Kepler's Laws
Three Rules That Govern Every Orbit
Left: ellipse with the star at one focus and the second focus empty. Centre: equal-area wedges swept in equal times — the planet visibly accelerates near perihelion. Right: three orbits with T = a^1.5 timing.
Kepler derived these laws from raw positional data of Mars gathered by Tycho Brahe — without telescopes, without Newton's gravity. They worked.
The 3rd law is especially powerful: measure a body's orbital period and distance and you can compute the central mass. This is how we weigh stars, planets, and the supermassive black hole at the Milky Way's centre.
Newton later showed all three laws fall out of his inverse-square gravity — and a tiny extra correction from general relativity explains the perihelion precession of Mercury.
The system map shows orbital periods that scale by Kepler's 3rd law — outer planets take exponentially longer years. Pinning down a body's mass from orbital data drives the values you see when scanning.
Frame Shift Drive cheats Kepler entirely — you cross AU in seconds — but the native bodies still orbit by these rules. A planet at 2 AU always has roughly a 2.8-year year (around a Sun-mass star).
Highly eccentric orbits (e > 0.5) are common in ED — comets, captured bodies, and outer planets in disturbed systems all show clear elongation in the map view.
| Law | Statement | Year | Note |
|---|---|---|---|
| 1st Law | Each orbit is an ellipse with the star at one focus | 1609 | Replaced perfect-circle dogma; works for any 2-body bound orbit |
| 2nd Law | A line from star to planet sweeps equal areas in equal times | 1609 | Equivalent to conservation of angular momentum |
| 3rd Law | T² = (4π² / GM) × a³ — period squared proportional to semi-major axis cubed | 1619 | Lets you compute mass from orbital geometry — 'celestial weighing' |
| Newton | Universal gravitation F = GMm / r² — derived all three Kepler laws from first principles | 1687 | Plus a small relativistic correction for tight orbits (Mercury, GR) |