Leap Seconds Explained
Why occasionally we add an extra second to our clocks
What is a Leap Second?
A leap second is an added second to UTC (Coordinated Universal Time) to keep it synchronized with Earth's rotation. It's added when Earth's rotation slows down slightly, making our atomic time slightly ahead of solar time.
Quick Fact
Unlike leap years (add 1 day), leap seconds are unpredictable because Earth's rotation varies due to tidal forces, climate changes, and geological events.
Recent Leap Seconds
- 2016 Dec 31+1 second added
- 2015 Jun 30+1 second added
- 2012 Jun 30+1 second added
- 2009 Jan 1+1 second added
- 2006 Jan 1+1 second added
Why Do We Need Them?
- Earth slows down - Tidal friction gradually slows Earth's rotation
- Atomic time is fixed - Cesium atoms always vibrate at same frequency
- Solar vs atomic time - After years, the difference would affect sunrise/sunset
UTC vs TAI
TAI (International Atomic Time) never adjusts - it keeps perfect seconds forever. UTC is TAI adjusted to stay within 0.9 seconds of solar time.
The Future
In 2035, the world's timekeepers plan to let UTC drift further from solar time, potentially eliminating leap seconds entirely. For now, they continue every few years when needed.
Computers handle leap seconds differently - some insert the second at 23:59:60, others simply skip it.