It’s not just you – the days really are getting longer. More than a billion years ago, the moon used to be about 40,000 kilometres closer, which made Earth spin faster. Back then, the days were less than 19 hours long.
Over the course of many thousands of years, Earth’s way of moving through the solar system goes through cyclical changes. The planet’s elliptical orbit shifts around the sun like a hula hoop, and the shape of the orbit itself wobbles between elliptical and nearly circular. Earth’s axis of rotation also tilts back and forth, and gradually moves in a circle like the handle on a spinning top.
Recorded in the chemical makeup of sedimentary rocks is evidence of the corresponding changes in the environment caused by these shifts.
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“All of those astronomical cycles influence how sunlight is distributed on the planet’s surface, and sunlight is what drives our climate systems,” says Stephen Meyers at the University of Wisconsin. He and Alberto Malinverno at Columbia University in New York used a statistical approach to work backwards from two ancient rocks – one about 55 million years old found in the Atlantic Ocean, and one 1.4 billion years old found in China – to determine the Earth’s position in the early solar system.
“When we look up in the sky at other stars, we’re looking back millions or billions of years into the past. We can’t do that with our solar system,” says Meyers. “If we want to look at the ancient solar system, we have to look at the rock record.”
Don’t look up, look down
They used a complex statistical program to compare the rock data to a range of theoretical models of the early solar system. The best match was a model in which, when the rock deposit was formed 1.4 billion years ago, the moon was about 341,000 kilometres away and receding at a slower rate than it is now.
The moon’s location in particular has a strong influence on how long days are on Earth. The closer the moon is to Earth, the faster the planet rotates. This in turn affects the climate, which changes the chemical makeup of the layers of sedimentary rock. They found that 1.4 billion years ago, days were only about 18.7 hours long.
“It’s a huge step forward for geology,” says Linda Hinnov at George Mason University in Virginia. Without this statistical method, the only way to examine the Earth’s rotation rate and relationship to the moon more than 500 million years ago is to look for signs of early Earth’s tides in an extremely rare kind of rock called a tidalite, of which fewer than 10 have been found, she says.
“Now we can use this method to go back and fill in, for very specific times, what the day length and Earth-moon distance was,” Hinnov says. “It’s just awesome.”
PNAS
Read more: The moon may have formed in a vaporised, doughnut-shaped Earth
