500. When a light is first seen on the horizon it will disappear again if the height of eye is immediately lowered several feet. When the eye is raised to its former height the light will again be visible. Which term is given to this process?

The repeated disappearance and reappearance of a distant light as the observer's height of eye changes is termed 'bobbing a light' in seamanship. This phenomenon occurs when the light is near the geometrical horizon, and small changes in height of eye move the line of sight above or below the Earth's curvature.