The EJS Stellar Aberration 3D 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 (ie in the direction of Earth's motion) very slightly.  As a result the apparent location of the star on the sky is different from its true location.

The simulation illustrates the effects of aberration for a star at any location in the sky, during any time of year.  A 3D view shows the Earth orbiting the sun, the star, and the apparent position of the star on the Celestial Sphere.  This view can also display vectors detailing how the velocity of Earth combines with the velocity of light from the star to produce a new relative velocity vector that indicates the apparent location of the star.  A separate 2D view shows the "true" location of the star as well as the apparent location for an observer looking up from Earth.

Note that some features have been simplified or exaggerated.  The Earth's orbit is treated as a circle.  The size of Earth, Sun, and Earth's orbit are greatly exaggerated compared to the distance to the star.  The speed controls allow the user to set Earth's orbital speed to an appreciable fraction of light speed, which is not realistic.  Finally, the simulation illustrates the "classical" aberration effect, not the (more correct) relativistic aberration.

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