The Abolition of Leap Seconds

The Royal Astronomical Society want a public debate over the leap second. What’s a leap second? Here’s a rundown:

  • We think of days as 24 hours long, and this is how our clocks view time.
  • However, the Earth wobbles on its axis, and the day is not actually 24 hours long. The actual length is unpredictable as the wobble can be affected by many variables, including tectonic movement. However, for the past 200 years the spin has been slowing. The last time the average length of a day was 24 hours was in the 1800s.
  • Many systems are time-critical, and it’s extremely convenient to have a definition of 1 second that isn’t linked to the unpredictable movement of the Earth. In the 1960s the official definition of the second was changed to the time taken for 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Cesium-133 atom. This is easily measurable, and atomic clocks use this measurement to keep extremely accurate time. This is called UTC, or universal coordinated time (no, I don’t understand the acronym either.)
  • So there are two systems of timekeeping, and they’re not the same. If you watch the position of the sun and time exactly how long it takes for the Earth to rotate seven times, it will take longer than the seven days as measured by an atomic clock. Although the difference is 0.002 seconds, over a year this adds up to 0.7 seconds. This is enough to cause problems.
  • Leap seconds are added to UTC (the time as measured by atomic clocks) in order to keep them within 0.9 seconds of the time as measured by rotations of the Earth. As you’d expect, they’re added just under once a year. So far, leap seconds have always been extra seconds, but if the Earth started speeding up (as is currently thought is happening) there could be a second removed.

So, why abolish them? The reasons, taken from here, are as follows:

  • leap seconds could cause disruptions where computers are tightly synchronized with UTC
  • leap seconds are a rare anomaly to deal with, which is a worry in particular with safety-critical real-time systems (e.g., new concepts for air-traffic control entirely based on satellite navigation)
  • exact astronomical time plays no significant role in most people