Earth's Rotation Accelerates, Leading to Shorter Days This Summer

Summer is synonymous with longer days. Sunrises come earlier and sunsets linger later, providing extended hours for leisurely beach outings and slow-cooked barbecues beneath the sun's gentle arc.

However, when considering a full astronomical day—one complete rotation of Earth marked by the hour hand circling a standard clock twice—some of the shortest days this year have occurred in July and August.

This week has recorded the shortest days so far. According to data from the U.S. Naval Observatory and the International Earth Rotation and Reference Systems Service, Tuesday's rotation was approximately 1.34 milliseconds shorter than the conventional 24-hour period.

Additional instances of faster spins are anticipated later this week, as well as toward the end of July and early August, based on projections from the Time and Date website.

Such variations are not entirely unusual: Earth's rotation has been trending slightly faster than average recently. Over the past decade, the average day length has generally shortened, and in the last five years, full rotations have frequently lasted just under 24 hours. Factors influencing these shifts include movements within Earth's core, atmospheric changes, and the Moon's position.

Long-term patterns, however, indicate that days will not keep shortening indefinitely. In fact, the opposite trend has prevailed for millions of years. Studies suggest that a Tyrannosaurus rex living 70 million years ago experienced an average day length of about 23 and a half hours.

Clark R. Wilson, a research professor at the University of Texas at Austin’s Center for Space Research, notes that the trend toward longer days is expected to continue, albeit on a timescale far beyond human lifespans.

The primary cause lies in tidal energy loss. The Moon's gravitational tug generates Earth's tides, and tidal currents slightly warm the oceans, dissipating energy that slows Earth's rotation. This process also causes the Moon to drift away at a rate of about four centimeters per year.

In physical terms, Earth and the Moon form a single system with a constant total angular momentum—a measure of rotational motion. As the Moon's orbit expands and its angular momentum increases, Earth’s angular momentum correspondingly decreases.

As a result, the length of days gradually increases.

Nonetheless, this is a process unfolding over billions of years and is complicated by short-term fluctuations.

Historical data spanning centuries demonstrate that Earth's rotation speed has never been steady. Periods of slower rotation, such as in the early 1990s or 1970s, saw days regularly exceed 24 hours by more than 2 milliseconds. Conversely, periods like the present show faster rotation.

Various internal and external factors contribute to these changes and can vary daily or over decades. Movements of material inside Earth—from its core to mantle and crust—affect rotation speed. Long-term meteorological and climatic shifts also play a role.

Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory, describes untangling these contributing factors as an extremely challenging problem.

Collectively, these influences create a complex interference pattern explaining why the duration of a terrestrial day has oscillated since humans began precise measurements.

Research involving Adhikari also suggests that recent human-induced climate change could become a significant factor in lengthening days. Melting polar ice redistributes water toward the equator, making Earth more oblate, which can slow its rotation—similar to how an ice skater spins slower with arms extended and faster when arms are pulled in.

Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory, explains that these effects interact in complicated ways. Given Earth's vast size and complexity, combined with the minuscule scale of the changes measured, predicting Earth's movements remains a difficult task.

Nevertheless, recent data clearly show that Earth’s rotation speed has increased this season. For those in the Northern Hemisphere just entering summer, this serves as an astronomical reminder that time can seem to fly even as long summer days stretch out.