Description

This simulation illustrate's Kepler's model of planetary motion. The upper view shows the Sun sitting stationary at the center of the system. The Earth and one other planet orbit around the Sun in ellipitcal orbits (Kepler's first law) such that the area swept out by the planet-Sun line is equal in equal times (Kepler's second law). Options allow the user to display the plane of Earth's orbit (ecliptic plane), the plane of the other planet's orbit, stars, the ecliptic grid, and lines illustrating the line of sight from the Earth to the Sun and the other planet. The paths of Earth and the other planet, as well as the apparent path of the planet as seen from Earth, can be traced. Note that although the relative sizes of the Earth and planet orbits are accurately depicted, the size of the Sun, Earth, planet, and celestial sphere of stars is NOT accurately depicted on that same scale.

Below this mean view is a view showing the portion of the sky along the ecliptic as seen by an Earth observer. The apparent position of the Sun and the other planet are shown against the background of the zodiacal constellations. The apparent motion of the planet relative to the stars can be traced (the trace automatically clears when the planet moves past either end of the viewing window).

Controls allow the user to select any of the five naked eye planets (those that Kepler knew), or a user-defined planet for which the user must specify the semimajor axis (a), the eccentricity (e), the inclination of the orbit (i), the longitude of the ascending node, and the argument of perihelion. Once a new planet is selected, or orbital parameters changed, the Set Scale button must be clicked to rescale the view to the proper size (this may take a few seconds).

Below the controls is a plot of the other planet's ecliptic latitude versus ecliptic longitude. The plot is shown as a series of disconnected points in order to avoid spurious horizontal lines as the planet moves past 360 degrees longitude.