what is a horizon?

Answer 1

It is the line that we see that seperates the Earth and the Sky. Look below this line, and everything you see is on the Earth. Look above and everything is in the sky and beyond. Its easiest to see when on the ocean, because anywhere else is probably obscured by trees or buildings.

Answer 2

the line at which the sky and Earth appear to meet

More precisely, it is the line that divides all of the directions one can possibly look into two categories: those which intersect the Earth, and those which do not. At many locations, the true horizon is obscured by trees, buildings, mountains and so forth. The resulting intersection of earth and sky is instead known as the visible horizon.

Answer 3

Horizon is the line where the earth's surface and the sky apparently join.Neglecting the refraction,the distance of the horizon from an observer at height 'h' is given by√(2ha) where 'a' is the earth's radius.