The apparent angular displacement of a celestial object due to a change in the position of the observer. With a baseline of known length between two observations, the distance to the object can be determined directly. The topic of parallax is divided into several subtopics: geocentric parallaxes, which use the surface of the Earth as a baseline to observe solar system objects; heliocentric or trigonometric parallaxes, which use the diameter of the Earth's orbit around the Sun to observe objects in the solar neighborhood; statistical and secular parallaxes, which are statistical methods used to derive distances based on the proper motions of the objects; and photometric and spectroscopic parallaxes, which rely on the normal astrophysical properties of stars to derive distances to them indirectly. Parallaxes have been determined from space satellites, including HIPPARCOS and the Hubble Space Telescope, and additional space projects are planned, such as Gaia and SIM.