When an object is viewed from two separate locations, that object appears to shift its position relative to the background. This is called parallax, and a perfect example of parallax is evident by placing your thumb in front of y our face and looking at it with one eye, then with the other eye. What happens? The thumb looks like it jumps around when you switch eyes from one to the other . Note that when your thumb is closer to your face, it appears to jump around m ore as compared to when your thumb is further away.
Thus, we say that the angle of parallax (P) is inversely proportional to the distance (d) of the observed object to the observer. This relationship c an be expressed mathematically via the following equation:
... where d is the distance in parsecs (1 parsec = 3.26 LY) and P is t he parallax angle in seconds of arc (there are 3600 seconds of arc in one degree ). Until recently, the limit of resolution of the parallax method was only about 100 parsecs; that is, stellar parallax could only determine the distance to objects up to 100 parsecs (about 326 LY) away. However, recently a wealth of data has come from the Hipparcos Astrometry Mission allowing the parallax method to be used with a resolution of 1000 parsecs!
As shown in the following figure, observers from Earth can use the size of the Earth's orbit as a method of determining the parallax angle (and thereby the distance) of very remote astronomical bodies.