The Stellar Aberration 2D JS Model illustrates the phenomenon known as the aberration of starlight, first reported by James Bradley in 1729.  Aberration occurs because light has a finite speed, and thus light from a star takes a finite amount of time to travel through the tube of a telescope.  During this time, the telescope moves as a result of Earth's rotational and orbital motions (in this case, the orbital motion is more important because it is faster).  Therefore, if the telescope is pointed directly at the star the starlight will hit the sides of the tube before reaching the eyepiece.  To see the star the telescope must be pointed forward (i.e. in the direction of Earth's motion) very slightly.

The simulation shows a telescope (depicted as a red rectangle) and a star (white point) directly overhead.  When the simulation is run a pulse of light is emitted from the star and travels straight downward to Earth.  The telescope moves to the right due to Earth's motion.  As a result, if the telescope is pointed straight up (ie toward the actual location of the star) the pulse of light will not reach the bottom of the telescope.  Controls allow the user to set the speed of Earth and the tilt of the telescope.  The user can modify the tilt until the starlight reaches the bottom of the telescope.  Alternately, the user can use the Options menu to set the telescope to the correct tilt for the current speed setting.

The Stellar Aberration 2D JS Model was developed using the Easy Java Simulations (EJS) version 5.  It is distributed as a ready-to-run html page and requires only a browser with JavaScript support.