You may not notice it, but today will be one of the shortest days ever. Indeed, Tuesday 5 August will be a full 1.25 milliseconds shorter than a normal day.
While many may say that 21 December is the shortest day of the year (when the sun rises latest and sets earliest), the shortest days of 2025 actually fall in July and August this year; the planet is rotating exceptionally quickly on its axis this summer.
Normally, the Earth takes 24 hours (or 86,400 seconds) to do this, but this Tuesday it will take "only" 86,399.9875 seconds – 1.25 milliseconds fewer.
Previously, scientists noted that the Earth is rotating remarkably quickly this summer, with outliers on 10 July (-1.36 milliseconds) and 22 July (-1.34 milliseconds).
Leap seconds
The fact that a day on Earth lasts 24 hours is not a regular pattern in the life cycle of the planet: when the Earth formed roughly 4.5 billion years ago, a day lasted about 10 hours. When looking at 1.5 billion years ago, it lasted just under 20 hours, and around 70 million years ago, a day lasted 23.5 hours – meaning that the days have only gotten longer.
This is because the Earth's rotational speed is primarily determined by its distance from the sun and the moon.
"Over time, the moon recedes slightly from our Earth," said astronomy professor Leen Decin (KU Leuven). As a result, the moon's gravitational pull on the Earth weakens slightly, and the Earth rotates more slowly. "The slower the planet rotates, the longer the days. This is why we sometimes add a leap second."
Credit: Canva
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time measured by atomic clocks – also called 'International Atomic Time' (TAI) – and imprecise observed solar time (UT1, or 'Universal Time'). Due to irregularities and long-term slowdown in the Earth's rotation, the latter varies.
Therefore, a leap second is added occasionally to ensure that the TAI does not run ahead of the (more imprecise) observed solar time. Such a second has been added 27 times since 1972, when the atomic clock enabled the adoption of Coordinated Universal Time (UTC) and measurements began.
Before 2020, the shortest day was only 1.05 milliseconds shorter than 24 hours, but the Earth has broken several speed records since then. The shortest day ever recorded was on 5 July last year (-1.66 milliseconds).
A new millennium bug?
Scientists are puzzled by this break in trend. While several studies on its influence on the melting of the ice caps are being conducted, Decin said that there is no scientific consensus on the results or even on the cause of the phenomenon.
"There are many hypotheses. Atmospheric changes may play a role, or there may be slowdowns occurring in the Earth's liquid core," she said. "If there is a decrease in energy there, we see an increase on the outside, as a result of the law of conservation of energy. But for now, we do not know exactly what is behind this."
Whatever the reason, a buildup of these "missing" milliseconds could lead to the introduction of a negative leap second (a second subtracted from UTC). Scientists estimate a 40% chance of this happening before 2035, but it could have major consequences.
"It is quite worrying, because we do not know for sure how computer systems, satellites, and telecommunications would handle it," said Decin. "It is reminiscent of the so-called 'millennium bug', when scientists feared that the turn of the century would lead to serious disruptions to computer systems."
