Which inaccuracy arises due to refraction when measuring celestial body altitude near the horizon using an 18th-century sextant?

💡 Explanation

When sighting near the horizon, atmospheric refraction bends light rays, causing the celestial body to appear higher than its actual position, leading to an underestimation of the true altitude angle with the sextant. Therefore the true altitude is underestimated rather than other errors which would depend on observer's positioning and optical properties.

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