Earth At Twilight. NASA.

Earth’s day lasted 19.5 hours for over a billion years

New research reveals that between two billion and 600 million years ago, an atmospheric tide induced by the sun played a crucial role in counterbalancing the lunar influence, ensuring a consistent rotational rate for Earth and a fixed day length of 19.5 hours. Had this remarkable phenomenon not occurred, the gradual deceleration of our planet's rotation would have continued uninterrupted, resulting in our present-day 24-hour day expanding to a staggering duration of more than 60 hours.


Did you know that Earth’s day lasted 19.5 hours for over a billion years? A group of astrophysicists from the University of Toronto (U of T) have uncovered the surprising explanation of why Earth’s day remained locked at 19.5 hours for more than a billion years. They highlight the ancient tug-of-war between the sun’s atmospheric tide and the moon’s gravitational pull, keeping Earth’s spin rate constant and the day’s duration steady.

Earth’s Day Lasted 19.5 Hours for Over a Billion Years

If it weren’t for this billion-year halt in Earth’s rotational slowing, a day in our modern lives would extend to over 60 hours. The groundbreaking study titled, “Why the day is 24 hours long; the history of Earth’s atmospheric thermal tide, composition, and mean temperature,” was released in the respected journal Science Advances today. The researchers leveraged geological clues and atmospheric science to shed light on this stalemate between the Sun and Moon, resulting from an unexpected but significant interplay between Earth’s atmospheric temperature and its rotational speed.

The Role of the Moon and Sun in Earth’s Rotation

Since its formation roughly 4.5 billion years ago, the moon has been applying a brake to Earth’s rotation through its gravitational influence, progressively lengthening our days. This deceleration continues even now, adding roughly 1.7 milliseconds to our day every century.

The moon’s gravity shapes oceanic tides, generating a braking effect on Earth’s rotation. Similarly, the sun’s gravity influences atmospheric tides, accelerating Earth’s spin instead of slowing it. For the most of Earth’s geological past, the moon’s influence dominated, causing Earth’s day to lengthen slowly.


Resonating With the Past

Around two billion years ago, the atmospheric tides magnified due to a warmer atmosphere and its natural resonance – the rhythm at which waves traverse it, matched the day’s length. This resonance depends on several factors, including temperature, much like the consistent note produced by a bell at a steady temperature.

In the billion-year period under examination, the warmer atmosphere resonated with a ten-hour period, matching with Earth’s rotation then slowed to 20 hours by the moon. When these factors became synchronized, the atmospheric tide grew, and the sun’s gravitational pull matched the moon’s, balancing the forces.

Learning from the Past for a Warmer Future

Using atmospheric circulation models, similar to those employed for studying global warming, the research team successfully predicted the atmospheric temperature during this period. These models validate the climate crisis, showing that changes in atmospheric temperature could have repercussions on Earth’s day length in the future.


Meta Description: An exciting study reveals why Earth’s day was 19.5 hours for over a billion years, and warns of the potential future impact of global warming on day length. Explore this billion-year standoff between the sun and moon that shaped our 24-hour day.

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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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