Here’s Why Summer Days are Getting Slightly Shorter

As summer reached its middle point in the week of July 7, 2025, the daily 24 hours were shortened by a few milliseconds. Strangely, this phenomenon occurs due to liquids in the Earth’s core moving, atmospheric changes, and the moon’s changing position in the sky.

According to the U.S. Naval Observatory and the International Earth Rotation and Reference Systems Service, Earth’s rotation was about 1.34 milliseconds less than 24 hours on July 9th. Astronomers predict shorter days on July 22 and August 5 as well.

Why this process occurs is because the tides pulled by the Moon have lost energy. One entire system, consisting of the Moon and Earth, has a generally constant total angular momentum (the tendency of an object to rotate). As the moon expands its orbit, its angular momentum increases, causing the Earth’s angular momentum to decrease. As a result, the days become shorter during the summer (this process only occurs midway through the year because the Moon regains its full control on the tides when it’s not summer).

In simpler terms, when the Moon’s gravity pulls on oceans, creating changes in tides, those tidal changes release a tiny amount of heat into the water. This slight heat causes energy to “dissolve,” therefore making the Earth’s rotation slower and allowing the Moon to travel farther away during the increased time. As a result, we humans, on Earth, see a shorter day. Currently, the rate of the Moon’s retreat is four centimeters per year.

Clark R. Wilson, a research professor at the Center for Space Research at the University of Texas, expects shorter days to continue throughout late summer, although the phenomenon is “well beyond human time scales.” This isn’t a surprise to scientists. Earth has been spinning faster than 24 hours since the year 2020, especially during the summer of every year since then.

For many millennia before 2020, days had been growing longer. For instance, a dinosaur living during the Late Cretaceous Period would have undergone an average of 23-and-a-half hours each day, says a study conducted by paleoclimatologist Niels J. de Winter and geologist Steven Goderis. On the other hand, during the early 1970s and 1990s, regular days were 2 milliseconds more than 24 hours.

Since the summer days are becoming shorter, every millisecond is worth cherishing before the school year starts.

Sources:

https://www.nytimes.com/2025/07/11/science/earth-speeding-up-summer-days-shorter.html

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019PA003723

https://www.timeanddate.com/news/astronomy/earth-fast-rotation-2025