Washington, Dec 23 : Scientists have made the first ever direct measurement of the Earth’s rotation.

Using the world’s most stable ring laser, the new study pinpoints the orientation of the Earth’s axis.

The Earth wobbles. Like a spinning top touched in mid-spin, its rotational axis fluctuates in relation to space. This is partly caused by gravitation from the sun and the moon.

At the same time, the Earth’s rotational axis constantly changes relative to the Earth''''''''s surface. On the one hand, this is caused by variation in atmospheric pressure, ocean loading and wind. These elements combine in an effect known as the Chandler wobble to create polar motion.

Named after the scientist who discovered it, this phenomenon has a period of around 435 days. On the other hand, an event, known as the “annual wobble” causes the rotational axis to move over a period of a year.

This is due to the Earth’s elliptical orbit around the sun. These two effects cause the Earth’s axis to migrate irregularly along a circular path with a radius of up to six meters.

Capturing these movements is crucial to create a reliable coordinate system that can feed navigation systems or project trajectory paths in space travel.

“Locating a point to the exact centimeter for global positioning is an extremely dynamic process – after all, at our latitude, we are moving at around 350 meters to the east per second,” Karl Ulrich Schreiber, who directed the project in TUM’s Research Section Satellite Geodesy, said.

The orientation of the Earth’s axis relative to space and its rotational velocity are currently established in a complicated process that involves 30 radio telescopes around the globe.

Scientists assume that these galaxy nuclei never change their position and can therefore be used as reference points. The geodetic observatory Wettzell, which is run by TUM and Germany’s Federal Agency for Cartography (BKG), is also part of this process.

In the mid-1990s, scientists of TUM and BKG joined forces with researchers at New Zealand''''''''s University of Canterbury to develop a simpler method that would be capable of continuously tracking the Chandler wobble and annual wobble.

Yet at the end of the 1990s, work on the world’s most stable ring laser got underway at the Wettzell observatory. The installation comprises two counter-rotating laser beams that travel around a square path with mirrors in the corners, which form a closed beam path, hence the name ring laser.

When the assembly rotates, the co-rotating light has farther to travel than the counter-rotating light. The beams adjust their wavelengths, causing the optical frequency to change.

The scientists can use this difference to calculate the rotational velocity the instrumentation experiences. In Wettzell, it is the Earth that rotates, not the ring laser. To ensure that only the Earth’s rotation influences the laser beams, the four-by-four-meter assembly is anchored in a solid concrete pillar, which extends six meters down into the solid rock of the Earth’s crust.

The Earth’s rotation affects light in different ways, depending on the laser’s location.

“If we were at one of the poles, the Earth and the laser’s rotational axes would be in complete synch and their rotational velocity would map 1:1,” Schreiber said.

“At the equator, however, the light beam wouldn’t even notice that the Earth is turning,” he added.

The scientists therefore have to factor in the position of the Wettzell laser at the 49th degree of latitude. Any change in the Earth’s rotational axis is reflected in the indicators for rotational velocity. The light’s behaviour therefore reveals shifts in the Earth’s axis. (ANI)