For 1,500 years astronomers argued about what sits still at the centre of everything. Choose a model and watch what the planets would have to do to make it true.
Drag across the sky to move through time; pinch or scroll to zoom.
Aristarchus of Samos suggests the Earth goes round the Sun. Almost nobody follows him.
Ptolemy writes the Almagest: a still Earth, planets on epicycles. It lasts 1,400 years.
Copernicus puts the Sun in the middle and the big loops vanish.
Tycho Brahe keeps the Earth still but sends the planets round the Sun, and round us with it.
Kepler, working from Tycho’s Mars measurements, finds that orbits are ellipses.
Galileo sees Venus show a full set of phases. Ptolemy’s arrangement can’t produce them.
Newton’s gravity explains why Kepler’s ellipses happen.
Proof the Earth moves. Bradley finds stellar aberration: every star is nudged slightly towards the direction Earth is travelling, the way rain seems to slant towards you when you run through it, and the nudge flips every six months as Earth’s direction reverses. Then Bessel measures stellar parallax: a nearby star shifts against more distant ones as we swing from one side of our orbit to the other, like your thumb jumping when you close one eye then the other. The shift is about the width of a coin seen from 12 km away, which is why it took until 1838 to see.
Every model shows the same real planetary motion, using Kepler’s ellipses flattened into one plane. Only what’s held still, and the machinery each astronomer needed to explain it, changes.