Our clocks, calendars, and other measurements of time are all based on the length of our solar day and year. While it’s generally believed that these are fixed quantities, new research has shown that the length of one day on Earth is actually getting longer. According to a recent publication in Proceedings of the Royal Society A, the Earth’s rotation is slowing down year after year. The calculations were made after analyzing historical records of solar eclipses made by ancient astronomers, which revealed a significant decrease in Earth’s rotational speed:
New compilations of records of ancient and medieval eclipses in the period 720 BC to AD 1600, and of lunar occultations of stars in AD 1600–2015, are analysed to investigate variations in the Earth’s rate of rotation. It It is found that the rate of rotation departs from uniformity, such that the change in the length of the mean solar day (lod) increases at an average rate of +1.8 ms per century.
These calculations began with a written account of a solar eclipse made in 720 B.C.E. by astronomers in Babylon, or modern day Iraq. Since this is the oldest-known account of an eclipse, astronomers used it as a baseline and worked forward to present day based on the current speed of the Earth’s rotation.
If the speed of rotation had remained constant in the nearly 1300 years since this eclipse, it would have not been visible in Babylon at that time. Thus, they concluded that the speed of rotation must have changed. Current theories speculate that the force of the Earth’s oceans slamming against landmasses accounts for the loss of rotational speed, although current calculations would have this tidal force slowing the Earth down more than it is. The discrepancy in calculations means there are potential unknown factors at work.
As always, climate change is being eyed as a possible culprit, since melting sea ice and the drying up of bodies of water contribute to changes in Earth’s center of gravity. While 1.8 milliseconds might not mean much in human time, that amount is significant on a planetary scale. After several more millennia of this deceleration, the effects on our planet will begin to be more significant.