Earth's rotation is slowing down, making each day longer than before—Here's why it's happening

Longer Days Helped Oxygen Fill Earth’s Skies, Say Scientists (Image: Canva)

Each day we live is a little longer than before. But few of us ever pause to think how that extra time may have once changed the course of life on Earth. Now, scientists suggest that the lengthening of Earth’s days played a key role in how our atmosphere gained its life-giving oxygen.

A team of researchers has linked Earth's slowing rotation to the Great Oxidation Event, which made the planet more habitable over 2 billion years ago. Their findings were published in Nature Geoscience in 2021.

The Moon’s pull and the rise of oxygen

The Earth has been slowing down for billions of years. This is due to the Moon’s gravitational tug, which causes the planet’s spin to ease gradually. As a result, the length of a day has grown over time. Around 1.4 billion years ago, a day lasted only 18 hours. Just 70 million years ago, it was 30 minutes shorter than now. On average, we're gaining about 1.8 milliseconds every century.

While this may seem minor, it has made a major difference across vast timescales. The change gave more daylight hours for cyanobacteria – tiny microbes that use sunlight to make oxygen – to thrive. These bacteria first appeared around 2.4 billion years ago and were key players in the Great Oxidation Event.

The cyanobacteria connection

The study was led by Gregory Dick, a microbiologist at the University of Michigan. He explained that the length of Earth's day may have shaped when and how the atmosphere became rich in oxygen. Scientists believe without this oxygen boost, life as we know it may never have emerged.

To test this idea, researchers studied cyanobacteria in Lake Huron’s Middle Island Sinkhole. These modern microbes are similar to the ancient ones that once dominated the Earth’s waters. At night, white microbes that feed on sulfur rise to the top of the microbial mat. When the Sun rises, the purple cyanobacteria slowly climb to the surface and begin to photosynthesise. But they take a while to get started.

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Judith Klatt, a geomicrobiologist at the Max Planck Institute, said the bacteria are “late risers.” This slow start means they need longer days to produce more oxygen. Marine scientist Brian Arbic wondered if longer days in the past could have helped these bacteria make more oxygen.

From the lakebed to planetary history

To explore further, the team ran experiments in the lab and field. They also built models linking sunlight and microbial activity to Earth's oxygen levels. Arjun Chennu of the Leibniz Centre explained that even if two 12-hour days seem equal to one 24-hour day, oxygen output doesn’t follow evenly. That’s because oxygen moves slowly through microbial mats, creating a time delay.

These findings suggest Earth's longer days directly influenced oxygen production on a planetary scale. They also explain a second rise in oxygen, the Neoproterozoic Oxygenation Event, which happened between 550 and 800 million years ago.

Chennu said the team connected processes on vastly different scales – from bacteria on the lakebed to the orbit of the Moon. "We show a link between day length and how much oxygen microbes release,” he said. “It’s exciting to connect the motion of molecules to the motion of our planet.”