One of five points at which small bodies can remain in the orbital plane of two massive bodies; also known as libration points. Three of the points lie on the line joining the two massive bodies: L1 lies between them, while L2 and L3 have the two bodies between them. These three points are unstable, slight displacements of a body from them resulting in its rapid departure. The fourth and fifth points (L4 and L5) each form an equilateral triangle with the two massive bodies, 60° ahead of and behind the smaller body in its orbit around the larger one. A well-known example of bodies lying at the L4 and L5 Lagrangian points are the Trojan asteroids in Jupiter's orbit. Among Saturn's satellites, Telesto and Calypso lie at the L4 and L5 Lagrangian points in the orbit of the much larger Tethys. In similar fashion, tiny Helene precedes Saturn's satellite Dione, keeping 60° ahead of Dione. The Lagrangian points are named after the French mathematician J. L. de Lagrange, who first calculated their existence. See also equipotential surface.
Lagrangian point: The Lagrangian points are five equilibrium points in the orbit of one body around another, such as a planet around the Sun