Why Earth is in a hurry and July 9 saw one of the shortest days ever

On July 9, 2025, the Earth spun 1.38 milliseconds faster than usual, completing a full rotation in less than 24 hours, making it one of the shortest days ever recorded. Scientists at the International Earth Rotation and Reference Systems Service (IERS) predict even shorter days ahead: on July 23 and August 6, the Earth is expected to complete its rotation 1.388 milliseconds and 1.4545 milliseconds faster, respectively, compared to the average 24-hour day. A millisecond is a unit of time equal to one thousandth of a second, which is considerably less than a blink of an eye, which lasts around 100 milliseconds.

What is a ‘day’

We commonly think of a day as exactly 24 hours, equal to the time it takes for Earth to complete one full rotation. However, here's something surprising: that's not entirely accurate. When Earth rotates 360 degrees, a distant star observed overhead will return to the same position after about 23 hours, 56 minutes, and 4 seconds. This duration, known as a sidereal day (meaning "star day"), is roughly 4 minutes shorter than our usual 24-hour day.

However, if we measure time using the Sun's position, for example, from one noon to the next, the length of this solar day changes throughout the year. Because Earth orbits the Sun in an elliptical path, it moves faster when it is closer to the Sun (around January) and slower when it is farther away (around July). Additionally, Earth's tilted axis also affects the duration of the solar day. As a result, the actual solar day varies, sometimes stretching to about 24 hours and 30 seconds (around late December) and shrinking to roughly 23 hours, 59 minutes, and 38 seconds (around mid-September).

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To make timekeeping practical, we use the 'mean solar day', an average of all those variations, which gives us the consistent 24-hour cycle (or 86,400 seconds) that we rely on in daily life. This is the foundation of our clock timekeeping system. When your clock counts exactly 86,400 seconds, we consider that one full day has passed.

This implies that on 9 July 2025, the actual length of the day was only 23 hours, 59 minutes, 59.9985793 seconds or 86,399.9986154 seconds rather than 86,400 seconds (which is 24 hours).

Length of a day (LOD)

In everyday conversation, when we talk about the ‘length of a day’, we might mean either how long the Sun stays up (daylight hours) or simply 24 hours as shown by our clocks. But for scientists studying Earth's rotation, it is a measure to quantify how our planet is spinning faster or slower than usual.

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The International Earth Rotation and Reference Systems Service (IERS) uses Very Long Baseline Interferometry (VLBI) to track distant radio sources, primarily quasars. They meticulously time the successive return of these radio sources to the exact same position in the sky. From this, they compute the actual rotation speed of the Earth. This measurement is expressed as Universal Time (UT1), which is a type of 'mean solar time'.

They compare this real-world measurement (UT1) with the standard 86,400-second day we use for timekeeping. The difference between these two, measured in milliseconds, is what scientists officially refer to as the LOD.

A positive LOD value means Earth's rotation is slowing down, while a negative value indicates it is speeding up. So while our clocks tick away at a steady 24-hour beat, Earth itself might be dancing to a slightly different rhythm.

Fossils as timekeepers

If modern astronomers use distant quasars to detect variations in Earth's rotation in contemporary times, palaeontologists use ancient coral fossils to study Earth's rotation millions of years ago, thereby determining the time it takes for Earth to spin.

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Corals, such as rugose and scleractinian, grow daily and annual bands in their calcium carbonate skeletons—daily bands form due to variations in sunlight cycles. In contrast, thicker annual bands reflect seasonal changes. By counting daily bands within a yearly layer, researchers estimate the number of days that existed in a year long ago. For example, Middle Devonian fossilised corals (approximately 385 million years old) exhibit roughly 400 daily bands per year, indicating that the Earth completed 400 rotations annually. A younger Carboniferous coral (approximately 330 million years old) had around 390 daily bands, suggesting that Earth's spin had gradually slowed.

Ancient records reveal Earth's changing rotation

Historians, too, have joined the mission. Historical records of solar and lunar eclipses, dating back as far as the 8th century BCE, provide valuable data for understanding the historical variation in Earth's rotational speed. For example, a Babylonian clay tablet documents a total solar eclipse that occurred over Babylon on 15 April 136 BCE. Through modern computer simulations, researchers are able to reconstruct the exact trajectory of this eclipse. Tweaking the rate of Earth's spin in the simulation, they found at what speed the eclipse would have been visible at Babylon overhead, as described in the record. These findings enable scientists to estimate changes in Earth's rotational velocity over time. These suggest that the LOD has been increasing at a rate of approximately 1.74 to 1.8 milliseconds per century during this historical period.

Lengthening day

During the Middle Devonian period, a day lasted just 22 hours (78,891 seconds). By the time of the dinosaur age (over 50 million years ago), it had extended to 23 hours. Using the records of eclipses recorded by Chinese scribes who etched astronomical observations into animal bones, scientists infer that days are now 0.047 seconds longer than they were in 1200 BCE. Scientists predict that in 200 million years, a day might stretch to 25 hours. These findings show how Earth's rotation has been steadily slowing due to tidal forces and other factors.

Earth's inconsistent spin and how we adjust our clocks

Modern atomic clocks, developed in the 1960s, revealed that our planet does not rotate at a perfectly steady speed. When scientists timed distant radio signals, they found Earth's spin varies slightly. While generally reliable (with days staying within milliseconds of the standard 86,400 seconds), our planet sometimes speeds up or slows down unpredictably.

