Careful analysis of hundreds of accounts and measurements belonging to various ancient cultures going back 2,700 years ago have revealed the fact that Earth's spin is slowing down at a rate of 1.8 milliseconds per century. Although it may not seem significant, the time discrepancy between the time of the oldest accounts and present times would amount to seven hours.
Earth's Spin And Time's Fascinating Complexities
Both time and Earth's rotation seem to be quite stable and well-measured concepts. However, there are numerous complexities that need to be taken into account. With the help of highly precise atomic clocks, humanity has managed to measure a day (86,400 seconds) as well as a second (exactly 9,192,631,770 oscillations of a cesium-133 atom). These measurements define the Terrestrial Time.
But this is not the only time that exists. Universal Time is defined and measured based on the dynamic gravitational movements of the moon, sun and Earth. The two times do not synchronize perfectly due to the fluctuations detected in Earth's spin, which influence the latter. As a result, leap seconds are added on June 30 or Dec. 31 whenever the two times are not perfectly coordinated.
Earth's spin itself is influenced by many factors. Very powerful winds, volcanic activity or earthquakes can all slow down or speed up the spin. The amount of polar ice is also a significant factor in this regard, so it is very likely that the changes registered in recent years will not have gone unnoticed.
Ancient Eclipses And Modern Answers
A small team of scientists from Great Britain decided to measure precisely the changes related to Earth's spin. The task proved to be quite challenging, as the only evidence to help the scientists was found in ancient accounts from around the world.
The team was most interested in eclipses and their exact dates, which could then be compared with modern data. Besides the need to learn over 1,500 Chinese characters to decipher the ancient writings, the scientists were also faced with other challenges, such as missing accounts or faulty measurements.
European accounts proved to be disappointing at times, but Arab observations, such as those of Ibn Yunus, al-Battani and al-Biruni proved to be of great help. After 1600, when telescopes became better, modern scientists could also rely on lunar occultations (observations of the moon covering certain stars at certain moments).
"People recording these things never had the slightest notion that what they were doing would lead to people in our generation actually studying changes in the Earth spin. We are very much at the mercy of these ancient chroniclers and astronomers," noted Richard Stephenson, an astronomer that has worked on this project for 40 years.
Although the progress made by researchers is quite impressive, the work is not done yet. The scientists are still looking for missing accounts that would help refine the calculations even more.