The horizon forms a right angle between the line of sight to the observer and the planet's radius.
Use the Pythagorean theorem to calculate the distance to the horizon: one leg of a right triangle
is the planet's radius, the hypotenuse is the radius plus the observer's height above the surface,
and the line of sight to the horizon is the remaining side.

Some ads would go here if you were connected to the Internet.
Or if you weren't using an ad blocker. Or something.
It's just a placeholder, dammit.
