If you know the distance to your horizon then the instant anything comes over the horizon you know its distance off.
There are a lot of equations for calculating distance to the horizon. They all give much the same answer and most are based on Pythagoras and his right-angled triangle. You need only two: one working with the height in feet and one with the height in metres. Both give answers in nautical miles.
Distance to Horizon = 1.144 x ^Height of Eye in feet. Distance to Horizon = 2.072 ^Height of Eye in metres.
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