Definition
The heliocentric model is the Copernican cosmology that places the Sun at the center of the planetary system, with Earth and the other planets orbiting it.
Nicolaus Copernicus published it in 1543 in De revolutionibus orbium coelestium. It was refined into its modern form by Johannes Kepler's three laws of planetary motion (1609, 1619), confirmed observationally by Galileo Galilei's 1610 telescopic discoveries, and given a dynamical foundation by Isaac Newton's Principia in 1687.
Why it matters
How it works
Copernicus argued that placing the Sun at the centre dramatically simplifies the geometry. Retrograde motion — the puzzling apparent reversal of Mars or Jupiter against the stars — is naturally explained as a parallax effect of Earth overtaking the slower outer planet on the inside track. The order of the planets falls out of their orbital periods. Copernicus still used circular orbits and a few residual epicycles, so his system was no more accurate than Ptolemy's, but it was geometrically cleaner.
Kepler eliminated the epicycles by replacing circles with ellipses. His three laws — orbits are ellipses with the Sun at one focus; equal areas are swept in equal times; the square of the period is proportional to the cube of the semi-major axis — describe planetary motion with no free epicycles and remarkable accuracy. These laws came from Tycho Brahe's precise pre-telescopic observations, which Kepler inherited.
Galileo's 1610 Sidereus Nuncius added decisive physical evidence. He saw the four largest moons of Jupiter, an obvious counter-example to the claim that everything orbits Earth. He observed Venus going through a full set of phases like the Moon — impossible under the Ptolemaic arrangement but a clean consequence of Venus orbiting the Sun inside Earth's orbit. Newton's Principia then derived Kepler's laws from a single inverse-square law of gravitation, unifying the heavens with terrestrial mechanics.