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Index Correction The sextant error caused by a misalignment of the two mirrors when the index arm is set to 0° and the micrometer is set to 0.0'. The correction is measured by aligning the direct and reflected views of the horizon or a star and then reading the micrometer. If the reading is a small number, the IC is "on the scale" and the correction is to be subtracted. If the reading is a large number such as 58.2' the correction is 60' minus 58.2', or 1.8' "off the scale" and the correction is to be added. The rule is, "If it's on, take it off; if it's off, put it on." It is important to measure IC carefully at some point, and then check it at each sight. The primary disadvantage of plastic sextants are unavoidable variations of the index correction, which means it must be checked frequently during any sight session. With metal sextants this value rarely changes. See also notes under Index Error and note on terminology in the cel nav general discussion group. Abbreviation:  IC

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