Here’s Why Earth Just Had Its Shortest Day on Record

Yes, there are 24 hours in a day. Almost. Most of our days don't hit this number at all. Earth's rotation varies from decade to decade, season to season, and even day to day. Beyond just being a quirk of a rotating planet, these variations in day length are also influenced by ancient ice sheets, powerful winds and the dynamics of our planet's core.

Some days are more extreme than others. The shortest day ever recorded was June 29, 2022. It came as no surprise to those who are familiar with it. The Earth's average rotation has been speeding up for more than 50 years. This long-term truncation of day length, seasonal effects on Earth's rotation and a little extra kick from daily climate effects made June 29 a record breaker.

Many of us understand the principle in physics. Imagine popping on a pair of ice skates and taking a spin in an office chair. Your rotation will slow when you stretch out your arms. You'll spin faster when you pull your arms in.

The mass of the rotating object, the speed at which it moves and the distance from the point it is rotating around are components of angular momentum. When you extend your arms in the office chair, they are farther away from the seat, which increases the strength of your body.

Mass and energy can be conjured out of thin air, but the same cannot be said for angular momentum. The only remaining option is to slow down. Earth is also bound by the same rules when it twirls around the sun.

Earth's rotation rate and day length are adjusting because it is constantly redistributing mass. The entire Earth is measured by the length of day. There is more than one number. There are many stories behind it.

The wind is one of the most powerful controls on Earth and contributes to June 29's 1.59-millisecond deficit. Sigrid Bhm is a geodesist at the University of Vienna who studies the effects of climate on the planet. The Earth rotates slower in the winter and faster in the summer.

The movement of air and land is caused by hills and mountains. The jet stream in the Southern Hemisphere is mostly over the ocean. Transient changes in wind and weather can be enough to change the length of a day by a fraction of a millisecond.

The planet's rotation is impacted by Gradual movements of mass. At the peak of the most recent ice age, huge, multi-mile-thick ice sheets covered most of the Northern Hemisphere. Our planet's mantle was squeezed out from beneath the ice sheets because they were so large. The phenomenon was similar to how pressing your thumb into a soft caramel candy would cause it to break. After the ice sheets have melted, the earth is slowly bouncing back, and shortening in day length.

Scientists think that the shortening days we have seen since the 1960s come from far deeper, where the rock in the mantle meets the hot, dense metal of Earth. There isn't yet a consensus about what is happening in the metallic parts of our planet to cause this long-term increase in Earth's rotation. The core touches the mantle at the boundary.

There is a chance that the decades-long trend is the result of the side effects of Earth not being spherical. The axis around which our planet rotates doesn't line up with the actual axis of symmetry, which causes the rotation axis to keep trying to adjust. It's not clear how important a role that changing axis might be in determining day lengths.

Scientists use mathematical models and measurements to unpack the contributions of all these processes. The movement of mass on Earth is measured by satellites and missions. Predicting the impact of air and water is helped by models of the planet's climate. Bhm says that they always try to figure out if they can close the budget.

Predicting how day lengths will change in the future is very difficult. First we need to understand the long-term variability. We can estimate the contribution from the atmosphere and ocean. We can estimate when the next shortest day will be. The climate phenomenon that helped June 29 break the record was likely just a change in wind speeds high in the atmosphere.

Chen thinks we are going to see a short day in the near future. We will have to wait and see when the shortest day will be.