Amateurs can make occultation measurements that are used to refine the orbits of minor planets. The planets were originally thought to have formed in or near their current orbits. Perturbations by Jupiter send bodies straying there into unstable orbits. Titan orbits Saturn at 20 Saturn radii. All KH-11 satellites are in either of two standard planes in Sun-synchronous orbits. Other methods are use for spacecraft in higher orbits. Analogous statements are known for perturbations of more complicated orbits. Stellar distances can be calculated by measuring parallaxes as Earth orbits around the Sun. Equatorial low Earth orbits (ELEO) are a subset of LEO. An artificial moon orbits the planet, upon which the Lunarians live. Development of an infrastructure for altering asteroid orbits could offer a large return on investment. In 1809 Carl Friedrich Gauss published his method of calculating the orbits of celestial bodies. Venus orbits the Sun every 224.7 Earth days. For a discussion of the Kepler problem specific to radial orbits, see Radial trajectory. These were flown at low inclination orbits.