What Is a Leap Second?
Every now and then, we add a leap second to hour clock to synchronize them with the Earth's slowing rotation.
Why Do We Need Leap Seconds?
We add leap seconds from time to time to ensure our clocks reflect the Earth's rotation speed as accurately as possible. The speed at which our planet rotates around its axis fluctuates daily, and it slows down very slightly over time.
By adding an extra second to the time count, we effectively stop our clocks for that second to give Earth the opportunity to catch up.
Adding a leap second creates a minute with 61 seconds and a day with 86,401 seconds instead of the usual 86,400.
About One Second Every 1.5 Years
An average Earth day is about 0.002 seconds longer than 24 hours. The difference grows to one second in about 1.5 years. However, the Earth's rotation speed fluctuates constantly, so the actual frequency of leap seconds can vary.
How It Works?
- The Science Behind Leap Seconds
Coordinated Universal Time (UTC) is the world's time standard, and each time zone is defined by being a certain number of hours or minutes ahead or behind UTC.
At its core, UTC follows International Atomic Time (TAI). This time scale combines the output of some 200 highly precise atomic clocks maintained in laboratories around the world.
Leap Second is added to Coordinated Universal Time (UTC) and clocks worldwide in order to compensate for the slowing of the Earth's rotation.
Coordinated Universal Time (UTC) is the time standard used to determine local times in time zones worldwide. It is primarily based on the combined output of several highly precise atomic clocks, a statistical time scale called International Atomic Time (TAI).
A normal day has 86,400 seconds, but in the atomic time scale 1 second is not defined as one 86,400th of the time it takes Earth to rotate around its axis but rather as the time it takes a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times.
- “More Precise” than Earth
The advantage of this definition is that it is extremely precise: atomic clocks deviate only 1 second in up to 100 million years. On the other hand, the Earth's rotation, which is expressed by the time standard Universal Time (UT1), is far less reliable. It slows down over time, which means that days get longer. On average, an Earth day is about 0.002 seconds longer than the daily sum of the 86,400 seconds measured by the atomic clocks. This makes for a discrepancy between TAI and UT1 of around 1 second every 1.5 years.
Leap seconds are added to our clocks (UTC) so this discrepancy does not get too large over time and the time we use is synchronized as much as possible with the Earth's rotation. Before the difference between UTC and UT1 reaches 0.9 seconds, one second is added to UTC. This means that the time difference between TAI and UTC amounts to an integral number of seconds because whole seconds are added, while the time difference between UTC and UT1 is always less than 0.9 seconds.
In practice, a leap second is scheduled when the difference between UTC and UT1 is predicted to exceed 0.9 seconds in the next 12 months. This means that the discrepancy is usually considerably smaller, approximately between 0.4 and 0.6 seconds, at the time the leap second occurs.
Since 1988, upcoming leap seconds are announced by the International Earth Rotation and Reference Systems Service (IERS) in Paris, France.
- Add or Subtract?
Leap seconds can be positive or negative 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).
- Atomic Clocks Are Too Accurate
The extraordinary accuracy of atomic time has one decisive disadvantage: it does not accurately reflect the length of a day on Earth. Due to the slowing of Earth's rotation, most days are very slightly longer than 24 hours.
Meanwhile, atomic clocks tick away at pretty much the same speed over millions of years. Compared to the Earth's rotation, atomic clocks are simply too consistent.
- Earth's Lag Is Kept Below 0.9 Seconds
The average speed of Earth's rotation is measured by Universal Time (UT1). When the difference between UTC and UT1 is predicted to reach 0.9 seconds within 12 months, a leap second is added to UTC and clocks worldwide. In other words, our clocks are always kept within a second of the average length of a day.
In practice, the discrepancy is usually considerably smaller than 0.9 seconds when the leap second occurs. It is generally kept between 0.4 and 0.6 seconds.
The International Earth Rotation and Reference Systems Service (IERS) in Paris, France, is responsible for monitoring the Earth's rotation and deciding when a leap second is to be inserted.
- How Far Are UTC, UT1, and TAI Apart?
Because whole seconds are added, the time difference between TAI and UTC amounts to an integral number of seconds, while the time difference between UTC and UT1 is always less than 0.9 seconds.
- 27 Seconds Added So Far
UTC was already 10 seconds behind TAI before the first leap second was added in 1972. Since then, the Earth has slowed down an additional 27 seconds, and a total of 27 leap seconds have been added (see table).
This means that UTC is currently 37 seconds behind TAI.
- Negative Leap Seconds
Leap seconds can be positive (1 second added to the schedule) or negative (1 second omitted). So far, all leap seconds have been positive. However, if the Earth's rotation keeps accelerating, as it has done in 2020, the IERS may decide to announce the world's first negative leap second at some point in the future.
- 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 Warring-ton, from Australia’s National Measurement Institute.
At the moment, days are 0.002 seconds longer than the sum of 86,400 seconds measured by atomic clocks; in 100 years, each day is predicted to be 0.004 seconds longer on average.
Next Leap Second
Latest Update for next Leap second Check on this website:- International Earth Rotation and Reference Systems Service (IERS).
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