How The Moon Is Tugging At Time
Daily Tech and Science #-1408
The Day Is Getting Longer—Very, Very Slowly
Did you know that a day on Earth wasn’t always 24 hours long? And in the future, it won’t be 24 hours long either! Thanks to a process called tidal braking, our planet’s rotation is slowing down—so slowly that we barely notice, but over millions of years, the difference adds up.
What Is Tidal Braking?
Tidal braking happens because of the gravitational pull between Earth and the Moon. The Moon’s gravity tugs on Earth’s oceans, creating tides—those rising and falling waves we see at the beach. But here’s the cool part: Earth is spinning faster than the Moon orbits around it. This means the ocean’s tidal bulge is slightly ahead of the Moon in its orbit.
Because of this misalignment, the Moon pulls back on the tidal bulge, slowing down Earth’s rotation little by little. At the same time, Earth’s pull on the Moon causes the Moon to drift about 1.5 inches (3.8 cm) farther away from us each year. So not only is our day getting longer, but the Moon is also escaping—very, very slowly!
How Long Until We Have 25-Hour Days?
Right now, Earth’s rotation slows by about 1.7 milliseconds per century due to tidal braking. That’s tiny, but over millions of years, it adds up. Scientists estimate that at this rate, it will take about 200 million years before an Earth day stretches to 25 hours.
Imagine if people in the far future have an extra hour in their day! Would they sleep more? Work longer? Watch more TV? Too bad none of us will be around to find out.
What’s the Longest a Day Can Get?
Tidal braking won’t last forever. The process will continue until Earth and the Moon reach tidal locking—a state where Earth’s rotation matches the Moon’s orbit, so that one side of Earth always faces the Moon, just like how one side of the Moon always faces us.
If that ever happens, one Earth day would last about 47 current days! That means the Sun would take almost six weeks to rise and set. But don’t worry—it won’t happen for billions of years, long after the Sun has changed so much that Earth might not even be habitable.
What About Leap Seconds?
Even though tidal braking is slow, it still affects timekeeping. Because atomic clocks are incredibly precise, we sometimes have to add leap seconds to keep our clocks in sync with Earth’s actual rotation. This has been done over 27 times since 1972! Scientists keep track of this using super-sensitive measurements of Earth’s rotation speed.
A Future With Longer Days
Tidal braking is a perfect example of how cosmic forces shape our world, even if we don’t notice them. So, while we live in a time of 24-hour days, the distant future holds longer and longer days—if Earth and the Moon stick around long enough to see them. For now, though, enjoy your 24-hour day… because it’s slowly changing, whether we notice or not!