Negative Leap Second 

News Excerpt: 

The melting of polar ice due to global warming is affecting Earth’s rotation and could impact precision timekeeping, according to a paper in the journal Nature.

Making the leap:

  • The establishment of two versions of time — astronomical and atomic came more than 55 years ago when atomic clocks were adopted as the official time standard.
  • In atomic time, a second is defined as 9,192,631,770 oscillations of a Cesium atom and thus, these clocks do not slow down. 
  • In the early 1970s, Earth was slowing down in its rotation, and a gap formed between atomic time and astronomical time. Astronomical time fell behind atomic time by 2.5 milliseconds every day. 
    • Thus, the “leap second” was born to adjust that the “day” was getting longer.

What is a Leap Second?

  • Coordinated Universal Time (UTC) is based on International Atomic Time (TAI) but is adjusted by seconds to account for the difference between the definition of the second and the rotation of the Earth. 
  • Occasionally, 1s is added to the UTC time scale. This second is called a leap second. 
  • Its purpose is to keep the UTC time scale within ±0.9 s of the Universal Time (UT1) time scale, which changes slightly due to variations in the rotation of the Earth. UT1 refers to the astronomical time.
  • Twenty-seven leap seconds have been added to Universal Coordinated Time since 1972. The addition of a leap second happens at the last tick of the clock on the night of Dec. 31 or June 30.
  • In a leap second, instead of 11:59 and 59 seconds turning to midnight, there is another second at 11:59 and 60 seconds. 

Need of the adjustment:

  • Timekeeping is an exact science in a highly technological society, which is why a need was felt to invent the concept of the “leap second” and “negative leap second”.
  • Global Positioning System (GPS) as well as Stock Trading requires split-second accuracy thus, this new difference of a leap second could impact them vastly.


  • But now, the Earth is not slowing down anymore. There hasn’t been a leap second added since the end of 2016.
  • Earth's rotation has been speeding up, overtaking atomic time. This means that to bring the two measurements in sync, timekeepers may have to introduce the first-ever negative leap second.
  • The melting of the ice caps in Antarctica and Greenland shifts mass (i.e. all that extra liquid is redistributing weight across the planet) toward the equator. That process increases the equatorial bulge of the planet.
  • A negative leap second would go from 11:59 and 58 seconds directly to midnight, skipping 11:59:59.
  • When we do eventually need to implement this adjustment, it could cause major headaches for computer systems not designed to handle a subtraction of time like that. They are designed to add not subtract time.
  • Overall, the Earth’s rotation is too unpredictable to definitively say when (or if) we’ll need that negative leap second in 2026, 2029, or anytime soon.