The simulation has two different modes. The default is an Earth Rotation Mode. In this mode the simulation illustrates the parallax caused by Earth's rotation on its axis. The top window shows an observer (indicated by a green dot) on the equator of Earth (blue disk). The green line shows that observer's line of sight to an object in space (indicated by a red dot). This line of sight extends to show where the object would appear against a more distant background. When the simulation is played the observer moves around the Earth as Earth rotates. Parallax causes the apparent position of the object to move back and forth between two extreme locations (indicated by open red circles). The bottom window shows the apparent motion of the object against the stars, as seen by the observer. [Note: to keep the simulation simple it is assumed that Earth's equator is aligned with the ecliptic plane (the plane of Earth's orbit). In actual fact they are tilted by 23.5 degrees, but this cannot be represented in a simple 2D simulation.]
The other mode for the simulation is Earth Orbit Mode. In this mode the simulation illustrates the parallax caused by Earth's orbit around the Sun. Now the observer moves along Earth's orbital path (shown as a blue circle with the orange Sun in the center). The bottom window now shows the apparent motion of the Sun as well as that of the object being observed.
Controls are provided to adjust the baseline (the diameter of the circle through which the observer moves), the distance to the object, and the location of the observer along the observer's circular path. The parallax angle (half of the angle between the two extreme apparent positions) is given. Menus are provided to let the user control which visual elements are shown and which mode is used.