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
- Viewing Options Menu
- Show Celestial Sphere: show the grid representing the
celestial sphere.
- Show Great Circles: show a set of great circles on the
celestial sphere that pass through the Apex and Antapex.
- Show Apex Line: show a line passing through the Apex and
Antapex.
- Show Final Position of Sun: show the displaced location
of the sun.
- Show Initial Line of Sight: show line from sun's initial
location (at center) through star to celestial sphere.
- Show Final Line of Sight: show arrow from sun's final
location to the star.
- Show Celestial Position Line: show arrow from center to
star's final position on celestial sphere.
- Visual Elements
- Open orange circle: initial position of Sun.
- Filled orange circle: final position of Sun.
- Open white circle: initial position of star on celestial
sphere.
- Filled white circle: actual location of star in space.
- Magenta point: apparent location of star on celestial
sphere.
- White arrow: line of sight from final Sun to star.
- Magenta arrow: line of sight from center to final
position of star on celestial sphere.
- Pink line: arc from initial to final position of star on
celestial sphere.
- White line: line from initial sun through star to
star's initial celestial location.
- Blue grid: celestial sphere.
- Green NCP/SCP: labels for North and South Celestial
Poles.
- Cyan line and labels: line from Apex to Antapex.
- Red circles: great circles on celestial sphere through
Apex and Antapex.
- Controls
- Original RA: initial right ascension of the star (in hours).
- Original Dec: initial declination of the star (in
degrees).
- Star Distance: distance to star (in AU).
- Displacement: displacement of sun toward Apex (in AU).
- Final RA: final right ascension of star (in hours).
- Final Dec: final declination of star (in degrees).
Todd K. Timberlake (ttimberlake@berry.edu)