If we were to travel back in time to a much earlier time in Earth’s history—let’s say one million years before the present day—we might be surprised to see how different things were long ago. Of course, our planet looked very different, as did the varieties of life that lived on it in ancient times. However, there were other things we would notice if we traveled back to this world of the distant past, such as the length of each day on our planet: one million years ago, a typical day lasted only 18 hours.
What would have caused the days of long ago to have been so much shorter? It actually had to do with a phenomenon that is still occurring today, and one that is causing days over the slow passage of time to get longer and longer.
According to new research published in the Proceedings of the National Academy of Sciences, the fact that the moon has slowly been moving further away from the Earth has caused our planet’s rotation to slow down gradually over time. The result of Earth’s spin slowing down, of course, has been lengthier days.
All objects in space exert gravitational influence over one another, in relation to their proximity. The same can be said of all the objects in our solar system, ranging from our sun to the constituent planets each of their moons.
For the study, researchers relied on the science of astrochronology, a process that gauges the age of soil deposits and other sedimentary features astronomically, often in relation to precession—a key factor in what are called Milankovitch cycles—or other astronomical data. With the help of this process, researchers aimed to learn valuable geological about the ancient past, which would produce data nearly as reliable as that which can be studied in the present day.
In the early 20th century, Serbian Mathematician Milutin Milankovitch calculated the periodicity of glacial periods on Earth using information from geological proxies. He determined that the likely governing factors included variations in the shape of the Earth’s orbit around the Sun (eccentricity), as well as its precession, and the Earth’s axial tilt.
Milutin Milankovitch
According to the researchers, “beyond about 1.5 billion years ago, the moon would have been close enough that its gravitational interactions with the Earth would have ripped the moon apart.” Earth possess only one natural satellite, while other planets, such as Mars or Jupiter, have numerous satellites in orbit around them, which are often visible from Earth with the visual aid of a telescope. However, Earth’s moon has played a key role in many of the natural forces at play on our planet over the course of the 4.53 billion years since it was formed.
Combining geological data collected via astrochronological processes with statistical data, the study’s authors were able to gauge periods of “maximum volatility” in the ancient past, partially by focusing on what were known as grapoloids, an extinct form of marine invertebrate once a major zooplankton species during the Early Paleozoic.
“We infer that these cycles influenced graptolite speciation and extinction through climate-driven changes to oceanic circulation and structure,” the study’s authors note. “Our results confirm the existence of Milankovitch grand cycles in the Early Paleozoic Era and show that known processes related to the mechanics of the Solar System were shaping marine macroevolutionary rates comparatively early in the history of complex life.”
Another key focus of the study was a 1.4 billion-year-old area in Northern China known as the Xiamaling Formation, as well as 55 million-year-old proxy samples from Walvis Ridge, a deep ocean feature located in the southern Atlantic. Results of the study revealed variations in the direction of the Earth’s axis of rotation (precession), as well as changes in rotational features over time. This data was used to reliably determine that days on Earth during these earlier periods were much shorter when compared with today, since the distance between Earth and the moon is greater now than it was during the Paleozoic Era.
There are times where the remarkable unity between separate scientific disciplines, and their complementary nature to one another, becomes simply fascinating. Here, we learn about ancient astronomy from the study of geology, though it can be said to some degree that the same applies in reverse as well, ever-broadening our overall understanding of geological deep time. The study, titled “Pacing of Paleozoic macroevolutionary rates by Milankovitch grand cycles,” was co-authored by James S. Crampton, Stephen R. Meyers, Roger A. Cooper, Peter M. Sadler, Michael Foote, and David Harte.
As a final note, for more on Milankovitch cycles and their use in determining climate changes on Earth throughout prehistory, see here; and for more on the concept of geological “deep time,” see here.