Parallactic Proper Motion


General Description

This simulation illustrates the phenomenon known as parallactic proper motion. Proper motion refers to the progressive (non-oscillatory) apparent motion of a star relative to more distant background stars. If this apparent motion is due to actual motion of the star through space then it is known as "true proper motion." However, this apparent motion can also be due to the motion of Earth (along with the Sun and the rest of the solar system) through space. This motion is known as "parallactic proper motion." In 1783 William Herschel detected parallactic proper motion in several stars and used these motions to determine that the solar system was moving toward a point called the "Solar Apex" in the constellation Hercules.

The window displays a celestial sphere (blue), the sun (orange), and one star (white). The initial location of the sun is at the center of the sphere. The sun can be moved in the direction of the Solar Apex by adjusting the Displacement slider. This alters the line of sight from the sun to the star and thus changes the apparent location of the star on the celestial sphere (which is assumed to be infinitely distant). The user can adjust the distance to the star as well as the star's (initial) celestial coordinates. The final celestial coordinates of the star are displayed at the bottom of the window.

Note that the line from the (final) sun to the star does NOT point to the star's apparent location on the celestial sphere. This is because the celestial sphere is supposed to be infinitely far away, but of course it is not shown that way in the simulation. To find the star's final location on the celestial sphere one must construct a line parallel to the sun-star line, but passing through the center of the celestial sphere. This line intersects the celestial sphere at the star's final celestial location. These lines can be displayed using the Viewing Options menu.

Note that some features have been simplified or exaggerated. The sun is assumed to move along a straight line toward the Apex point. The star is assumed to be stationary. The distance to the star is much smaller than that of any real star. Finally, the size of the sun and star have been exaggerated to make them visible.


Parallactic Proper Motion Frame


Todd K. Timberlake (ttimberlake@berry.edu)