Leap seconds have helped keep clocks synchronized with Earth’s rotation, but their time may be over – here’s why

Humans haven't always counted time down to the second. mrs/Moment via Getty Images

You’re probably used to the month of February having an extra day every four years, during the leap year. Timekeepers added that extra leap day because Earth completes one orbit around the Sun in about 365 and one-quarter days. If we didn’t add leap days, the equinoxes and solstices that mark the beginning of seasons would occur later, slowly shifting the days on which each season began.

For millennia, a day has been defined as one full rotation of the Earth. But the Earth’s rotation isn’t completely uniform, and in order to be more precise, modern timekeeping systems have stopped using the Earth’s rotation as their basis.