If the sextant index error is ON the arc, what correction is applied?

• A. Double the error
• B. Ignore the error
• C. Subtract the error (away from you)
• D. Add the error (toward you)

Answer: (C)

Index error is the sextant’s own offset that must be corrected to turn the instrument reading into true altitude. When the index error is ON the arc, the reading is too large by that amount. To compensate, you apply a negative correction—subtract the index error from the observed altitude. This brings the value down to the true altitude (the magnitude of the correction remains the same, just with the opposite sign). For example, if the index error is 2 minutes on the arc and the observed altitude is 50°15', the corrected altitude would be 50°13' (subtracting 2'). Ignore, doubling, or adding would not produce the correct true altitude in this case.