Astronomical Parallax 2D

General Description

This simulation illustrates the phenomenon of parallax in astronomy. Parallax is the apparent displacement of an object relative to the background that is caused by the motion of the observer (rather than the motion of the object itself, or of the background). This simulation illustrates the parallax of an object in space that results from the Earth's rotational or orbital motions.

The simulation has two different modes. The default is an Earth Rotation Mode. In this mode the simulation illustrates the parallax caused by Earth's rotation on its axis. The top window shows an observer (indicated by a green dot) on the equator of Earth (blue disk). The green line shows that observer's line of sight to an object in space (indicated by a red dot). This line of sight extends to show where the object would appear against a more distant background. When the simulation is played the observer moves around the Earth as Earth rotates. Parallax causes the apparent position of the object to move back and forth between two extreme locations (indicated by open red circles). The bottom window shows the apparent motion of the object against the stars, as seen by the observer. [Note: to keep the simulation simple it is assumed that Earth's equator is aligned with the ecliptic plane (the plane of Earth's orbit). In actual fact they are tilted by 23.5 degrees, but this cannot be represented in a simple 2D simulation.]

The other mode for the simulation is Earth Orbit Mode. In this mode the simulation illustrates the parallax caused by Earth's orbit around the Sun. Now the observer moves along Earth's orbital path (shown as a blue circle with the orange Sun in the center). The bottom window now shows the apparent motion of the Sun as well as that of the object being observed.

Controls are provided to adjust the baseline (the diameter of the circle through which the observer moves), the distance to the object, and the location of the observer along the observer's circular path. The parallax angle (half of the angle between the two extreme apparent positions) is given. Menus are provided to let the user control which visual elements are shown and which mode is used.

Space View Frame

• Options
  • Earth Orbit Mode: display Earth Orbit Mode (rather than Earth Rotation Mode) - see above.
  • Show Line of Sight: show line from observer through object to apparent position of object.
  • Show Axis: show line from center of observer's motion (center of Earth or Sun, depending on the mode) through the object.
  • Show Extreme Lines of Sight: show the extreme apparent positions of the object and the lines of sight for these apparent positions.
• Visual Elements
  • Blue disk (or circle): Earth (or Earth's orbit).
  • Red dots: the dot on the left is the object being observed, the dot on the right is that object's apparent location.
  • Green line: observer's line of sight to the object.
  • Orange disk (if shown): Sun.
  • Red open circles: extreme apparent positions of the object.
  • Red lines: extreme lines of sight for the object.
  • White line: axis from the center of Earth (or Earth's orbit) through the object.
  
  
• Controls
  • Play/Pause: start and stop the simulation.
  • Baseline: adjust the diameter of Earth (or of Earth's orbit).
  • Distance to Object: adjust the distance from the center of Earth (or Earth's orbit) to the object being observed.
  • Location: adjust the observer's location on Earth (or Earth's orbit).
  • Parallax: the parallax angle for the object, as viewed by the observer, in degrees.

Sky View Frame

• Visual Elements
  • Orange disk (if shown): Sun.
  • White points: other stars.
  • Red disk: the object being observed (shown at its apparent position).
  • White line: reference point to help see apparent motion.