NavList:

When I teach about index correction, I like to bring out a little postal scale. If it reads 1 ounce when there's nothing on it, then it's clear to everyone that if I place an object to be weighed on it, and it displays 7 ounces, then the actual weight is 6 ounces. You subtract the "phantom" weight. The index correction for a sextant is analogous. It's just the offset from zero when it SHOULD be reading zero, like the "tare weight" of an empty scale. And if it shows some positive value (or as navigators call it, if it's "on arc"), then the offset has to be subtracted from any and all readings of the sextant before you even think about what they mean (and if it's "off arc", it has to be added, of course). It's just the instrument's scale offset. It doesn't matter if you're measuring a horizontal angle between buoys, an altitude of the Sun off the sea horizon, a lunar, or a doubled altitude from an artificial horizon. The index correction gets applied first, before you consider the meaning of the angle in any way.

-FER

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