Why a Year Is 365 Days: Earth's Orbit

Earth's year is 365.25 days long for one reason: it sits about 150 million kilometers (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 — fixes the orbital period from those two numbers alone. This top-down simulator lets you drag the semi-major axis, central star mass and orbital eccentricity and watch the "year length" genuinely recompute in real time, or click anywhere in the orbital plane to drop the planet at that exact distance and see its period instantly, with a faint reference ellipse always showing Earth's real orbit for comparison. Drag to pan, scroll or pinch to zoom.