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.

This switch created a discrepancy between how a single second used to be defined, and how it is defined today.

Over the course of several years, modern time systems can become a few seconds out of sync with the Earth’s movement. To solve this issue, timekeepers use what’s called a leap second. Just as leap years keep our calendars in alignment with Earth’s orbit around the Sun, leap seconds have kept our timekeeping aligned with Earth’s daily rotation.

But in October 2026, an international organization governing timekeeping may change its strategy for adding leap seconds.

As a space scientist, I know how measuring time is fundamental to physics research. The precision of physicists’ measurements has gotten better as the way we measure time has changed.

The systems humans use to measure time have evolved over the past few centuries.

The origin of the 24-hour day can be traced to the second millennium B.C.E., when ancient Egyptians divided day and night into 12 parts each. Because the length of day and night varies during the year, the length of each hour of daylight and each hour of night did as well.

The Greek astronomer Hipparchus, who lived in the second century B.C.E., later adopted a system using 24 equal hours for astronomical calculations.

The division of an hour into 60 minutes of 60 seconds each also has its roots in astronomy. Division by 60, or a base-60 system, traces back to the Sumerian and Babylonian civilizations of Mesopotamia. The number 60 is particularly convenient because it can be divided up evenly into smaller parts.

The Greek astronomer Claudius Ptolemy adopted the base-60 system while measuring........

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