Keplerian Orbits Explained
Planets do not move in perfect circles. They follow ellipses, governed by three laws Johannes Kepler worked out 400 years ago. Wanderstar uses these real orbital elements to place every body — here is what that means.
Kepler’s three laws
- Orbits are ellipses with the Sun at one focus (not the centre).
- Equal areas in equal times: a planet sweeps out the same area each day, so it moves faster when closer to the Sun and slower when farther away.
- T² ∝ a³: a planet’s orbital period squared is proportional to the cube of its average distance — which is why outer planets take so much longer to go around.
Eccentricity: how stretched an orbit is
Eccentricity measures how far an orbit is from a circle (0 = a perfect circle). Earth’s orbit is nearly circular (0.017), while Mars (0.09) and especially comets and Pluto are noticeably stretched. In the simulator you can select a body to see its perihelion (closest point) and aphelion (farthest point) marked on the orbit.
Why it matters
Real orbits explain things flat circles cannot: why Mars’ distance from Earth varies so much, why retrograde motion happens, and why spacecraft launch in narrow windows. Honest note: Wanderstar uses a simplified two-body Keplerian model — great for intuition, but not a spacecraft-grade ephemeris. See Data Sources.