Most of us can probably agree that the passage of time has gotten a little weird lately.
While it’s been a few years since the COVID-19 pandemic started, it sometimes feels like it’s only been a few months and then it feels like a decade at the exact same time.
While that’s really all in our heads, the length of a day is constantly changing. And a couple months back, we recorded the shortest day ever at least since atomic clocks started keeping track for us. But while some events on and in Earth might be trying to shorten the day, others are trying to make it longer.
We typically think of a day as being exactly 24 hours long. That’s how we defined hours. But in reality, it’s sometimes a teeny bit shorter, or a teeny bit longer, depending on how fast the Earth is actually spinning.
And on June 29th, 2022, the Earth was spinning even faster than normal, so the day was a record-breaking 1.59 milliseconds shorter. But that record is only for Earth’s recent history ever since humanity started keeping track of time with super accurate atomic clocks.
If you go even further into the past, Earth’s day was hours shorter than it is now. How do we know this?
Well, scientists can use mollusk fossils to measure how long the days were millions of years ago. These mollusks added daily layers of calcium carbonate to their shells, so in general, thinner layers means shorter days.
By combining those measurements with how much the ratio of different elements varied in the environment, research suggests that about 72 to 84 million years ago, the average day on Earth was a whole half hour shorter than it is now.
In other words, the Earth was spinning so fast that there were 372 days in a year, not 365…or 366 in a leap year. And if you go back all the way to 1.4 billion years ago, days were less than 19 hours long.
How is that possible? Well, it all comes down to Earth’s moon, and the fact that the Earth spins faster than the Moon orbits around us.
Because of that difference in speeds, the Moon’s gravity exerts a kind of braking effect, slowing the Earth’s spin by a couple of milliseconds each century. But the length of a day can change over even smaller time frames not just years, but individual days.
Usually, it has to do with how Earth’s mass is distributed. It’s the same physics behind an ice skater changing their spin by pulling their arms in, or by squatting and holding out one of their legs. And Earth has lots of matter to move around.
Air, water, even rock itself moves daily thanks to tidal forces Earth shares with the Sun and Moon. And ice growing at the poles helps squish the Earth so it’s a little less spherical.
According to one hypothesis, this record-setting day may be partially due to climate change. Since the ice sheets and glaciers at the poles have been melting without refreezing to what they were in past winters, they’re squishing the Earth less and less each year.
And like an ice skater spinning faster after pulling in their limbs, the Earth spins a tiny bit faster, too. The days get a little bit shorter, in contrast to the general pattern of days getting longer over millions or billions of years.
It’s a different kind of wibbly wobbly timey wimey. Now, you and I did not notice that missing 1.59 milliseconds, but our technology did. For example, GPS works by sending signals from a series of satellites to a receiver like your smartphone. It uses the Earth’s rotation rate, along with the time that passes between the signals it sends, to calculate your location or how far you’ve traveled. If the Earth is spinning at minutely different rates each day, that can significantly mess up where the GPS tells you you are.
Which can spiral into some serious consequences depending on the technology. For example, scientists can use GPS to monitor where storms, floods, and even wildfires may head, giving people time to evacuate if need be. Now, to compensate for these ever changing day lengths, the scientific community usually adds a leap second to the year every few years.
But if this shortest day on record is any indication that Earth will experience even more days that are shorter than average, we might need to remove a second. And that rate might depend on factors like how fast the polar ice caps continue to melt.
But the Earth can only do so much to keep its days short in the short term, astronomically.
The Moon is still up there, slowing us down. And if the lengthening continues the way it has over the last 1.4 billion years, an Earth day will be a whole 25 hours long in only about 250 million years. Thank you for reading