This one's a right bastard: you can clearly see it, you can photograph it, you can tell friends about it and they can see it clearly too, the sun even disappears behind it if you're facing in the right direction.
And yet … it doesn't exist.
The thing about the horizon is that it's all a matter of perception. Which sort of puts it in the same box as that other non-existent yet ever-present concept: the present.
The horizon is merely the furthest point that you can see. So my horizon will be different to another person's horizon if they are somewhere else. Or even different to what I would see were I to move position or location.
Of course when I say "the furthest point you can see" I mean the furthest point you can see horizontally. Because the lazy bastard horizon is always lying down.
The horizon is therefore the geographic equivalent of a couch potato.
If the horizon is a geographic couch potato, I wonder what the geographic equivalent of its remote control would be. Or the geographic equivalent of the TV set that the geographic equivalent of the couch potato is watching its geographic equivalent of football on. Or what the geographic equivalent would be of the beer it is drinking. Or the potato chips it is eating as it scratches the geographic equivalent of its bum.
But one thing is certain: the geographic equivalent of its couch is our planet.
The planet Couch.