Some days really are longer than others. And now scientists know by just how much.
Earth’s rotation isn’t perfectly steady. Sometimes the planet twirls slightly faster or more slowly. That will make a day shorter or longer — by several milliseconds.
Many of the reasons for Earth’s faster or slower spin are well understood. Take the gravitational pulls of the moon and the sun. These “tidal forces” mess with the planet’s spin by stretching Earth out a tiny bit. Scientists know how to predict those effects on Earth’s rotation. But other factors that affect Earth’s spin are harder to estimate. One example is the churning of air in Earth’s atmosphere or the water in its oceans. Earthquakes can do it, too.
Now, scientists have measured changes in Earth’s rotation by a rate smaller than a millionth of a percent. They used a ring laser gyroscope. The new technique could help scientists understand the complex flows of water and air that cause the tiniest tweaks to our planet’s spin.
Meet G
The gyroscope is known simply as “G.” It’s located at the Geodetic Observatory Wettzell in Germany. And here’s how it works.
Within the gyroscope, laser beams travel around a square-shaped ring. It’s four meters (13 feet) on each side. One laser beam travels around the ring clockwise. Meanwhile, another circles the ring counterclockwise. Since those beams travel in opposite directions, Earth’s rotation affects them differently.
Light is made up of waves. Just like waves in water, light waves can have longer or shorter wavelengths. When the laser light inside G travels in the direction of Earth’s rotation, its light waves get stretched out. The wavelength lengthens because the planet’s turning elongates the path along which these light waves have to travel. But waves of light inside G traveling against the direction of Earth’s spin get squished. That shortens their wavelength.
When combined, these light beams…
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