Earth Spun Faster Today. Here's How We Know.

 

Hold on! (emarto/Getty Images)

On Tuesday, August 5, Earth will finish a spin 1.33 milliseconds sooner than normal. With a duration of 86,399.99867 seconds, it is among the shortest days of 2025.

Your mind may be racing over how it occurs and how we can even quantify it so precisely.
A sidereal day is when the Earth physically revolves in an average of 23 hours, 56 minutes, 4 seconds, and 90.5 milliseconds. It is the "real" rotation of the Earth with respect to far-off objects in deep space, such as stars.

A solar day, on the other hand, is a 24-hour day that most people observe; it is the period of time between two sunrises or successive noons.  The additional four minutes are caused by the fact that in order for the Sun to reappear at the same location, Earth must revolve by one more degree, to 361 degrees.

 Due in significant part to the Earth's atmosphere's winds, fluid circulation in the ocean and magma, and even the Moon's gravitational influence, both types of days are marginally shorter on August 5, 2025.

 Since the 1970s, atomic clocks and astronomy have been used to measure deviations from the 24-hour clock with accuracy.

These variations accumulate over the course of a year; in 1973, for instance, the total discrepancies came to +1,106 milliseconds, which meant that the Earth's rotation was behind schedule by just more than a second.  In order to account for this, leap seconds were used that same year. At 23:59:60, one second was added to the clock at the conclusion of the day.

 In timekeeping, absurdly high levels of accuracy are required.  There is no issue with global positioning systems, or GPS as it is more often known, being able to locate you in space.  However, an uncorrected GPS won't be aware of this, and your position won't match your map if the planetary surface you are on has physically rotated a little bit faster or slower than anticipated that day.

If left unchecked over the course of the year, the cumulative drift of 1973 would have resulted in GPS errors of almost half a kilometer, since a 1.33 millisecond variation corresponds to a location inaccuracy of roughly 62 cm at the equator.

Why doesn't the Earth stay still?

You must locate a reference frame where, ideally, nothing is moving in order to determine how quickly the Earth is rotating at all. The farther we gaze, the more motionless things appear, even though everything in space moves in relation to everything else. This is similar to how close farms fly by and far-off hills seem to move more slowly when you're riding a train.

Fortunately, some things are so dazzlingly bright that they can outshine entire galaxies. These are quasars, which can be seen from billions of light years away throughout the cosmos.

The Milky Way galaxy as a whole emits 100–10,000 times less light than quasars, which are supermassive blackholes with masses up to billions of times that of the Sun.  As cosmic beacons, quasars can be seen from billions of light years away in the universe, where everything is basically motionless.

 By measuring our position in relation to these, radio telescopes are able to determine the true rotation period of the Earth with sub-millisecond accuracy.

In order to anticipate the duration of the day, computer models that incorporate movements of the atmosphere, oceans, celestial motions, and more are based on those incredibly accurate observations.  This allows us to anticipate when a day will be shorter and adjust GPS accordingly.

 Due to their collisions with the ground surface, especially mountain ranges, winds in the Earth's atmosphere have the greatest impact on the duration of each day.  As unbelievable as it may seem, wind actually slows the Earth's rotation.

Even though we often refer to the summer months as the "longest" days in the northern hemisphere due to their longer daylight length, the prevailing winds on Earth are at their slowest from June to August and at their fastest during the winter months in the northern hemisphere.

These seasonal and daily fluctuations are only transient dips on top of longer-term slowdowns. The Earth's rotation has been slowed over decades by the melting of the polar ice caps. Consider a spinning dancer who retracts her outstretched arms to see why; they start to spin considerably more quickly. The same is true of a spinning ball, such as Earth.

Because the Earth is oblate, the equator's surface is 21.5 kilometers from the planet's center, but the poles' surface is closer.  The ocean carries meltwater from the poles to the equator as a result of climate change melting the polar ice caps.  Similar to the dancer extending her arms back out, rising sea levels cause water to be farther below the surface, which facilitates Earth's slowing.  Earthquakes and other forms of mass redistribution alter our rotation in comparable ways.

Despite its beauty, the moon can be a major hindrance over billions of years.  The Moon's gravity raises the waters on Earth, yet the increased oceans are carried a little bit ahead of the Moon in its orbit when the Earth rotates.  However, we slow down because the Moon keeps tugging on those oceans, drawing them backwards against the Earth's anticlockwise spin.

 The Moon receives Earth's rotational energy instead of being lost, which increases its orbital speed and helps it defy Earth's gravity. As a result, it is moving away from us at a rate of 3.8 centimeters each year.  Due in major part to the Moon consuming Earth's angular momentum over the ages, our day has grown longer than it was 17 hours and 2.5 billion years ago.

From 1973 to 2020 (when accurate measurements are available), the Earth's rotation slowed annually, adding hundreds of milliseconds of lag per year, which has already been adjusted for by adding 27 leap seconds.

 Beginning in 2020, the Earth began to rotate more quickly rather than more slowly each year. This was likely caused by the exchange of angular momentum between the Earth's core and mantle, but it was also influenced by the many other motions we have studied.

Ahead of time, July 5, July 22, and August 5 were identified as some of the quickest days of the year because, in addition to the Earth's internal movements and seasonal variations in atmospheric winds, the Moon's orbit slows the Earth twice every two weeks.

 This is because the Moon's tidal drag acts east to west when it is directly above the equator, but on certain dates, it is farthest north and south, which lessens that influence.

 You won't notice that the sunrise is 1.33 milliseconds earlier, but it will be clear to quasar-referenced astronomical observations and precision atomic clocks.

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