International Atomic Time facts for kids

International Atomic Time (TAI) is a super-accurate way to measure time. It's like the master clock for the whole world! TAI is based on the incredibly steady "ticking" of over 450 special atomic clocks. These clocks are located in more than 80 countries around the globe.

Imagine a perfect, continuous timeline. That's TAI. It never stops or jumps. This makes it super stable and reliable. TAI is the foundation for Coordinated Universal Time (UTC). UTC is the time standard we use every day for things like our phones and computers.

You might wonder why we have two times, TAI and UTC. The main difference is that UTC sometimes adds a "leap second". This is done to keep UTC closely aligned with the Earth's slightly slowing rotation. TAI, however, never adds leap seconds.

Currently, UTC is exactly 37 seconds behind TAI. This difference has been constant since January 1, 2017, when the most recent leap second was added. The 37 seconds come from an initial 10-second difference in 1972, plus 27 leap seconds added to UTC since then. In 2022, a big decision was made. The General Conference on Weights and Measures decided to stop adding leap seconds by 2035. This means the difference between TAI and UTC is planned to become fixed in the future.

TAI uses traditional ways to mark days, like Julian days and the Gregorian calendar. TAI was matched with Universal Time (which is based on Earth's rotation) in 1958. But because Earth's spin is slowing down, TAI and Universal Time have slowly drifted apart since then.

How International Atomic Time Works

Many Clocks, One Super Time

TAI isn't just one clock. It's a "weighted average" of hundreds of atomic clocks. This means that the time from each clock is carefully combined. Some clocks, which are known to be more stable, get a bit more "say" in the final average.

Most of these clocks are caesium clocks. The official definition of a second in the International System of Units (SI) is based on the vibrations of a caesium atom. This makes caesium clocks incredibly precise.

Comparing Clocks Across the Globe

How do all these clocks in different countries stay in sync? They use advanced methods. These include GPS signals and special satellite links. These methods allow scientists to compare the clocks very accurately.

By combining the readings from so many clocks, TAI becomes much, much more stable. It is far more stable than any single clock could be on its own. It's like having many musicians playing together to create a perfect, steady rhythm.

The Role of the BIPM

The International Bureau of Weights and Measures (BIPM) in France is in charge of calculating TAI. Every month, the BIPM collects all the measurements from the participating laboratories. They then use these measurements to figure out the most stable and accurate time scale possible.

This official time scale is published monthly in a document called "Circular T". Once TAI is published in Circular T, it is considered final and is not changed. Even if scientists later find tiny errors, the published TAI stays as it is. Instead, these new findings help improve other important time scales, like Terrestrial Time (TT).

A Brief History of TAI

The Start of Atomic Timekeeping

The idea of using atomic clocks for timekeeping began in the mid-1950s. Early efforts used quartz clocks that were checked against a single atomic clock. The first caesium atomic clock used for this was at the National Physical Laboratory in the UK, starting in 1955.

Other countries, like the United States, also began their own atomic time scales around this time. These early efforts showed how much more accurate atomic clocks were. They were much better compared to older methods based on Earth's rotation.

TAI Becomes Official

In 1967, the second was officially defined using the caesium atom. This was a huge step for atomic timekeeping. Between 1971 and 1975, international groups officially named the time scale calculated by the BIPM as International Atomic Time (TAI).

Adjusting for Gravity

In the 1970s, scientists realized something fascinating. Gravity affects how fast clocks tick! This is called gravitational time dilation. Clocks at higher altitudes (further from Earth's center) tick slightly faster than clocks at sea level.

Since the atomic clocks contributing to TAI were at different heights, TAI was an average of these different ticking speeds. So, starting on January 1, 1977, corrections were made. TAI was adjusted to represent time as it would pass at sea level (the geoid). This meant TAI slowed down just a tiny bit, by about one part in a trillion.

This adjustment was important for other fundamental time scales used in the Solar System. One example is Terrestrial Time (TT). After this change, TAI became a way to "realize" or show Terrestrial Time. It has a fixed difference: TT(TAI) = TAI + 32.184 seconds.

TAI and Coordinated Universal Time (UTC)

TAI is a continuous, smooth flow of time. Coordinated Universal Time (UTC), however, is a bit different. UTC is designed to stay close to Universal Time (UT1). UT1 is based on the actual rotation of the Earth. Because Earth's rotation isn't perfectly steady, UTC sometimes needs small adjustments.

These adjustments are called leap seconds. When a leap second is added, UTC pauses for one second. This helps it catch up with Earth's rotation. This makes UTC a "discontinuous" time scale, meaning it has tiny jumps.

The use of leap seconds in UTC is a compromise. It allows a publicly broadcast time scale to be very stable (like TAI). But it also keeps it useful for things like navigation that rely on the Earth's exact position. The decision in 2022 to stop adding leap seconds by 2035 will change this relationship. It will make UTC continuous like TAI in the future.

See also

• Clock synchronization
• Time and frequency transfer