A year lasts about 365 days because Earth needs that long to orbit the Sun once.

The longer version

One circuit around the Sun takes Earth roughly 365 days. The planet sweeps along its orbit at high speed while sunlight strikes each hemisphere at a shifting angle, which is what produces spring, summer, autumn and winter. The figure is awkward because the circuit does not finish after a whole number of spins.

That remainder is why calendar makers had to invent corrections. Early observers fixed the year by watching stars return to the same position or by noting when a noon shadow reached its shortest. Farming depended on such markers, so a year even slightly wrong slowly pushed planting and harvest away from the conditions that suited them best.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.