The top half of each frame shows the appearance of the sky as seen from the Earth (ignoring the Sun), and the bottom half shows a fixed view looking down from above onto the plane of the Earth's orbit around the Sun (the ecliptic). A red star is shown located some distance to the right (also in the ecliptic plane). In this simulation, the star is fixed in space with respect to the Sun, and its proximity to the Sun is greatly exaggerated to help make its parallax easy to see.
In the first half of the movie, the parallax motion of the red star over the course of one year is shown. Note that the star is not moving through space, as can be seen in the bottom panel, only the Earth is moving. The star's parallax motion is simply a reflection of the Earth's orbital motion. When viewed from the moving Earth (top panel), the red star appears to move first west (towards the right) then east (towards the left) with respect to the distant background stars which are so far away that their parallax motions are too small to be seen at this scale.
In the second half, we move the star 2x farther away (as indicated by the scale bar at the bottom) and run through another year. Now the annual the trigonometric parallax motions are 2x smaller because the distance to the star is 2x greater. This fact, that the trigonometric parallax of a star is inversely proportional to its distance from the Sun gives us a direct measurement of the star's distance.
Note that the parallax motion of the star is an illusion due to the orbital motion of the Earth around the Sun. Real stars are much more distant than shown here (the nearest stars, the Alpha Centauri triple star system, is about 150,000 AU away, resulting in a maximum parallax amplitude of about 1.3 arcseconds).
[Credit: R. Pogge, OSU]
Updated: 1998 January 5 [rwp]
Copyright © 1998 Richard W. Pogge, All Rights Reserved.