Flights, aircraft, basically all engineering is based on Euclidean geometry (basically the plumb & level). The earth is level as in orientation, not flat.

Key Principles

Euclidean geometry is based on five main postulates, which provide the foundation for the study of plane (2D) and solid (3D) figures.

The most famous is the parallel postulate, which asserts that given a line and a point not on it, exactly one line through the point can be drawn parallel to the original line.

Euclidean geometry describes flat (planar) space, where familiar concepts like points, lines, angles, distance, and shapes such as triangles and circles, are rigorously defined and related