The Days are Getting Longer: It’s Recorded in the Fossils


Dr. Rowland back for our first Fossil Friday of 2021!


Does it seem like the years are flying by, with each year going faster than the one before? Well, the years are actually going by more slowly. As we celebrate the beginning of a new year, it is a good time to explore how some fossils record celestial dynamics. Let me start by saying that the length of time it takes for each trip around the sun does not change. What changes is Earth’s spin rate; it is slowing down. Fluids sloshing around on Earth―the oceans and the fluid outer core―are a drag on Earth’s spin rate. So, Earth spins slightly slower each year than it did the previous year. “How much slower?” you might ask. So, I’ll tell you: 0.0018 seconds per year slower! Okay, it’s a tiny amount, but it adds up over geologic time.


Over the past several hundreds of millions of years, as the spin rate has slowed, the number of days per year has been decreasing. And the results are recorded in fossil corals. The first image shows a rugose coral (also called a horn coral) that is approximately 400 million years old, from the Devonian Period. Notice the conspicuous ridges, or ‘rugosities,’ on the shell. The presence of these rugosities is what gives these corals their name rugose corals. Each ridge represents one year of growth.

The second image schematically shows tiny growth lines within a rugose coral, seen by cutting open the fossil and looking at it with a microscope. These microscopic growth lines record daily increments of shell growth. You see where I’m going with this: the presence of daily growth lines, together with annual ridges, allow paleontologists to determine how many days were in a year when this coral was growing its shell. The results indicate that each year was about 400 days long in the Devonian Period, 400 million years ago.


‘Happy New Year’ from the Las Vegas Natural History Museum. This year you will have 180,000 more nanoseconds to visit the museum!

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