For thousands of years, eclipses were interpreted as signs from the gods, announcements of wars, omens of misfortune or events capable of changing the destiny of a king. Today we know that they were also something else: they were watches. Every eclipse observed by a human being centuries ago left a small mark on the history of our planet’s movement. And when modern astronomers compare where and when the phenomenon should have occurred with the place and time someone wrote that they saw it, they can discover something that doesn’t show up in any telescope: how the Earth rotated in the past.
It is one of the most extraordinary stories hidden behind eclipses because the Earth is not a perfect mechanism. Our planet rotates on its axis, but its rotation speed does not remain exactly constant. The main cause of long-term slowing is tidal friction caused by the Moon. The gravitational interaction between both bodies slowly transfers energy and angular momentum from the Earth’s rotation to the Moon’s orbital motion.
The result is that, on very long time scales, the Earth rotates more and more slowly. At our scale, the difference is tiny, but when we go back hundreds or thousands of years, those small accumulated differences begin to be enough to considerably shift the position in which an eclipse should have appeared. And that’s where ancient astronomers unknowingly become chronometers.
The trick is knowing where the shadow should have fallen. We can calculate with extraordinary precision the movements of the Moon and the Earth. If we know their orbital positions and assume a certain speed of Earth’s rotation, we can reconstruct where the Moon’s shadow would have passed during an eclipse. The problem appears when we try to make this calculation for an eclipse that occurred two or three thousand years ago. Orbital movements can be reconstructed quite accurately, but we also need to know what time the Earth actually kept.
Astronomers call this difference between uniform astronomical time and time based on the Earth’s rotation ΔT. And the further back we travel, the greater the uncertainty. NASA explains that ΔT values prior to the era of telescopes have been precisely reconstructed from historical observations of eclipses and occultations. That is to say: to know how the Earth rotated thousands of years ago, we need to read what those who were there wrote.
One of the most fascinating examples comes from China. More than 3,000 years ago, Chinese astronomers recorded eclipses on animal bones and turtle shells used for so-called oracular inscriptions. Some of them described andhe phenomenon with an extraordinarily graphic expression: “the Sun has been devoured.”
Much later, those records acquired a completely different value. Astronomers at NASA’s Jet Propulsion Laboratory studied eclipses recorded in Anyang, including those of 1226, 1198, 1172, 1163 and 1161 before our era. If the Earth had rotated at exactly the same speed as it does today, the shadow of those eclipses would have passed thousands of kilometers away from the place where observers reported seeing them.
But the eclipses occurred there. The explanation is that the Earth was spinning slightly faster then. From these records, scientists were able to estimate how the Earth’s rotation had changed over the past 3,200 years. The NASA calculates that, in that interval, the length of the day has increased by approximately 47 thousandths of a second. It seems insignificant. But multiplied by thousands of years they are enough to move the shadow of an eclipse thousands of kilometers.
And before there were telescopes, modern observatories or mechanical clocks, there was another civilization that turned the heavens into a gigantic astronomical archive: the Babylonians. Their tablets recorded for centuries the movements of the Sun, the Moon and other celestial bodies. Over time, these records made it possible to recognize patterns in eclipses and develop methods to anticipate them.
One of these patterns is what is known as the Saros cycle: approximately 223 synodic months, about 18 years and 11 days, after which the geometries of the Sun, Earth and Moon once again produce an eclipse with similar characteristics. Saros does not mean that the next eclipse will occur in exactly the same place. The fraction of a day that the cycle contains means that the Earth has rotated a little more when it is repeated, shifting the path of the shadow to another region of the planet. The Babylonians learned to recognize this rhythm long before they knew the physics that produced it. AND Their observations ended up serving something they never imagined: helping to measure the Earth’s rotation.
And here appears the paradox that makes eclipses so special. To predict a modern eclipse we use atomic clocks, gravitational models and computer calculations. But to know how that same cosmic clock worked 2,000 or 3,000 years ago, we have to turn to documents written by people who didn’t know any of those things. A phrase on a clay tablet, an annotation in a Chinese chronicle, a record of a medieval astronomer monk… Each observation contains a temporal and geographic coordinate. And by comparing it with modern calculations, astronomers can determine how much the Earth’s actual rotation has deviated from an ideal clock. History thus becomes a tool of astronomy.
But there is something even more interesting in this story. Eclipses not only allow us to reconstruct the past. They also remind us that the Earth is not an isolated sphere rotating in a perfect vacuum. Its rotation is connected with the Moon, with the oceans, with the atmosphere and with processes that occur inside the planet. Tidal slowing is only part of the story. The Earth’s rotation also undergoes variations on much shorter time scales, caused by atmospheric and oceanic phenomena and internal processes of the planet.
That is why astronomers cannot simply construct an eclipse calendar and project it into the past as if the Earth were a perfect mechanical clock. The uncertainty in the Earth’s rotation is precisely one of the main factors that determine the reconstruction of ancient eclipses. And here is the last lesson of this series: this phenomenon will disappear As the Moon moves further away from our planet, the perfect distance will no longer be “perfect” and we will not only miss out on one of the few privileges of inhabiting Earth.also a cosmic clock.