NavList:

Yes. Two different aspects of the same thing, and we don't need separate terminology for them. This is not unique to sextants. It applies to any simple manual measuring instrument (and not only manual instruments but we'll limit to that for now).

Consider an old-fashioned postal spring scale. I use one in my navigation classes to illustrate exactly this issue. The postal scale has a little platform on top. You place a package on that platform. The platform sinks down, and a pointer indicates how many ounces (or grams) it weighs. Suppose your postal scale has a "zero-ing" error. When there's nothing on the scale, suppose it reads with the pointer at 1.0 ounces. That's wrong, but it's a simple pointer zero-ing or "index" error. And what do we do with this error? We correct it by subtracting 1.0 ounces from the measured weight of any package we place on the scale. That's obvious, right? The scale displays too much when empty. That incorrect zero reading is the index error or I.E. if we're in a mood for jargon-y abbreviations. We fix that by applying a correction which is equal in magnitude but opposite in sign. Every time I put a package on the scale, I subtract 1.0 ounces from the displayed weight. In other words the index correction or I.C. is the same as the I.E. in absolute value, but if the index error is positive, then the index correction is negative. That's the only difference! This index error and index correction on a postal scale is completely analogous to the IE and IC for a sextant.

What else can we do to deal with index error (zero-ing error) on a simple postal spring scale? Most such scales included a little thumb screw that allowed us to adjust out the index error. It's better when it's zero. If the I.E. is zero, then of course the I.C. is zero. That's easier. Likewise sextants can be adjusted via thumb screws or screw-like adjustment points on the back of the horizon mirror (usually) so that they have zero index error and require no index correction. Whenever possible, zero out your sextant's index error.

Frank Reed