Why Leap Years Exist: Orbital Remainder

This top-down simulation drives Earth around a real orbit at its true 365.2422-day period while a calendar counter ticks off years using the leap-year rule you pick: none, the simple Julian "every 4 years" rule, or the real Gregorian rule. Every calendar rollover drops a red mark at Earth's true orbital position and adds a point to the drift chart below, plotting the accumulated seasonal drift in days against calendar year — a direct, visual record of how quickly an uncorrected calendar would fall out of step with the seasons, and how effectively each leap-year rule corrects it. Drag the orbit view to pan and scroll or pinch to zoom.