Dip of the Sea Horizon.
Dip, Depression or Inclination of the Visible Horizon (Dip of the Sea Horizon) is the vertical angle between the True Horizon Plane and the direction to the Visible Horizon.
Subtract from the observed altitude of a celestial body to obtain the altitude above the true horizon.
This happens due to Terrestrial Refraction, because the Earth’s atmosphere is not all the same density. It depends on air pressure, temperature, humidity, purity, etc.
Coefficient of Terrestrial Refraction.
It characterizes the terrestrial refraction ability and has a variable value in various books or nautical table.
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