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.
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 400 highly precise atomic clocks. It provides the exact
speed at which our clocks tick.
How is TAI Measured?
International Atomic Time is an extraordinarily precise means of time-keeping. Atomic clocks deviate only 1 second in up to 100 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; a cloud of atoms that is tossed upwards by lasers in the Earth's gravitational field. If one could see an atomic fountain, it would resemble a water fountain.
If TAI is so precise, why use leap seconds?
To achieve the highest possible level of accuracy, the International
Bureau of Weights and Measures combines the output of about 400 atomic
clocks in 69 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.
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.
How do leap seconds work?
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.