Equation of Time Calculator

About this calculator

A sundial and a clock rarely agree. The equation of time is the day-by-day difference between apparent solar time (what a sundial shows, based on the sun's actual position) and mean solar time (the uniform 24-hour day that clocks keep), and it swings between roughly −14 and +16 minutes over the course of a year. The discrepancy comes from two effects layered together: Earth's elliptical orbit, which speeds and slows the sun's apparent motion (Kepler's second law), and the tilt of Earth's axis, which skews the sun's path relative to the celestial equator.

Given a day of the year, this calculator returns how many minutes fast or slow the sundial reading is compared to clock time. It's essential for sundial design, where a correction table or analemma curve lets a sundial be read against real clock time, and it's also used in solar energy calculations, where predicting the sun's exact position throughout the year affects tracking-panel angles and solar noon timing.