Kepler’s Laws

Ellipses, equal areas, and T² ∝ a³

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• First law: orbits are ellipses, with the Sun at one focus.
• Second law: equal areas in equal times, so planets are fastest when closest.
• Third law: $T^2 = a^3$ (in years and AU). It follows from Newton's gravity.

Steps

1. Not quite a circle
2. Equal areas in equal times
3. Comet speed
4. The third law
5. Year on Mars
6. Distant planet