All tools
Planetary Orbit Simulator (NAAP)
Note: Shares Kepler SWF with Kepler's Third Law — different UI focus.
All three Kepler laws: elliptical orbit, faster near perihelion, P² = a³.
r = 0.983 AU · P = 1.00 yr · perihelion 0.983 AU
Before you start
NAAP Kepler lab: ellipse, changing speed, and P² = a³ in one place.
Key idea
Planets move faster near perihelion. Earth’s e is only 0.017, so our speed barely changes.
What you see
- Ellipse with Sun at a focus
- a, e, and true-anomaly sliders
Try this
- Raise e to 0.6 and play — speed change is obvious
- Set a = 1, e = 0.017 for Earth
Common mistakes
- Putting the Sun at the geometric centre of the ellipse
Glossary
- Perihelion
- Closest point to the Sun.
- True anomaly
- Angle from perihelion along the orbit.