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The Science Behind Leap Seconds

Leap seconds are added to Coordinated Universal Time (UTC) - and clocks worldwide - in order to compensate for the slowing of the Earth's rotation.

Illustration image

An atomic clock in Greenwich, U.K.

In an atomic clock second is defined by exactly 9,192,631,770 oscillations of a cesium atom.

©itimeanddate.com/Steffen Thorsen



“More Precise” Than Earth



Add or Subtract?

Leap seconds can be positive (one second added) or negative (one second omitted) - at least in theory: so far, all leap seconds were positive, and given the slowing of the Earth's rotation it is unlikely that a negative leap second will ever occur.

The Earth Rotates Unevenly

The speed of the Earth's rotation differs from day to day and from year to year, so the difference between UT1 and TAI varies accordingly. For example, the accumulated discrepancy over one year was 0.28 seconds in 2011, but only 0.02 seconds in 2001 (based on data from IERS).

Days Grow Longer

Not only do days become longer, but the rate at which day lengths increase also grows over time – but only by about two thousandths of a second per century, according to Dr Bruce Warrington, from Australia’s National Measurement Institute (NMI). This means that at the moment days are 0.002 seconds longer than the sum of 86,400 seconds measured by atomic clocks; in 100 years, they are expected to be 0.004 seconds “too long”.

Some scientists suggest abolishing leap seconds, effectively redefining the way we measure time.

Will leap seconds be used in the future?

Topics: Timekeeping, Clocks

In This Article


Leap Seconds

  1. What's a Leap Second?
  2. How Leap Seconds Work
  3. How Long is a Day on Earth?
  4. The Future of Leap Seconds
  5. Intl Atomic time (TAI)

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