Which error increases in 18th-century sextant measurements if the horizon mirror develops parallelism error relative to the frame?

💡 Explanation

When the horizon mirror on an 18th-century sextant is not perfectly parallel to the frame, a parallelism error is introduced because the angle of incidence at the horizon mirror no longer reflects accurately, leading to consistent bias. Therefore consistent angular height biases result, rather than bearing inaccuracies, zenith errors, or misleading ship speeds which arise from entirely different navigation errors and mechanisms.

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