Days on Earth have grown longer - but it didn’t happen overnight
- Research on ancient rock samples suggests that for about a billion years, the Earth rotated every 19 hours
- The period corresponds to the ‘boring billion’ when there were few environmental and biological changes
More than a billion years ago, a day on Earth lasted 19 hours, rather than the 24-hour period we know today, according to a study of ancient rock samples from southwest China.
Between 1 billion and 2 billion years ago, days were shorter than they are now because the Earth’s natural satellite – the moon – was closer.
Over time, the moon used the planet’s rotational energy to move into a higher orbit farther from Earth. This slowed the Earth’s rotation, resulting in longer days.
In the study, scientists from Australia and China found that day length did not change steadily over the Earth’s 4.5 billion years of history.
Instead, they found that the length of a day stalled for about a quarter of the Earth’s history because the sun’s push balanced the moon’s pull, temporarily stabilising the Earth’s rotation in a 19-hour day cycle.
“The day length stalled at about 19 hours for about 1 billion years during the mid-Proterozoic,” the researchers wrote in an article published in Nature Geoscience on Monday. The Proterozoic period of Earth’s history began 2.5 billion ago and ended 541 million years ago.