DAVIS Mark 15, 25 Sextant - Manual (Page 11)

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Figure 11
Due to the height of the eye of the observer, the visible horizon (H) falls below the
plane tangent to the earth at the point where the observer is standing (P).
Correcting for the Height of the Eye
When measuring the altitude of the sun, you need to measure the angle
formed by a ray from the sun and a plane tangent to the earth at the point
where the observer is standing. However, due to the height of the eye of the
observer, the visible horizon actually falls below this theoretical place (Fig. 11).
This requires that a “dip correction” be made.
To apply a “dip correction” for the height of the eye:
Apply a correction as shown in the table below. Dip correction increases
as the eye is raised further above the surface of the water.
Dip correction must always be subtracted from the sextant reading.
Height of Eye Dip Correction
5 ft. (1.5 m) 2'
10 ft. (3.0 m) 3'
15 ft. (4.5 m) 4'
25 ft. (7.5 m) 5'
40 ft. (12.0 m) 6'