Building a Copernican Orrery — A Mechanical Model of the Heliocentric Solar System

An orrery is a mechanical model of the solar system — a clockwork device that shows the relative positions and motions of the planets as they orbit the Sun. The name comes from Charles Boyle, 4th Earl of Orrery, who commissioned one of the first such devices in 1713, but the concept traces directly to Nicolaus Copernicus (1473–1543), whose heliocentric model placed the Sun at the centre and the planets in circular orbits around it. Building an orrery makes the Copernican revolution tangible: instead of abstract diagrams, you hold a physical machine where turning a crank makes Earth and the inner planets orbit a central brass Sun at their correct relative speeds. The key engineering challenge is the gear ratios — Mercury orbits the Sun in 88 days, Venus in 225, Earth in 365, and Mars in 687, so the gear train must produce these ratios from a single input shaft. This blueprint builds a simplified desktop orrery showing the Sun and the four inner planets (Mercury, Venus, Earth, and Mars) using brass gears, rods, and a wooden base — accurate enough to demonstrate retrograde motion and planetary conjunctions.