Astronomy

Earth accelerates: the shortest days ever arrive

The shortest days of the century are expected in July and August, with minimal but significant effects on weather measurement

27 June 2025

3' min read

We may soon have the shortest days ever, forecasts indicate 9 or 22 July or even 5 August. This is according to the authoritative IERS, International Earth Rotation and Reference System Service, the international body responsible for monitoring the Earth's rotation since 1987.

As our measuring instrumentation becomes more precise and sophisticated, in fact, what seemed peacefully always equal to itself, such as the duration of a solar day, 86,400 seconds, creates non-trivial problems for us.

Infinite time differences

These are no longer the days when one got up at dawn, went to work in the fields until sunset and then returned home, today life is frenetically very different and we are able to appreciate, with our instruments, infinitesimal differences in time for us. In the three days we have just mentioned, in fact, the day will be shorter than the canonical 86400 seconds by 1.5 or so milliseconds, thousandths of a second, a tiny amount of time that we cannot even imagine.

La Terra avrà presto due Lune

We might easily think we were being fooled, were it not for the fact that our present civilisation, which uses GPS signals for a host of important functions, from finding the restaurant we want with the map programme, to giving the precise location of a plane or ship, uses times that are spasmodically smaller and smaller and must be precise. Think, for example, of the quantity of orders to buy or sell shares, which run on fibre-optic cables to the world stock exchanges, and are all stamped with a time stamp indicating the exact moment when the value of the transaction has to be calculated.

In short, why is this small but important shift from the average happening, we may ask. The fact is that the Earth rotates around its own, imaginary axis of rotation that does a bit of everything: it describes a cone in thousands of years, it oscillates even on the small scale and, since 2016, we have also noticed thanks to atomic clocks that the planet is accelerating its rotation.

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The reasons for this phenomenon are complex and not entirely clear: in the case we are reporting, undoubtedly the Moon's position relative to the Earth, far from our equatorial plane, plays a role. But that is certainly not all.

A crushed spheroid

We in fact think that the earth, as we were taught in primary schools, is a sphere, like the globe we may have at home or had in class. This is not so, in fact the closest figure to the shape of the earth is that of a flattened spheroid, and then there is the important question of the distribution of mass, which is not homogeneous: at least 70 per cent of the surface is water, and in any case the gravity measurements carried out by a little-known but exceptional European satellite, Goce, show us a very lumpy earth in its surface, rather far from the ideal spheroid we have just introduced.

La vera forma della Terra se si tiene conto della distribuzione della Massa ( GOCE/ESA)

The distribution of mass and oceans also has its share of responsibility, not just the Moon, and recent studies have introduced hypotheses that were unthinkable until recently, such as the melting of glaciers, which would change the distribution of mass on our planet.

The physical law behind the phenomenon

The physical law that explains the phenomenon is that of conservation of momentum of the earth.

When masses move on the Earth, the angular velocity, with which it rotates, can vary. If this explanation is a bit tricky, to make it easy let us think of a skater who rotates more or less quickly depending on whether he spreads his arms out or not: when he spreads them out, changing the distribution of his mass, the speed with which he rotates decreases. For example, the same seems to happen with the melting of the poles.

La Terra avrà presto due Lune

Currently we rely on the time marked by atomic clocks, which have a measurement accuracy of 1 second every couple of billion years, to get an idea, but astronomical time is still important and this irregularity is being remedied by introducing a 'leap second', an interleaving second, which brings the two times, atomic and terrestrial, back into alignment. On 30 June or 31 December, one minute of 61 seconds is counted, and everything falls back into place, the last time being in 2016.

The solution, however, is becoming less and less popular, and the strong proposal is to omit astronomical time and rely only on atomic time, but the idea finds many opponents for essentially historical reasons.