HomeNAAP LabsPlanetary Orbits › Instructor Resources

Planetary Orbits Lab — Instructor Resources


The NAAP Planetary Orbits Lab pedagogical objectives are aimed increasing an understanding of Kepler's Laws of Planetary Motion and how basic Newtonian concepts relate to orbits.


Course Integration

The Planetary Orbits Lab can be used either when Kepler is introduced or when the solar system is introduced. The simulator is essentially self-sufficient for explanation. Little to no physics experience is required.


  • Units of Kepler's 3rd law equation: P2 = a3 as written is valid only when P is in years and a in AU. The form P2 = (4π2/(G(M+m))) × a3 is needed to use SI units.
  • The planets are assumed to be points of no mass. Since the sun is over 300,000 times the mass of earth and over 1000 times the mass of Jupiter. Celestial mechanics uses the reduced mass (m = 1 / (1/m1 + 1/m2)) in orbital calculations. When m1 is the sun and m2 is a planet, the reduced mass is the same as the sun to at least 3 digits.
  • Inclination in the orbits has been suppressed. While this does not change the orbital period, the actual view of the planets of our solar system when viewed from above will be ever so slightly different than what is viewed in this simulator. The flattened view allows Neptune and Pluto to “collide” which can't happen because of inclination.
  • No attempt was made to make the sizes of the sun and planets accurate relative to each other. Moreover, the sizes do not change with a change in scale. The only indication scale is changing is the background grid (and the value on the semimajor axis slider).
  • The semimajor axis of all the solar system planets are aligned (they are horizontal on the screen). The semimajor axes of the solar system are not aligned. This simplification was introduced to easily allow the matching of the orbit of the fictional planet to the solar system planets.