Leap years and Earth's orbit

Every four years, February gains an extra day due to a leap year. This adjustment compensates for the fact that Earth takes about 365 days and a quarter to orbit the Sun. Without it, the timing of seasons would shift over time.

Modern time and Earth's rotation

The day was once defined by Earth’s rotation, but modern systems now rely on atomic measurements. This shift has created a small discrepancy between the old and new definitions of time, which is corrected using leap seconds.

Origins of the 24-hour day

The 24-hour day originated with ancient Egyptians, who divided the day and night into 12 parts each. Later, Greek astronomer Hipparchus standardized the day into 24 equal parts, and the division into 60 minutes and 60 seconds came from the sexagesimal system used by Mesopotamian civilizations.

Evolution of time measurement

As mechanical clocks improved, the system of measuring time in hours, minutes, and seconds became widely adopted. However, it was not until the invention of the pendulum clock by Christiaan Huygens in 1657 that time could be measured accurately to the second.

Atomic clocks and leap seconds

In 1967, the second was redefined using the oscillations of a cesium-133 atom. This atomic second is more precise than Earth's rotation, leading to the need for leap seconds to keep atomic time in sync with Earth's movement.

Challenges with leap seconds

Adding a leap second can disrupt computer systems that assume time flows predictably. To avoid issues, companies like Google and Meta use "smearing" to gradually adjust time instead of adding a second all at once. These companies support eliminating leap seconds altogether.

Earth's faster rotation

Since 2015, Earth has been rotating faster than expected, leading to shorter days. In 2022, the shortest day measured was 1.59 milliseconds shorter than the atomic second. This could lead to the first "negative leap second" in the future.

Future of time measurement

The General Conference on Weights and Measures will meet in 2026 to consider ending leap seconds entirely. If approved, this change would allow a larger gap between atomic time and Earth's rotation, potentially leading to a "leap hour" in the distant future. However, this change would likely go unnoticed in everyday life.