In the 1850s British railway companies started introducing a single standard time to make their timetables consistent. Before that, every city would set its own clock based on the observation of the position of the sun. Nowadays, precise time standards are not only needed so people don’t miss their trains but also make modern communication technologies and satellite navigation work.
Generally, there are two methods of defining time, one is based on the local passage of time as measured by atomic clocks, while the other relies on the exact measurement of Earth’s rotation. The latter is not an easy exercise that involves extragalactic radio sources or huge laser-based gyroscopes. The constant survey of the Earth’s spin tells us that days are constantly getting longer, but surprisingly, severe earthquakes and weather phenomena can also take little discrete bites out of the planet’s supply of rotational kinetic energy.
How do we keep our ultra precisely measured time, the rotation of the Earth, and our position in the heavens in line?
TAI, UTC, and UT1
There are a confusingly large number of different time standards but the most fundamental is International Atomic Time (TAI). It is defined by the International Bureau of Weights and Measures in France and calculated from the weighted average of over 400 atomic clocks located worldwide that tick away at an incredibly constant rate.
On the other end of the spectrum is Universal Time (UT1) which in turn is purely based on Earth’s rotation.
Between these two is Coordinated Universal Time (UTC). UTC seconds are the same length as TAI seconds, but UTC is adjusted to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds, which is where leap seconds come in. Because Earth’s rotation is slowing down, TAI leads UTC: currently, TAI is 37 seconds ahead.
Monitoring Earth Rotation with Quasars and Laser Gyroscopes
If keeping a network of atomic clocks in sync is hard, measuring UT1 is even harder. The permanent monitoring of the Earth’s 3D rotation vector is the task of the International Earth Rotation and Reference Systems Service (IERS). The gold standard for this measurement is very long baseline interferometry (VLBI). VLBI measures the time differences between the arrival of microwave signals from various extragalactic radio sources (mostly quasars) with a global network of Earth-based radio telescopes. Thereby, the relative positions of the telescopes within the celestial reference frame can be determined with an accuracy of a few millimeters.
Currently, the Earth’s rotation is not monitored in real-time because there is some latency due to the required observation time for each measurement. Add, the involved telescopes can only dedicate a fraction of their up-time to the VLBI measurement. Further, the VLBI measurement technique is not directly linked to the rotational axis of Earth but needs to be calculated from the telescope positions.
There are other measurement techniques that address these problems and one of them is the recently completed laser gyroscope ROMY, located at an underground facility near Munich, Germany. ROMY consists of four 12 m long triangular ring lasers arranged as a tetrahedron. In each triangle, two separate laser beams circulate in opposite directions. A rotation introduces a frequency shift between both lasers according to the Sagnac effect and the resulting interference can be used to determine the angular velocity. From all four triangles, the full 3D rotation vector of the Earth can be reconstructed. Although the long-term stability is still not comparable to VLBI, ROMY can provide complementary real-time observations with high resolution.
Why is Earth’s Rotation Changing?
The rotation of the Earth is influenced by various factors. First of all the rotation axis changes relative to the Earth’s surface which is referred to as polar motion. The main component of polar motion is the so-called Chandler wobble. Although it was already discovered by American astronomer Seth Carlo Chandler in 1891 the exact cause of this change of Earth’s spin with a period of about 14 months is still being debated. One of the more recent publications surmises that it is dominantly excited by pressure fluctuations in the ocean.
Apart from the orientation, the velocity of Earth’s rotation also varies with other periodicities. As the tides move the mass distribution of the oceans around the solid parts of the Earth, its rotational inertia varies slightly with a period of 12 hours. There are also seasonal variations due to atmospheric circulation. Even slower variations are caused by flows in the Earth’s liquid core, and long-term climatic variations in the atmosphere.
On top of all this, Earth’s rotation is constantly slowing down due to tidal acceleration of the moon. The moon raises tides on the Earth, effectively stretching the earth a little bit. But because this water is spinning along with the Earth’s rotation, the asymmetry leads the moon’s orbit a little bit, pulling it along ever so slightly. This gravitational torque angular momentum transferred from the Earth to the Moon lengthens the day by 2.3 ms / century.
The length of a day can even be influenced by single large catastrophic events. One example is the 2004 Indian Ocean earthquake, which according to calculations by NASA scientists shortened the length of the day by three microseconds and shifted the position of the North Pole by 2.5 cm. An even more drastic example is the 1982-83 El Niño event which stretched the length of day by 0.2 ms.
With great accuracy comes great responsibility. On one hand, we can measure our position in the universe to within the breadth of a pencil eraser. On the other hand, the TAI’s averaged atomic clocks tick with an accuracy of one part in 1014. Nobody likes leap seconds, but they reflect the messy reality of living on a moving planet that’s subject to the laws of physics. It’s amazing that we can notice these tiny differences at all. Hooray for science!