Stellar Aberration 3D


General Description

This simulation 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 Space View window shows the Earth (blue point) orbiting the Sun (orange point). The open white circle shows the location of the star on the celestial sphere. The colored (initially magenta) point shows the apparent location of the star on the celestial sphere, which is shifted from the true location because of the aberration effect produced by Earth's orbital motion. Options allow the user to display the Earth's velocity vector and its orbital path, the vector sum that produces the aberration effect (the velocity of light from the star minus the velocity of Earth gives the apparent velocity of light from the star), the equatorial and ecliptic planes and axes, direction arrows, and a trace of the star's apparent motion due to aberration. There are also options to display plots of the apparent right ascension and declination of the star as functions of time. A menu allows the user to select a particular day (equinox or solstice) of the year. Controls allow the user to adjust the time of year, speed of Earth, and coordinates of the star. The color of the apparent star (and its traces, etc) can be changed so that multiple traces can be shown at once, each in a different color.

The Sky View window shows the apparent location of the star on the sky as seen by an Earth observer. The "true location" (the location as seen from the Sun) is shown as an open white circle, while the apparent location (effected by aberration) is shown as a colored disk.

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.


Space View Frame

This frame shows a view of the Earth orbiting the sun, the "true" and apparent locations of the star to be observed on the Celestial Sphere, as seen from outside the Sphere.

Sky View Frame

This frame shows an observer's view of the "true" and apparent locations of the star as seen from Earth.

RA Plot Frame

This frame shows a plot of the apparent right ascension of the star as a function of time (with time given in years since the vernal equinox).

Dec Plot Frame

This frame shows a plot of the apparent declination of the star as a function of time (with time given in years since the vernal equinox).
Todd K. Timberlake (ttimberlake@berry.edu)