**Hull down**. The horizon at sea is seen as a curve reflecting the shape of the earth. As a ship approaches another, the tallest part of her becomes visible above the horizon first and an observer sees first the mast, then the sails, then eventually her hull. Thus for a ship to be hull down over the horizon, if any part of her is visible it will be just that part of the rigging sticking above the horizon and she will be difficult to see.

The higher one stands above sea-level, the further over the curve one can see and thus the farther away the horizon moves. Standing at sea level with one’s eyes about five feet above the surface of water, the horizon is about 3 – 4 miles distant. Climb a 75 foot mast and the horizon moves to about 8 – 9 miles away. The formula for computing this in nautical miles, is 1.15*(root H + root h) where H is the height of the object at sea being observed, and h is the height of the observer’s eye above sea-level, both heights in feet.