Kepler’s first law
Planets travel in ellipses with the Sun at one focus — not perfect circles. Earth’s path is nearly round (eccentricity 0.017); Pluto’s is much more oval (0.25).
Celestial mechanics
Why planets don’t fall into the Sun
Gravity pulls every world inward. Sideways speed keeps them missing the center — forever falling, never landing. That balance is an orbit.
Kepler’s second law
A line from the Sun to a planet sweeps equal areas in equal times. Near perihelion a planet speeds up; near aphelion it slows. Comets show this dramatically.
Kepler’s third law
The square of the orbital period scales with the cube of the semi-major axis: P² ∝ a³. Double the distance and the year grows by about 2.8×.
Period vs distance
Drag the distance slider. Watch how long a full orbit takes when the semi-major axis changes. The demo uses Kepler’s third law with Earth’s year as the unit.
Orbital period: 1.00 Earth years
Inner track ≈ Mercury; mid track ≈ Earth; outer tracks approach Jupiter’s distance in this simplified scale.
Eccentricity playground
Raise eccentricity and the ellipse stretches. The Sun stays at a focus, so the planet skims close then coasts far away.
*Halley’s comet is more eccentric than this slider allows; the button sets a strong ellipse for comparison.
Orbital periods at a glance
| Body | Mean distance (AU) | Period | Eccentricity | Inclination |
|---|---|---|---|---|
| Mercury | 0.39 | 88 days | 0.206 | 7.0° |
| Venus | 0.72 | 225 days | 0.007 | 3.4° |
| Earth | 1.00 | 365.25 days | 0.017 | 0.0° |
| Mars | 1.52 | 687 days | 0.093 | 1.9° |
| Jupiter | 5.20 | 11.9 years | 0.049 | 1.3° |
| Saturn | 9.58 | 29.5 years | 0.057 | 2.5° |
| Uranus | 19.2 | 84 years | 0.046 | 0.8° |
| Neptune | 30.1 | 165 years | 0.010 | 1.8° |
| Pluto | 39.5 | 248 years | 0.249 | 17.2° |
Watch periods live
In the simulator, crank the speed and count how many Earth years pass while Neptune barely moves.