Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek, meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances. In astronomy, parallax is the only direct method by which distances to objects beyond the Solar System can be measured.