With all the events going on in the world it passed almost unnoticed that a decision was made recently to abolish the leap second. The resolution was made on 18 November 2022 during a meeting held in Paris held by the International Bureau of Weights and Measures.
What is a leap second?
The length of a day for civil timekeeping purposes is precisely 24 hours. However, due to the slowing of the Earth’s rotation,the mean solar day, or average “natural” day measured by the Earth’s rotation, is now slightly longer than this. In the year 1900 the mean solar day was almost exactly 24 hours, but averaged over the last 15 years it is now around 24 hours 68 milliseconds. The cumulative effect of using a civil day slightly shorter than the mean solar day means that standard time measured by atomic clocks would slowly drift away from the time standard defined by the Earth rotation (which astronomers call UT1) if no adjustments were made. Although, since the difference is so small, it would take 1500 days (roughly 4 years) for the time measured by atomic clocks to gain one second.
To bring the time measured by accurate atomic clocks to within a second of UT1, a leap second is added approximately every 1,500 days. The extra second is always added at the end of the day on 30 June or 31 December.
To complicate matters further, the speed of the Earth’s rotation is variable and events such as
- large earthquakes
- movements in the Earth’s crust
- melting of glaciers and
- changes in the mantle (the region of the Earth below the crust)
may temporarily speed it up. So although the trend over decades and centuries is that the days are getting longer, over shorter time-scales the average length of a day may actually decrease. The diagram below shows, in milliseconds, the amount by which the average length of a [mean solar] day was longer than 24 hours for the years 2008 to 2022. (1 millisecond equals one thousandth of a second.)
The day length averaged out over the years from 2008 to 2022 is 24 hours 0.00068 seconds, but interestingly in the most recent years 2021 and 2022 the average day was shorter than 24 hours.
This variation means that, unlike the case of leap years, where there are precise rules for determining whether or not a given year will be a leap year, it is not possible to say years in advance when there will be a leap second. A body called the International Earth Rotation and Reference Systems Service (www.iers.org) decides from accurate measurements of the Earth’s rotation when the next leap second will occur.
Leap seconds are always inserted at midnight (GMT) So the latest leap second was inserted at 23:59:60 at 31 December 2016, Greenwich Mean Time (GMT) . So in the UK which uses GMT in the winter months, it occurred just before midnight. In different parts of the world the leap second was added at different local times. New York is 5 hours behind GMT, so the extra second was be added at 18:59:60 on 31 December 2016 local time. Beijing is 8 hours ahead of GMT and the extra second was be added at 07:59:60 on 1 January 2017
This announcement is made around six months in advance. For example, the official announcement of the latest leap second which was introduced on December 31 2016 was made on 6 July 2016.
In the past when leap seconds have been applied, a number of data centres and websites around the world experienced system problems and crashed. This was because they could not cope with a minute which contains 61 seconds and interpreted this unforeseen event as a system failure.
The Google approach for the 2016 leap second- smearing time
For the December 2016 leap second Google (and many other organisations) used an interesting solution to the problem. For the 20 hour period from 2 pm on 31 December 2016 to 10 am on 1 January 2017 Google adjusted the normal rules of timekeeping. Instead of normal seconds, they used a “Google Second” which was fractionally longer than the usual second. So rather than of adding an extra second before midnight, which might have caused system errors, they spread the extra second throughout this 20 hour period.
What will happen in the longer term?
As the length of the day gradually increases, if we still wanted to keep UTC in step with UT1 to within a second, leap seconds would need to added more often.
- In 100 years time, the mean solar day would be on average 24 hours 0.003 seconds long and we would have to add a leap second every 330 days – slightly more than once a year
- In 1000 years time, the mean solar day would be on average 24 hours 0.03 seconds long and we would have to add a 11 leap second per year
However assuming the November 2022 resolution is implemented as planned we will no longer do this. Although it does not specific what the maximum difference between UT1 and UTC is, it is likely that a difference of a least a minute will be chosen. If we decided to go for maximum difference of one minute.
- In 100 years time, when the mean solar day is on average 24 hours 0.003 seconds long and we would have to add a leap minute every 55 years
- In 1000 years time, when the mean solar day is on average 24 hours 0.03 seconds long and we would have to add a leap minute every 5 and a half years. However I suspect the resolution would have been superseded long before then!!
1)The average length of the mean solar day when calculated over a entire year is just over 24 hours. However, over the course of a year the the actual length of a solar day varies. This variation throughout the year is due to entirely different effects than the slowing of the Earth’s rotation. It is at its longest – 24 hours and 30 seconds – around Christmas day and it is shortest at around 23 hours 59 minutes and 38 seconds in mid September. This is variation is described in more detail in my video
2) The term Greenwich Mean Time is no longer used by astronomers. Instead, they use two different times which agree with each other to within 1 second.
- Universal Time, often abbreviated to UT1, is the mean solar time, the time determined by the rising and setting of the Sun at the Greenwich Meridian, zero degrees longitude.
- Co-ordinated Universal Time, usually abbreviated to UTC, is the time measured by atomic clocks and is kept to within 1 second of UT1 by the addition of leap seconds.
In common use, Greenwich Mean Time (GMT) is often taken to be the same as UTC. However, it can also be taken to mean UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, GMT is normally avoided in precise writing.
The Greenwich Meridian- Image from Wikimedia Commons
Full details of the November 2022 resolution to abolish leap seconds on or before 2035 can be found below