A Day on Earth Used to Be Just 19 Hours Long

Earth gained only 0.047 seconds per day since the Bronze Age collapsed around 1,200 BCE.

Key Takeaways

• Earth’s day length wasn’t always 24 hours; it once paused at 19 hours for a billion years.
• The Moon’s gravitational pull usually lengthens days, but atmospheric forces counteracted this effect.
• The pause in day length coincided with the Great Oxidation Event, which altered Earth’s atmosphere.
• Scientists used sediment data and cyclostratigraphy to uncover why the pause occurred.
• Modern days lengthen at a glacial pace—just 0.000015 seconds per year.

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Earth’s Day Length: A Billion-Year Pause

Earth’s 24-hour day is a relatively recent development in geological terms. In the Mesozoic Era, roughly 66 million years ago, a day lasted 23 hours. Going further back to 1.4 billion years ago, Earth spun so quickly that a day was only 18 hours and 41 minutes long. This slowdown is primarily due to the Moon, which formed 4.5 billion years ago and has been steadily moving away from Earth, reducing its gravitational influence and causing longer days.

However, scientists have discovered a surprising deviation from this trend. For about a billion years—between 2 billion and 1 billion years ago—Earth’s day length remained static at around 19 hours. This period, known as the “boring billion” due to its relative lack of significant evolutionary developments, defied the assumption that Earth’s days always grew longer over time.

Atmosphere vs. the Moon’s Pull

Researchers from the Chinese Academy of Sciences and Curtin University investigated this anomaly using advanced cyclostratigraphy, which analyzes sedimentary layers for climate patterns caused by astronomical events. Their findings, published in Nature Geoscience, revealed that Earth’s atmosphere during this era created opposing forces that neutralized the Moon’s tidal influence.

The team discovered that atmospheric tides, intensified by changes in solar radiation and the composition of Earth’s atmosphere, counteracted the Moon’s gravitational effects. According to Curtin University’s Uwe Kirscher, this balance of forces caused a “tidal resonance,” effectively freezing the Earth’s day length.

The timing of this pause aligns with the Great Oxidation Event, a temporary rise in atmospheric oxygen that led to the formation of an ozone layer. This atmospheric shift likely enhanced solar atmospheric tides, contributing to the stabilization of day length.

Implications for Life and Beyond

The prolonged 19-hour day may have delayed the evolution of life on Earth. Early photosynthetic bacteria required sufficient sunlight to produce enough oxygen for more complex life forms, and longer days could have supported this process.

Today, Earth’s rotation is no longer stuck, and days are slowly lengthening by 0.000015 seconds annually. This rate is so gradual that an Earth day has only increased by 0.047 seconds since 1,200 BCE, making any significant impact far beyond human timescales.

The discovery sheds light on Earth’s dynamic history, emphasizing the intricate interplay between its atmosphere, celestial influences, and the evolution of life.