When you watch a distant object out of the corner of your eye, you can keep it in view without turning your head even though you walk some distance at the same time, as long as you walk in a straight line. However, if the object is about the same distance away as the length of your walk, you will end up looking over your shoulder if you insist on keeping an eye on it. This is the essence of parallax, the shift of the apparent direction of a source as the observer changes position - which might not seem to be much help to Astronomers, until you realize that the Earth moves quite some distance every year in its path about Sol. By carefully measuring the angular shift in a star's position throughout a year, Astronomers can gauge its distance from the Earth out to an impressive range.