Which error increases when a 18th-century marine sextant's index mirror is not perfectly perpendicular to the frame?

💡 Explanation

When a sextant's index mirror is misaligned, side error results because the reflected image is skewed relative to the direct image, leading to errors in angular measurement that vary sinusoidally as celestial bodies are tracked. Therefore observed altitude varies sinusoidally, rather than a constant errors or longitudinal drift, because these require different physical causes.

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