written by Todd Timberlake

The EJS Parallactic Proper Motion Model 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.

• Download ejs_astronomy_ParallacticProperMotion.jar - 2229kb Java Archive File

Published May 17, 2011
Last Modified June 11, 2014

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The source code zip archive contains an XML representation of the EJS Parallactic Proper Motion Model.   Unzip this archive in your EJS workspace to compile and run this model using EJS.

• Download ejs_ParallacticProperMotion.zip - 75kb Compressed File

Released under a This material is released under the GNU General Public License Version 3.

Published May 17, 2011
Last Modified June 11, 2014

This file has previous versions.