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Parallactic Proper Motion
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.

Please note that this resource requires at least version 1.5 of Java (JRE).
1 source code document is available
Subjects Levels Resource Types
- Astronomy Education
= Curricula
- Cosmic Time and Distance
= Parallax
- Historical Astronomy
= History of Astronomy
- The Sun
- Lower Undergraduate
- High School
- Instructional Material
= Simulation
Intended Users Formats Ratings
- Learners
- Educators
- General Publics
- application/java
  • Currently 0.0/5

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Access Rights:
Free access
This material is released under a GNU General Public License Version 3 license.
EJS, Easy Java Simulations, OSP, Open Source Physics, William Herschel, motion of the Sun, parallactic motion, proper motion
Record Creator:
Metadata instance created May 17, 2011 by Todd Timberlake
Record Updated:
June 11, 2014 by Andreu Glasmann
Last Update
when Cataloged:
May 17, 2011
Other Collections:

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Record Link
AIP Format
T. Timberlake, Computer Program PARALLACTIC PROPER MOTION, Version 1.0 (2011), WWW Document, (
T. Timberlake, Computer Program PARALLACTIC PROPER MOTION, Version 1.0 (2011), <>.
APA Format
Timberlake, T. (2011). Parallactic Proper Motion (Version 1.0) [Computer software]. Retrieved February 20, 2017, from
Chicago Format
Timberlake, Todd. "Parallactic Proper Motion." Version 1.0. (accessed 20 February 2017).
MLA Format
Timberlake, Todd. Parallactic Proper Motion. Vers. 1.0. Computer software. 2011. Java (JRE) 1.5. 20 Feb. 2017 <>.
BibTeX Export Format
@misc{ Author = "Todd Timberlake", Title = {Parallactic Proper Motion}, Month = {May}, Year = {2011} }
Refer Export Format

%A Todd Timberlake
%T Parallactic Proper Motion
%D May 17, 2011
%O 1.0
%O application/java

EndNote Export Format

%0 Computer Program
%A Timberlake, Todd
%D May 17, 2011
%T Parallactic Proper Motion
%7 1.0
%8 May 17, 2011

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Parallactic Proper Motion:

Is Based On Easy Java Simulations Modeling and Authoring Tool

The Easy Java Simulations Modeling and Authoring Tool is needed to explore the computational model used in the Parallactic Proper Motion.

relation by Wolfgang Christian

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