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For A Billion Years, A Day On Earth Was Only 19 Hours Long

At one point, not all of us had the same amount of hours in a day. Learn what controls the length of a day.

ByConor Feehly
Credit:Krishna Vihari/Shutterstock

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Days come and go. Our circadian rhythms are tuned to the rising and setting of the Sun and the push and pull of the Moon. Days appear stable to us, an endless cycle of day and night, unless we decide to change our clocks, there are always 24 hours in a day.

However, change is the norm when it comes to Earth's day length over geological periods. Earth's satellite, our Moon, steals rotational energy to travel farther and farther away from us over time. And as the Moon's gravitational effect on the Earth diminishes, so does its ability to affect our day length.

"It is certainly the pull of the Moon's gravity that causes the tides in the oceans to ebb and flow, and that is the dominant factor controlling Earth's day length.

There are indeed other forces that affect Earth's spin, but they generally all pale in comparison to ...

  • Conor Feehly

    Conor Feehly is New Zealand-based science writer who covers a wide range of topics, including astronomy and neuroscience, with an eye for research at the intersection of science and philosophy. He received a masters in science communication degree from the University of Otago. Conor is a regular contributor to Discover Magazine, with his work also appearing in New Scientist, Nautilus Magazine, Live Science, and New Humanist among others.

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