The Earth is trying to give you more hours in your day… but it wasn’t always

Pamela L. Gay, Ph.D.
4 min readJul 8, 2023

As we gear up for our yearly fundraiser, I feel confident in saying that just about everyone on our team is wishing we could somehow cram more hours into every day. At a certain level, our world is happy to oblige, and every year our days get the smallest fraction of a second longer and the Moon gets a few centimeters farther away.

Prague’s Astronomical Clock. Credit: Pamela L Gay

In science, we often find things related in the wildest of ways. In this case, conservation of momentum is trying to keep everything rotating with the same amount of energy: we have the Earth rotating about its axis, the moon rotating and revolving around the Earth, and the Earth and Moon going round and round the Sun. All things being constant, everything would stay exactly the same forever, but… things aren’t constant. Because our Earth isn’t perfectly spherical, the Moon’s gravity is pulling on mountains and others high-mass points a little bit harder, and in the process slowing our world’s rotation. As the Earth slows, the Moon moves outward. This is the same physics that describes how an ice skater can slow down by flinging their arms outward.

In this simple situation, everyday from here forward will get a little bit longer, and everyday — looking back through history — we thought the days would be getting a little bit shorter.

But the universe and our planet are rarely simple, and it turns out that our moon isn’t the only thing affecting our world. Our Sun also affects the Earth, and it has the ability to speed things up.

And new research shows that when the Earth’s day is about 19 hours long, the Moon’s habit of lengthening the day and the Sun’s efforts to shorten the day almost balance out.

According to a new paper by Ross N. Mitchell & Uwe Kirscher appearing in Nature Geoscience, fossil records indicate that from about 1 billion years ago to about 2 billion years ago, our Earth’s rotation rate stalled out at about 19 hours. During this time, evolution also stalled out in its own way: Photosynthetic bacteria weren’t able to create the oxygen-rich atmosphere we enjoy until the days grew longer.

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Pamela L. Gay, Ph.D.

Astronomer, technologist, & creative focused on using new media to engage people in learning and doing science. Opinions & typos my own.