The EJS Parallactic Proper Motion Model illustrates the phenomenon known as parallactic proper motion.  Proper motion refers to the progressive (non-oscillatory) apparent motion of a star relative to more distant background stars. If this apparent motion is due to actual motion of the star through space then it is known as "true proper motion." However, this apparent motion can also be due to the motion of Earth (along with the Sun and the rest of the solar system) through space. This motion is known as "parallactic proper motion." In 1783 William Herschel detected parallactic proper motion in several stars and used these motions to determine that the solar system was moving toward a point called the "Solar Apex" in the constellation Hercules.

The window displays a celestial sphere (blue), the sun (orange), and one star (white). The initial location of the sun is at the center of the sphere.  The sun can be moved in the direction of the Solar Apex by adjusting the Displacement slider. This alters the line of sight from the sun to the star and thus changes the apparent location of the star on the celestial sphere (which is assumed to be infinitely distant). The user can adjust the distance to the star as well as the star's (initial) celestial coordinates. The final celestial coordinates of the star are displayed at the bottom of the window.

Please note that this resource requires at least version 1.5 of Java (JRE).