Due to a complex interplay of Earth’s and the moon’s gravitational fields, our planet’s rotation has gradually slowed over the millennia. It hasn’t been the designated length of one solar day—the time it takes Earth to make a full rotation, or slightly more than 86,400 seconds—since about 1820.

As a result, our global standard of time, known as Coordinated Universal Time, or UTC, occasionally becomes misaligned with UT1—the marker used to measure the actual length of one mean solar day. UT1 is determined using very long baseline interferometry (VLBI), a technique that relies on signals from extremely distant quasars to measure Earth’s precise orientation in space. In 1972 a policy to add a small unit of time, called a leap second, to UTC was implemented to correct the minute discrepancies found using such precise measurements.

For reasons that are not entirely clear to scientists, however, the rate at which Earth’s rotation slows is variable, so leap seconds must be added with unpredictable frequency. In the first few decades following the adoption of the leap second approach, UTC adjustments were made about once a year, but today’s leap second is only the fourth since 1999.