NavList:

Paul Hirose, you wrote:
"Or maybe not. I don't have the current Nautical Almanac, but the 2020 edition says,
Calculate apparent altitude
     H = Hs + I - D
where I is the sextant index error."

And you asked:
"Is that a mistake?"

By certain narrow definitions over which many navigators obsess and unfortunately sometimes become pedantic, yes, this is an error. And it's in the latest 2024 edition of the Nautical Almanac exactly like that and has been for at least thirty straight years (see below). Personally, I recommend in cases where a simple sign flip can screw things up that navigators, and any players in the games of practical math, should have a means of choosing signs by "thinking" --what does the sign actually imply?? There are ways of driving this home using simple mnemonics, like the slightly daft "if it's on, it's off; if it's off, it's on", to visualizations that I won't get into right now.

Consider this though: while the Nautical Almanac's text quoted above is technically erroneous instruction, it seems that no ships have sunk, no navigation classes have collapsed in chaos that I am aware of, and apparently there haven't even been as much as a few letters to the editors complaining. That this text is wrong by narrow definition (which, arguably, it is), but has been in place for at least thirty years and is found, yes, in the current 2024 Nautical Almanac, tells us something very important. It tells us that this doesn't matter. The instructions are either ignored by real navigators, or real navigators intuitively understand what is meant by the text and gloss right over that slight error. I think that's the case. Myself, I have read that instruction many times, and it never caught my eye. I know what they mean without fussing over the exact definition.

It's also worth asking when this particular "fussiness" got started... When did the difference between "index correction" and "index error" first become formalized? When did the obsession start?! Before that narrowing of definitions, how did navigators cope?? What did they call this, for example, two hundred years ago, in 1824? In that era when few people understood positive and negative numbers, did it make any difference what terminology was employed? What did they call index error back in 1824?

I checked six years of modern issues of the Nautical Almanac. The section we're talking about with "calculator" algorithms was added in 1989, and I don't think it has changed at all in the 35 years since. Fortunately, I had stacked up a bunch of recent almanacs on my desk last month. In less than five minutes, I checked the specific section on index error that you quoted in volumes from 1994, 1998, 2004, 2010, 2016, 2021, and, yes, 2024. They're all identical, right down to the typography and details of printing. Incidentally, the volume for 1994 was accidentally scanned and placed online by Google many years ago, so I used that pdf to grab the page and highlight the text in the attachment below. Again, it's identical to what you quoted, and it seems to be harmless.

Frank Reed

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