How wind, ice and rock may have combined to give our planet its shortest day
There are 24 hours in a day, right? Well, almost. Though it mostly goes unnoticed, hardly any of our days hit this number exactly. Decade to decade, season to season and even day to day, Earth’s rotation speeds up and slows down, shaving off or adding milliseconds to our 24-hour count. Beyond just being a quirk of a rotating planet, these variations in day length are also affected by ancient ice sheets, powerful winds and the dynamics of our planet’s core.
Angular momentum has three components: the mass of the rotating object, the speed at which it moves and its distance from the point it is rotating around. In the case of the office chair, when your arms are fully extended, they are farther from the seat, which increases the angular momentum of this part of your body.
One of the most powerful controls on Earth’s rotation throughout the year—and a contributor to June 29’s 1.59-millisecond deficit—is the wind, particularly the strong jet stream winds in the Northern Hemisphere. “Whenever they get stronger, the angular momentum of the atmosphere increases and the solid earth angular momentum decreases,” explains Sigrid Böhm, a geodesist at the University of Vienna, who studies the effects of climate on the planet’s rotation.
But scientists suspect that the progressively shortening days we’ve seen since the 1960s—culminating in June 29’s record—come from far deeper: 1,800 miles beneath our feet, where the rock in the mantle meets the hot, dense metal of Earth’s core. We can’t observe the core directly, so there isn’t yet a consensus about what is going on in the metallic parts of our planet to cause this long-term increase in Earth’s rotation.
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