International Atomic Time

Special clocks keep time very well. These clocks are in many labs. They are all over the world.

Many of these clocks use tiny atoms. They work together to stay steady. They find a middle time. This is called atomic time.

This time is very smooth. It does not skip any seconds. It stays the same all the time.

Most people use a different time. We call that time UTC. It is based on atomic time.

UTC sometimes adds a second. This keeps it close to Earth's spin. Atomic time does not do this. It is a very steady way to track time.

Scientists need a very steady way to track time. They use International Atomic Time, or TAI. This time is made by many special clocks. There are over 450 atomic clocks in use. These clocks are in more than 80 labs around the world. Most of these are caesium clocks. A caesium clock uses tiny atoms to keep time.

To make TAI, experts take a weighted average. This means they find a middle time from all the clocks. They compare the clocks using GPS signals. They also use satellites to send time from place to place. This makes TAI very stable. It is even more steady than the best single clock.

Most people use a different time called UTC. UTC is based on TAI. But UTC is not a smooth scale. It uses leap seconds to stay close to Earth's spin. Because of these extra seconds, UTC is behind TAI. On January 6, 2026, UTC was 37 seconds behind TAI. Experts may stop using leap seconds by the year 2035. This would make the gap stay the same.

Scientists need a very steady way to measure time. They use a system called International Atomic Time, or TAI. This is a high-precision way to track time on Earth. It is much more stable than regular clocks. TAI is the main way we realize Terrestrial Time. This is a fundamental time scale for our planet.

How does this huge system work? It is a weighted average of many different clocks. There are over 450 atomic clocks in more than 80 national laboratories. Most of these are caesium clocks. The scientific definition of a second is based on caesium. Experts compare these clocks using GPS signals. They also use two-way satellite time transfer. This makes TAI more stable than any single clock.

People began working on this in the 1950s. Atomic timekeeping services started as experiments in 1955. The first caesium clock was at the National Physical Laboratory in the UK. This clock helped set a scale called Greenwich Atomic. In 1956, the United States Naval Observatory started its own scale. The NBS-A scale began in Colorado in 1957. By 1967, the SI second was officially defined using caesium.

There are important numbers to know about TAI. As of January 6, 2026, UTC is exactly 37 seconds behind TAI. This gap has been the same since January 1, 2017. The 37 seconds come from a 10-second difference in 1972. There have also been 27 leap seconds added since then. In 2022, experts decided to stop using leap seconds. They plan to do this by or before 2035.

Most people use a different time called UTC. UTC is used for civil timekeeping all over the world. It is based on TAI but it is not continuous. It uses leap seconds to stay close to Earth's rotation. This can be a hard job for some machines. TAI stays smooth without these extra seconds. This helps scientists keep a very steady record of time.

International Atomic Time, or TAI, is a high-precision time standard. It measures the passage of time on Earth's geoid, which is the shape the Earth takes due to gravity. TAI is essential because it provides a continuous, steady scale for scientific work. It serves as the primary way to realize Terrestrial Time, a fundamental scale used in the Solar System. Unlike the clocks we use in daily life, TAI does not use leap seconds to adjust for Earth's rotation. This makes it a perfectly smooth and constant measurement of time.

To create this standard, scientists use a weighted average of many different clocks. There are over 450 atomic clocks located in more than 80 national laboratories around the world. Most of these are caesium clocks. This is important because the International System of Units defines a second based on the caesium atom. To ensure all these clocks agree, researchers compare them using GPS signals and two-way satellite time and frequency transfer. Because the system averages so many different sources, TAI is much more stable than even the best single clock in the group.

Each participating laboratory broadcasts a frequency signal in real time. This signal includes timecodes, which represent that specific lab's estimate of TAI. These timecodes are often published in the form of Coordinated Universal Time, or UTC. Scientists use specific labels to identify these scales, such as UTC(NPL) for the National Physical Laboratory in the UK. It is important to distinguish TAI(NPL) from TA(NPL). The latter is an independent atomic time scale that is not synchronized to TAI or any other standard.

The International Bureau of Weights and Measures, or BIPM, manages the final calculation. The BIPM combines all the measurements from the various laboratories to calculate the most stable average possible. This official version of TAI is published every month in a document called "Circular T." This publication includes tables showing the differences between UTC and the time at each specific institution. Once these circulars are published, the TAI scale is considered definitive and is not revised. If errors are found later, scientists simply create better estimates for Terrestrial Time rather than changing the TAI record.

Atomic timekeeping has a long history of discovery. Experiments began in 1955 using the first caesium atomic clock at the National Physical Laboratory in the UK. This helped establish a scale called Greenwich Atomic. In 1956, the United States Naval Observatory started the A.1 scale. Shortly after, in 1957, the NBS-A scale began in Colorado. By 1967, the scientific community officially defined the second using the caesium atom. Over the decades, the name and methods for these scales evolved through various international decisions.

There are specific mathematical relationships between TAI and other time scales. For example, TAI and UTC are linked by a specific number of seconds. As of January 6, 2026, UTC is exactly 37 seconds behind TAI. This specific gap has remained unchanged since January 1, 2017. The 37-second difference comes from an initial 10-second offset in 1972 plus 27 leap seconds added since then. However, the General Conference on Weights and Measures decided in 2022 to abandon leap seconds by or before 2035. At that point, the difference between TAI and UTC will stop changing.

Gravity also plays a major role in how these clocks function. In the 1970s, scientists realized that clocks tick at different rates depending on their altitude due to gravitational time dilation. Because many clocks were located above sea level, they ticked slightly faster. To fix this, corrections were applied starting on January 1, 1977. These adjustments ensured TAI corresponds to the proper time at the geoid, or mean sea level. This correction actually caused TAI to slow down by about one part in a trillion to maintain accuracy.

Finally, TAI connects to much larger systems in space. The corrections made in 1977 also serve as the starting point, or epoch, for other fundamental scales. These include Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT). These scales represent the different ways time is measured across the Solar System. By maintaining a steady TAI, scientists can ensure that all these complex measurements across the universe remain synchronized and accurate.