These tiny daily variations, measured as Length of Day (LOD), might seem insignificant, but over months and years, they accumulate. Just like we add a leap day every four years to keep our 365-day calendar aligned with Earth's 365.2422-day orbit around the Sun, we also need to adjust for these rotational changes. This is where 'leap seconds' come in.

Introduced in 1972, leap seconds help synchronise our ultra-precise atomic clocks with Earth's actual rotation. When Earth's rotation slows, we add a positive leap second. If it were to speed up (though this has not happened yet), we would subtract a negative leap second. So far, all 27 adjustments have been positive additions, with the latest in 2016, showing our planet has been gradually slowing down in recent decades.

Why does Earth's spin behave so erratically?

Over millions of years, the Moon's gravitational pull has been slowing down Earth's rotation through tidal friction. This constant tug-of-war transfers Earth's rotational energy to the Moon, making our days longer over geological timescales while causing the Moon to recede by approximately 3.8 cm every year. This has led to the lengthening of the day on a geological scale and the repeated addition of leap seconds in the past 30-40 years.

But what explains the daily wobbles in Earth's spin? Scientists point to two key factors: celestial and terrestrial. The Moon does not just orbit Earth in a simple circle; its path is tilted and wobbly. The Moon's elliptical orbit shifts over time, and its gravitational effects change depending on whether it's north or south of Earth's equator. During special events like the "lunar standstill" around July 9, 23, and August 6, 2025, when the Moon reaches its farthest point from the Earth's equator closer to one of the poles, it delivers an extra "kick" to therth's spinven distant planets like Jupiter contribute tiny gravitational nudges.

Earth itself is no steady platform either. Massive earthquakes can literally reshape our planet's spin, as seen in the 2004 Sumatra quake, which shortened days by 2.68 microseconds by shifting the Earth's mass inward. Imagine a spinning office chair: when you tuck your arms in, you spin faster; stretch them out, and you slow down. Earth behaves similarly when its mass redistributes through quakes, winds or ocean currents.

The atmosphere plays its part too. Seasonal winds and climate patterns, such as El Niño, push against mountains and oceans, acting like invisible brakes or accelerators on Earth's rotation. Even ocean currents, by moving vast water masses, tweak our planet's spin rate. All these forces combine to make each day slightly different from the last.

Why do milliseconds matter?

To us humans, a millisecond is a flash; just the blink of an eye takes about 100 milliseconds! On 9 July, when Earth's rotation was 1.38 milliseconds shorter than usual, it meant our day was 23 hours, 59 minutes and 59.9985793 seconds long instead of 24 hours. Seems insignificant, right?

But in our modern, high-tech world, these tiny fractions of a second make a huge difference. In just 1.38 milliseconds, Earth rotates about 62.66 cm at the equator. For missile guidance systems or spacecraft docking, even nanosecond-level precision is crucial; without it, targets are missed and missions fail.

This precision becomes even more critical for the technologies we use on a daily basis. The atomic clocks in GPS satellites must be accurate to nanoseconds (billionths of a second) because even a microsecond error can cause your location to be off by 300 meters. A millisecond mistake could throw navigation systems off by hundreds of kilometres.

In our digital world, milliseconds rule everything. Computer networks use millisecond timestamps to monitor performance and troubleshoot issues. Internet Banking systems track transactions to the nearest microsecond to prevent fraud. Even in astronomy, studying millisecond pulsars requires exact time measurements.

Politics of time

For many years, India has relied on foreign GPS systems (American and Russian) to meet its crucial navigation needs in defence and technology, such as missile guidance. This dependence proved risky during critical moments, such as the Kargil War, when access to these services was suddenly denied by a superpower. This hard lesson prompted India to develop its own satellite navigation system, NavIC (Navigation with Indian Constellation), originally known as IRNSS.

However, creating an independent navigation system came with its own challenges. The system required ultra-precise atomic clocks onboard each satellite. Initially, ISRO imported atomic clocks from Western countries, but most failed after launch, in what appeared to be deliberate sabotage, leaving the navigation satellites nearly useless. This setback became a turning point for India's space program.

Determined to become self-reliant, ISRO took on the challenge of developing these critical atomic clocks within India. Working with academic institutions across the country, led by ISRO's Space Applications Centre in Ahmedabad, scientists successfully created indigenous Rubidium atomic clocks. The first made-in-India atomic clock was launched aboard the 10th NavIC satellite in May 2023. Since then, the new NVS series of NavIC satellites proudly carries these Indian-made atomic clocks.

Soon, Indian computers, laptops, smartwatches, and phones may synchronise their time using these indigenous atomic clocks instead of relying on foreign navigational and time service providers.

The Earth's spin mystery

For millions of years, Earth's rotation has been gradually slowing, resulting in our days becoming longer over time. However, something strange has been happening recently. Since 2020, scientists at the International Earth Rotation and Reference Systems Service (IERS) have noticed that our planet is actually spinning faster. In fact, 2020 saw 28 of the shortest days ever recorded since the 1960s, when atomic clocks began measuring time.

This unexpected speed-up is so significant that timekeepers might soon need to make a historic adjustment; instead of adding a leap second (as we've done 27 times before), we may need to subtract one for the first time ever to keep our clocks in sync with Earth's changing spin.

So, what is causing Earth to suddenly hurry up? Scientists suspect that changes in our planet's molten iron-nickel core may be redistributing mass and affecting its rotation. However, the truth is that we're still trying to fully understand why our world appears to be spinning faster these days. It remains one of the fascinating unsolved puzzles.