Earth curvature and horizon in modeling large scenes
Having the horizon fall off is simply dropping the ground in the distance down somewhat.

A point $x$ km away if you follow the curve of the planet will be $r-r\cdot \cos \frac{x}{r}$ down and $r\cdot \sin \frac{x}{r}$ out horizontally where $r$ is the radius of the planet you are modeling (~6.3k km for earth). In the vertex shader you can account for that before you apply the view and projection matrix.