Tuesday, July 22 will be the second-shortest day of the year, as Earth completes a full rotation in less time than usual.
The planet on Tuesday will experience a shorter rotation than the typical 24 hours, though not by much: 1.34 milliseconds less than usual, according to Timeanddate.com.
Typically, we think of the shortest day in the Northern Hemisphere as Dec. 21 or 22, which coincides with the winter solstice – July 21 or 22 in the Southern Hemisphere – which is the point at which the hemisphere is tilted farthest away from the sun. In those instances, however, it’s only the number of hours of daylight that are affected, and not the length of time it takes for the Earth to make a full rotation on its axis.
The 1.34-millisecond discrepancy in Tuesday’s length is explained by differences in the gravitational pull between the Earth and the moon, which causes the Earth to spin more quickly, according to the Lunar and Planetary Institute.
In recent years, Earth has broken its own speed records several times, including on July 5, 2024 – the shortest day ever measured, when the planet’s full rotation was completed 1.66 milliseconds faster than usual, according to Timeanddate.com.
This year, July 10 was measured as the shortest day of the year, with a full rotation occurring 1.36 milliseconds faster than normal. Aug. 5 is expected to be the third-shortest day of the year, projected to be 1.25 milliseconds shorter than usual.
Earth completes one full rotation on its axis in 24 hours, or about 86,400 seconds, plus or minus a millisecond or so, according to Timeanddate.com. Before 2020, Earth’s shortest length of day was just 1.05 milliseconds shorter than usual.
If current trends continue, a full second may need to be subtracted from atomic clocks in the near future to accommodate for a “negative leap second,” according to Timeanddate.com.
The system of leap seconds was introduced in the 1970s, according to Time and Date. While there have been dozens of positive leap seconds, in which a second was added, there has never been a negative leap second. The International Earth Rotation and Reference Systems Service, which monitor’s the Earth’s rotation, also determines whether to add or subtract a second.
In its early days – about 4.5 billion years ago – Earth was spinning at a much faster rate, according to the Lunar and Planetary Institute. A full rotation only lasted between three and six hours.
During the mid-Proterozoic period – between 2.5 billion years ago and 543 million years ago – the Earth had slowed such that a day lasted about 19 hours, according to a 2023 study published in Nature Geoscience.
Since then, the length of an Earth day has slowed to its present 24 hours due to the fact that the moon’s gravitational pull “steals” some of energy Earth uses to spin, in addition to causing tidal friction, according to the Institute.
Additionally, climate change is causing days on Earth to become longer due to the melting of ice at the poles, according to a study published in the Proceedings of the National Academy of Sciences. As the poles melt, the redistribution of mass from the resulting sea-level rise also is increasing the length of a day, the researchers found.