Understanding orbit simulator
Change the shape of an orbit and discover why planets move faster near their star.
The idea behind it
In astronomical units, solar masses, and Earth years, Kepler’s third law connects the size of an orbit to its period. Increasing eccentricity stretches the orbit without changing its period when the semi-major axis stays fixed.
The relationship to remember
P = √(1³ / 1) = 1 years. Closest approach: 0.7 AU; farthest: 1.3 AU.
Make your own discovery
Set eccentricity to 0.8, then press Play. Compare the speed near the star with the speed at the far end.
Try the interactive lab ↗What this model leaves out
An isolated star and a negligible-mass planet in a fixed, two-dimensional Keplerian orbit. The diagram rescales to fit; star and planet sizes are exaggerated.
Check your understanding
Why is the star off-center?
The star sits at one focus of an elliptical orbit, not at the ellipse’s center. A circular orbit has coincident foci.
Does an eccentric orbit take longer?
Not if the semi-major axis and stellar mass stay the same. Eccentricity changes the shape and the variation in speed.
Continue with the formula reference or test yourself in space practice.