90° - Ho = __________.
Official USCG Answer & Rule Citation
Correct Answer: Option A — zenith distance
As defined in Bowditch Volume I, zenith distance is the angular distance along the vertical circle from the celestial body to the observer's zenith. Since the horizon is at 90 degrees from the zenith, zenith distance is derived directly by subtracting the observed altitude (Ho) from 90 degrees.
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