Why Are Leap Seconds Needed?
The need for leap seconds arises from a conflict between two ways of measuring time:
- Atomic Time (TAI): This is the time kept by atomic clocks, which are incredibly precise and stable. Coordinated Universal Time (UTC) is based on TAI.
- Solar Time (UT1): This is time based on the rotation of the Earth. However, the Earth's rotation is not perfectly constant; it gradually slows down and has minor irregularities.
Over time, a discrepancy builds up between the ultra-steady atomic clocks and the slightly irregular rotation of the Earth. To prevent these two time scales from drifting too far apart, a leap second is added to UTC. This effectively pauses the atomic clocks for one second to let the Earth's rotation catch up.
How It Works
When a leap second is added, the sequence of time looks like this: 23:59:59 -> 23:59:60 -> 00:00:00. This extra second is usually added at the end of June 30 or December 31.
The decision to add a leap second is made by the International Earth Rotation and Reference Systems Service (IERS) when the difference between UTC and UT1 approaches 0.9 seconds.
The Future of Leap Seconds
Leap seconds can cause issues for computer systems and networks that require perfectly linear time. Because of this, there is an ongoing international discussion about whether to continue the practice. As of 2023, a resolution has been passed to cease adding leap seconds by the year 2035, which will represent a major shift in how we coordinate global time.
Conclusion
Leap seconds are a fascinating bridge between astronomical time and atomic time. While they add complexity, they have played a crucial role in keeping our modern clocks in sync with the natural world.