Here is a question. What lasts 86,400 seconds? Before you start to divide out this number to find the number of minutes and then hours, let me save you the trouble. It equates to 24 hours, the length of one complete day, and the time it takes the Earth to make one full rotation. For the majority of the population of this planet, this is a standard we can rely on throughout our lives. However, to scientists working in this area, things are not as reliable as we think.
The rotation of the Earth is subject to outside forces. These include the distance the Moon is from the Earth. Yes, this does vary, and as a result, the gravitational force between the two bodies changes, which in turn slightly alters our day. The same holds true for the gravitational effect of the Sun as well as the Earth’s own gravitational field.
For billions of years, we know the rotation of the Earth has been slowing down. We believe this is because the Moon is very slowly moving further away from the Earth. However, since 2020, the opposite has been noted. The spin of the Earth is speeding up, with the consequence that our day is getting shorter. We need to understand that the changing time periods and the change of distances involved are tiny and imperceptible to the average person without specialised measuring equipment.
The changes in time are just over one millisecond. In that case, can we ignore the changes? The answer is definitely not, as many areas of our technical world rely on being extremely accurate. Our GPS devices, electrical networks and the world banking system would not work if our measurement of time did not keep in line with any changes, no matter how small.
So, time has to be measured extremely accurately and to do so, there are 400 atomic clocks set up around the world. These clocks are accurate to one billionth of a second. They are all synchronised with each other, and their value of time is known as UTC or Coordinated Universal Time. They are independent of the length of a day because the latter has irregularities. Over time, these variations often cancel each other out.
However, when there is a discrepancy, then a leap second is added. We have all heard of a leap year, which occurs every 4 years, and it is when an extra day is added, the 29th of February, to the year. What has changed regarding the speeding up of Earth’s rotation is that a negative leap second may need to be incorporated for the first time. The purpose of a leap second is to equalise the Earth’s rotation, astronomical time, with UTC measured by the atomic clocks.
The idea of a leap second was first introduced in 1972, but it is far from perfect. Different countries have their own variations as to how it is implemented, and this causes havoc with some computer systems. One of the worst hit is airline scheduling. Therefore, it has recently been decided that the leap second will be withdrawn from use by 2035.
The summer of 2025 has witnessed three days on which the day got shorter. They varied from 1.2 milliseconds to 1.5 milliseconds. The shortest day occurred on the 5th of August 2025, and this was mainly due to the Moon being furthest away from the Earth. The problems with accurate timekeeping are not going to go away, and further research and new rules will have to be put in place, certainly before “time” runs out.
John Pullen 