed