Anyone who has done Celestial navigation has corrected for the angle you look down at the horizon. It is called 'dip'.
Height of Eye (HE)
height of a sextant user's eye above sea level
Dip Correction
Dip is the angle of the visual horizon, dipping below the true horizon due to your eye height above it. This is also tabulated in the nautical almanac. An approximate equation for dip correction that incorporates a standard horizon refraction is thus:
Corr DIP = - 0.0293 • SquareRoot(h) Decimal Degrees
Where h is the eye height above the water, meters. Corr DIP is always negative.