1Orbits are ellipses, not circles
The Sun sits at one focus of the ellipse, an off-centre point. The second focus is just empty space. Stretch the orbit:
Distances squeezed, sizes enlarged — everything fits.
Following
Drag to pan · Scroll to zoom · Click a planet for its story
Why orbits work this way
Kepler’s three laws
In the early 1600s Johannes Kepler found three rules hidden in decades of Mars measurements, sixty years before Newton explained them with gravity. They still describe every orbit we’ve found since.
2Planets speed up near the Sun
The line from a planet to the Sun sweeps out equal areas in equal times. Far away the planet crawls; close in, it sprints. The shaded wedges cover the same stretch of time. The small dots are equal time-steps, bunching up where the planet is slow.
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3Farther means slower, exactly
Square a planet’s year and cube its distance: every planet gives the same number. Measured in AU and years, a³ = T² exactly, so all eight land on one straight line. It’s also how astronomers weigh distant stars.
The data behind the motion
The numbers
An AU is one Earth–Sun distance, about 149.6 million km. Eccentricity is how stretched an orbit is: 0 is a perfect circle, closer to 1 is a long cigar.
| Planet | Distance (AU) | Orbital period | Eccentricity | Radius (km) |
|---|