Why a Year Is 365 Days: Earth's Orbit (3D)

Earth's year runs 365.25 days because Earth orbits about 1 astronomical unit from a star with the Sun's mass, and Kepler's third law — T² = a³/M in AU/year/solar-mass units — pins the orbital period down from exactly those two numbers. This 3D simulator renders a real Kepler ellipse you can orbit the camera around, with live sliders for orbital radius, central star mass and eccentricity that genuinely recompute the "year length" each frame — or click directly on the orbital plane to drop the planet at a chosen distance and watch its period update instantly, always alongside a faint reference ellipse showing Earth's real orbit. Drag to orbit the camera, scroll to zoom.