Non-Euclidean geometry , useful in modeling interstellar space, that rejects the parallel postulate , proposing instead that at least two lines through any point not on a given line are parallel to that line.
Though many of its theorems are identical to those of Euclidean geometry , others differ. For example, two parallel lines converge in one direction and diverge in the other, and the angles of a triangle add up to less than 180°.