

EarthOrbit
General Description
This simulation illustrates the Copernican theory of Earth's orbit around the sun. The top window shows a view from outside the celestial sphere. The simulation shows the moving Earth along with its axis or rotation and the line of sight from Earth, through Sun, to the Celestial Sphere. The end of the arrow indicates where, on the Celestial Sphere, Sun appears to be located as seen from Earth. The tilt of Earth's rotational axis (relative to the ecliptic plane) is adjustable. The bottom window shows the view of a portion of the sky (near the ecliptic) as seen by an observer on Earth.
Note that this simulation also illustrates Copernicus' theory of the seasons, since the North pole of Earth alternately tilts toward or away from the sun as the Earth travels through its orbit. A menu allows the user to position the Earth for the solstices/equinoxes.
Finally, the simulation can illustrate one of the major problems for the Copernican system: the problem of parallax. If Earth orbits Sun then the celestial poles should trace out circles through the stars over the course of each year. The simulation will trace the motion of the celestial poles to illustrate this parallax effect, and by reducing the radius of Earth's orbit the user can see how Copernicus explained why no annual motion of the poles had ever been observed.
Orbit Frame
- Display Options Menu
- Show Sky View: show the view from Earth in a new window.
- Show Equatorial Plane: show the plane of the celestial equator.
- Show Celestial Grid: show grid lines representing celestial coordinates (right ascension and declination).
- Show Ecliptic Plane: show the plane of Earth's orbit.
- Show Ecliptic Grid: show grid lines representing ecliptic longitude and latitude.
- Show Line of Sight Through Sun: show an arrow running from Earth, through Sun, to Sun's apparent location on the celestial sphere.
- Show Stars: show stars on the celestial sphere.
- Highlight Bright Stars: color six bright stars along the ecliptic to use as landmarks for Sun's apparent motion.
- Show Earth's Axis of Rotation: show a line representing Earth's rotational axis.
- Highlight North End of Axis Line: puts a red dot at the north end of Earth's rotational axis line.
- Trace Motion of Celestial Poles: traces the paths of the celestial poles on the celestial sphere.
- Use Transparency: make equatorial and ecliptic planes partially transparent. The alternative is to display them as a wire mesh.
- Select Day Menu: select a day to position the Earth for the winter solstice, vernal equinox, summer solstice, or autumnal equinox.
- Visual Elements
- Blue sphere: Earth.
- Green line: Earth's rotational axis. An optional red dot indicates the north end of the axis line.
- Orange sphere: Sun.
- Yellow arrow: line of sight from Earth, through Sun, to Celestial Sphere. The end of the arrow shows where, on the Celestial Sphere, Sun appears to be as seen from Earth.
- Red/green lines: traces of the motion of the north (red) and south (green) celestial poles.
- Yellow circular plane: the ecliptic plane.
- Magenta circular plane: the Celestial Equator.
- Blue grid lines: the celestial coordinate grid.
- Light gray grid lines: the ecliptic coordinate grid.
- White points: stars on the celestial sphere.
- Colored points: highlighted bright stars. The highlighted stars are:
Star |
Constellation |
Highlight Color |
Procyon |
Canis Minor |
Cyan |
Betelgeuse |
Orion |
Orange |
Aldebaran |
Taurus |
Magenta |
Antares |
Scorpius |
Red |
Pollux |
Gemini |
Yellow |
Regulus |
Leo |
Green |
- Controls
- Play/Pause: start and stop the simulation.
- Step: advance the simulation by one time step.
- Reset: reset sim to original state.
- Time: time of year (in years). Measured from the winter solstice.
- Tilt: tilt of Earth's rotational axis relative to the axis of its orbit (in degrees).
Sky View Window
- Display Options Menu
- Show Stars: show background stars along the ecliptic.
- Highlight Bright Stars: color six bright stars along the ecliptic.
- Show West Arrow: show an arrow the points generally westward along the ecliptic.
- Visual Elements
- Orange disk: Sun.
- White points: other stars.
- Colored points: the highlighted stars.
- Green arrow: indicates the direction along the ecliptic that is generally westward (rather than eastward - it may have a significant North/South component).
Todd K. Timberlake (ttimberlake@berry.edu)