The Moon’s Tug

Three weeks ago, for my Earth System Dynamics course, I listened to a Radiolab podcast about the distance between the Earth and the Moon. In “The Times They Are a Changin'” paleontologists talk about how coral shells taught us that the Earth used to have shorter days. Their shells have tiny bands, an alternating pattern of stripes, that have tiny lines in them, each of which records a day. Paleontologists first noticed that living corals each have 365 little lines in the bands on their shells. These shells are basically little calendars and clocks. When paleontologists went back to investigate how many of these little lines were on corals from the Ordovician Era, 450 million years ago, they found that there were over 415 little lines, meaning there were 415 days in the year. The Earth has not been speeding up around the Sun, slowing down our years. Instead, the days have been getting longer. The “celestial waltz” between the Earth and the Moon is the reason our planet spins slower than it once did. It’s the reason we have the 24-hour days we are accustomed to. Since the Earth is rotating faster than the Moon is orbiting it, the Earth tugs the Moon along and the Moon tugs the Earth back, slowing it down. This means that each day is 54-billionths of a second longer than the day before it. That means today is the shortest day of your life!

A rendering of a “coral moon” by WNYC’s Sahar Baharloo

Why does this matter? The Earth isn’t slowing enough to really affect me. The day I die will be only a few seconds longer the day I was born. But taking a step back, it helps me appreciate the delicate balance that allowed me to get here. All of the things that we take for granted about our planet that allow us to live here are constantly changing. Around the time that the first creatures moved out of the water and onto land (almost 400 million years ago), the Moon was a lot closer. That means that the tides would have been much more dramatic. Perhaps having much larger tides would have helped sea creatures make their way ashore. Perhaps it is the case that without the 415-day year the ancestors that led humans ashore would have never been able to get out of the comfort of the oceans. If it weren’t for the proximity of the Moon hundreds of millions of years ago would we be mermaids instead of humans? We’ll never know, but it is always nice to think of how many different things had to go exactly the way they did in order for us to be alive today.

If you found this topic interesting, check out another Radiolab podcast, “The Distance of the Moon,” in which actor Liev Schreiber reads a story written by Italo Calvino and telling of a time that the Moon was so close to the Earth that you could jump back and forth between the two. Thanks for reading!

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