dip of the horizon

Webster's Dictionary (1913)

Astronomy, the angular depression of the seen or visible horizon below the true or natural horizon; the angle at the eye of an observer between a horizontal line and a tangent drawn from the eye to the surface of the ocean.

From the entry Dip.

Astronomy, the vertical angle between the sensible horizon and a line to the visible horizon, the latter always being below the former.

From the entry Horizon.

Webster’s Revised Unabridged Dictionary (1913), from GCIDE 0.54