International Atomic Time (TAI)

International Atomic Time (TAI) is one of the main components of Coordinated Universal Time (UTC), the time scale used to determine local times around the world. It tells us at which speed our clocks should tick.

Illustration image
TAI keeps th pace of time.
©iStockphoto.com/Kuzma

TAI keeps the pace

Two components are used to determine Coordinated Universal Time (UTC):

  • International Atomic Time (TAI) is a time scale that uses the combined output of some 200 highly precise atomic clocks. It provides the exact speed at which our clocks tick.
  • Universal Time (UT1), also known as astronomical time or solar time, refers to the Earth's rotation. It is used to compare the pace provided by TAI with the actual length of a day on Earth.

How are UT1 and TAI used to determine UTC?

If TAI is so precise, why do we use leap seconds?

How is TAI measured?

International Atomic Time is an extraordinarily precise means of time-keeping. Atomic clocks deviate only 1 second in about 20 million years.

The secret to this impeccable precision is the correct measurement of the second as the base unit of modern time-keeping. The International System of Units (SI) defines one second as the time it takes a Cesium-133 atom at the ground state to oscillate exactly 9,192,631,770 times. Atomic clocks are designed to detect this frequency, most of them today using atomic fountains.

To achieve the highest possible level of accuracy, the International Bureau of Weights and Measures combines the output of more than 200 atomic clocks in over 50 national laboratories worldwide to determine TAI. The time scale is weighted, prioritizing the time signal provided by institutions that maintain the highest quality of primary cesium.

Why do we use UTC, not TAI?

The high level of precision achieved by using atomic clocks is both a blessing and a curse. On the one hand, accurate time-keeping is a necessity, for example for time-sensitive technology, such as modern air traffic control systems that rely on satellite navigation. On the other hand, TAI does not take into account the Earth's slowing rotation, which determines the length of a day. For this reason, TAI is constantly compared to UT1. Before the difference between the two scales reaches 0.9 seconds, a leap second is added to UTC.

Read more about leap seconds

Topics: Timekeeping

In this Article

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Leap Seconds Library

  1. How do leap seconds work?
  2. The Future of Leap Seconds
  3. Atomic time

What's a Leap Second?



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