Planeter bevæger sig i ellipser med solen på et fokus?
Kepler's laws of planetary motion
De fleste planetariske baner er næsten cirkulære, og det kræver omhyggelig observation og beregning at fastslå, at de ikke er perfekt cirkulære. Beregninger af Mars' bane, hvis offentliggjorte værdier er noget suspekte, viste en elliptisk bane. Johannes Kepler udledte heraf, at andre legemer i solsystemet, herunder dem, der befinder sig længere væk fra solen, også har elliptiske baner.
Kepler's laws of planetary motion
Most planetary orbits are nearly circular, and careful observation and calculation are required in order to establish that they are not perfectly circular. Calculations of the orbit of Mars, whose published values are somewhat suspect, indicated an elliptical orbit. From this, Johannes Kepler inferred that other bodies in the Solar System, including those farther away from the Sun, also have elliptical orbits.
Kepler's laws of planetary motion