In July and August 2025, we will witness an interesting astronomical phenomenon. Due to the special influence of the Moon, the length of the day on July 9, July 22, and August 5 will be shorter. However, the difference will only be noticeable for scientific instruments. Each of these days will be shortened by 1.3–1.51 milliseconds compared to the standard 86,400 seconds (24 hours). The reason for this is a specific gravitational “pull” from our moon.
The Moon as a regulator of rotation
The Earth is not a perfectly stable spinning top. Its rotation around its axis is constantly changing under the influence of external and internal forces. The key players here are the Sun and the Moon. Their gravity causes tides not only in the oceans, but also in the Earth’s crust and mantle. This creates friction, which slows down the planet’s rotation in the long term. When the Moon was closer to Earth 4 billion years ago, a day lasted only 19 hours.
Why these particular dates?
The conditional “records” for shortened days in 2025 are related to the Moon’s orbit. On July 9, July 22, and August 5, it will reach its maximum distance from the Earth’s equator. This is a key point. Imagine the Earth as a spinning top. If you hold it in the middle, it spins slower. But if you squeeze it at the top and bottom, closer to the “poles,” it spins faster. This is exactly how the Moon acts when it is higher above the equator — its gravitational pull “compresses” the planet closer to the poles, causing it to rotate slightly faster and “shorten” the days. The last time this happened was in 2016.
What else impacts the length of the day?
Although the position of the Moon is the main reason for the upcoming micro-accelerations, other factors also influence the long-term rotation speed. Human activity, particularly climate change, also plays a role. The melting of glaciers and the redistribution of groundwater lead to changes in the planet’s mass. For example, NASA researchers have calculated that between 2000 and 2018, these processes have already increased the length of a day by 1.33 milliseconds per century. Significant events, such as the powerful earthquake in Japan in 2011, can also have an impact, shortening the day by 1.8 microseconds.
How to measure the short days of 2025?
A reduction of one and a half milliseconds in a day will remain imperceptible in everyday life. Our clocks, synchronized with atomic time, continue to run at their usual pace. Adaptation of global time standards is only necessary in the event of a very significant difference accumulating – more than 0.9 seconds (900 milliseconds). This is precisely what the International Earth Rotation Service (IERS) does, occasionally adding a “leap second” to Coordinated Universal Time (UTC) to align it with the actual rotation of the planet. Therefore, the summer days of 2025 will be ordinary for everyone except the most accurate clocks and scientists studying the dynamics of our planet.
Earlier, we reported on how a huge man-made object was slowing down the Earth’s rotation.
According to livescience.com
