The length of a day on Earth isn’t set in stone. Over billions of years, our planet has been gradually slowing down. This slowdown mostly occurs because of tidal friction, where Earth’s rotation transfers energy to the Moon, making it drift further away. This process changes day length, little by little.
Recent studies show that the average day has stretched by about 1.7 milliseconds every century recently. If we look back over the last 2,700 years, this figure rises to about 2.3 milliseconds per century. Surprisingly, a day during the time of the dinosaurs was shorter than today!
A key piece of evidence comes from fossils, particularly coral. In 1963, paleontologist John W. Wells published research in Nature showing that modern corals add a daily growth layer to their skeletons, much like tree rings. By counting these growth lines in ancient corals, he found that corals from 385 million years ago recorded about 400 days in a year, suggesting their days were around 21.9 hours long.
As Wells’ method evolved, he studied corals from an even earlier time and discovered that days were still shorter. For instance, corals from the Pennsylvanian period (about 320 million years ago) showed about 385 to 390 days per year, indicating days of about 22.4 hours.
However, corals only existed for part of Earth’s history. For earlier periods, scientists rely on sedimentary layers and orbital patterns in rocks. Recent articles, like one in Nature Geoscience, suggest that during the mid-Proterozoic, days may have been around 19 hours long for about a billion years. This stalling happened due to the Moon’s gravitational pull and atmospheric tides balancing each other out.
Going even further back, around 500 million years ago when complex animals were emerging, researchers estimate days were about 21 hours long, with 420 days in a year.
Why does this slowing happen? Essentially, the Moon causes tidal bulges in Earth’s oceans, and because Earth spins faster than the Moon orbits, these bulges exert gravitational pulls that push the Moon outward. This interaction means Earth has to slow down a bit.
Interestingly, there are also short-term shifts. For instance, in 2020, Earth’s rotation sped up slightly, with June 29, 2022, marking a record for a day finished in 1.59 milliseconds less than the usual 24 hours. Yet, the long-term trend still shows a gradual slowdown.
So, what does this mean for how we view the past? While a day of 22 hours or 19 hours doesn’t change our daily lives, it challenges our understanding of geological time. Imagine a trilobite wandering a Cambrian seabed with a sun that zipped across the sky in roughly 21 hours, not 24!
Wells’ work in the 60s was groundbreaking. He showed that natural records, like coral growth, could tell us more about the Earth’s history. Understanding how our planet has changed adds depth to our view of time and life itself. For more on the fascinating connections between our planet’s rotation and coral growth, check out this detailed article on Paleontological Research.
