Billion-year conspiracy between Sun and Moon prevented 60-hour days
If you’ve ever felt like you need more hours in the day, well how does 60 sound? A new study suggests we’d have 60-hour days by now if a stalemate between the Sun and Moon hadn’t interfered with Earth’s rotation for over a billion years.
A day seems like a pretty stable amount of time, but in fact today is longer than this same date was a century ago – albeit by a mere 1.8 milliseconds. That’s because the Moon is very gradually drifting away from Earth, which is causing our planet’s rotation to steadily slow down, and of course if the world turns more slowly the day becomes longer.
Over the lifetime of the planet, the day has had many different lengths. Soon after the Moon first formed, around 4.5 billion years ago, the day was less than 10 hours long. Near the end of the reign of the dinosaurs, around 70 million years ago, it had stretched out to a more familiar 23.5 hours.
But that rate of change doesn’t quite add up, according to researchers at the University of Toronto. If the day had continued to lengthen at a consistent rate, it would be around 60 hours long today. It’s intriguing to ponder the cultural and even biological differences that would have invoked in humans and other animals.
So what happened? The researchers have investigated a hypothesis that a specific combination of factors from the Earth, Moon and Sun came together in just the right way to stall the slowing of the rotation for a long period of time. And by examining geological evidence and atmospheric research tools, there appears to be some truth to it.
We all know the Moon is responsible for the ocean’s tides, as its gravity pulls the water into “bulges” in different directions during the course of the day. This is also how the Moon slows down Earth’s rotation, thanks to that gravitational pull and the friction it causes between the tides and the seafloor. But it’s not the only force acting on the Earth’s spin.
“Sunlight also produces an atmospheric tide with the same type of bulges,” said Professor Norman Murray, corresponding author of the study. “The Sun's gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing down Earth’s rotation like the Moon, it speeds it up.”
The Moon’s effects, however, are around 10 times stronger than the Sun’s, so it wins the tug-of-war and the Earth’s spin slows. But the researchers on the study found that these two factors once reached an equilibrium, starting around 2.2 billion years ago.
At that time, the air in the atmosphere was warmer, so the bulges created by the Sun moved through it faster, taking about 10 hours to circle the planet. By a lucky coincidence, the Earth’s rotation had by then slowed to a 20-hour day. Because the two tides were in a 1:2 resonance, it reinforced the atmospheric tide and made the Sun’s influence stronger.
The end result was that the Sun and Moon balanced each other out, so the length of the day stopped growing. As such, it was locked at 19.5 hours for an astonishing 1.6 billion years. Without that extended pause, we’d be looking at 60-hour days and very different lifestyles by now.
The team says that this study suggests climate change could have intriguing implications on the length of the day going forward.
“As we increase Earth's temperature with global warming, we’re also making the resonant frequency move higher – we’re moving our atmosphere farther away from resonance,” said Murray. “As a result, there's less torque from the sun and therefore the length the day is going to get longer – sooner than it would otherwise.”
The research was published in the journal Science Advances.
Source: University of Toronto