Fact #29
June 29, 2026Earth's Spin Is Why We Need "Leap Seconds"
A day feels perfectly fixed at 24 hours, but the Earth's rotation is slightly irregular. Atomic clocks keep time more steadily than the spinning planet, so global timekeepers sometimes add a "leap second" to Coordinated Universal Time to keep clock time aligned with astronomical time.
Mini Facts
1
A leap second keeps UTC within 0.9 seconds of Earth-rotation-based time, called UT1.
2
Earth's rotation can vary by tiny amounts because the planet is not a perfectly steady clock.
3
June 29, 2022, was recorded 1.59 milliseconds shorter than a standard 24-hour day.
4
Leap seconds are announced by the International Earth Rotation and Reference Systems Service, or IERS.