The Earth speeds up: shorter days in the modern era

On 9 July it was the shortest day of the year, with a rotation completed 1.34 milliseconds less than the standard 24 hours. Both 22 July and 5 August also recorded historically short days. This acceleration of the Earth’s rotation is a complex and unexpected phenomenon. Although fluctuations are normal, the recent trend towards shorter days has been particularly marked over the past five years. Several factors contribute to influencing the speed of our planet’s rotation:

• Lunar influence: the Moon, through its gravity, exerts a force that can either slightly slow down or speed up the Earth’s rotation depending on its position relative to the equator or the poles.
• The atmosphere: the Earth and its atmosphere share a constant angular momentum. If the atmosphere slows down - as happens in summer due to jet streams - the planet must accelerate to compensate.
• The Earth’s core: something unusual is happening inside the Earth. Over the last 50 years the Earth’s core has slowed its rotation and, to maintain overall angular momentum, the outer parts of the Earth have sped up. Physicists do not yet have a complete explanation for this phenomenon.

In recent years we have seen the shortest days since the introduction of atomic time measurement (1955). However, in the Earth’s geological history, days were much shorter: 70 million years ago a day lasted 23.5 hours, and 430 million years ago only 21 hours. The long-term trend is for days to lengthen because of the gravitational influence of the Moon on the tides. A millisecond less is not perceptible in daily life (a blink lasts much longer). However, this tiny difference is crucial for astronomers and for technologies requiring extreme precision, such as GPS systems and telecommunications. Atomic time is constant, while Earth’s rotation varies, creating a discrepancy. To realign astronomical time with atomic time, leap seconds have historically been added. Now, with the Earth accelerating, it may be necessary to introduce an unprecedented negative leap second by 2029. This could create significant challenges for software, which usually assumes that time always moves forward.