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News has just been released all over my news feeds about how paleontologists now believe that early photosynthetic organisms became much more efficient at producing oxygen after the Earth started to slow down more and more, going from a 4- or 4.5-hour day-night cycle shortly after its creation to something closer to our current 24-hour cycle....

My question is; how do they know that the Earth used to rotate so much faster, so long ago?

Are they basing this assumption on computer models of planet formation?

Or, the fact that every few years the Grand International Timekeepers Guild (or whoever) adds a second to the end of the year, on New Years' Eve.....

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The Moon's gravitational force exerted on the Earth's tidal bulge results in a net torque opposite the direction of Earth rotation, pictured very nicely in the below wikipedia graphic.

enter image description here

The Earth's rotation rate is gradually decreasing due to this tidal interaction with the Moon. In order for the angular momentum of the Earth/Moon system to be conserved, the Moon's orbital momentum has to increase, which also causes the Moon's orbital distance from the Earth to increase. This theory of tidal torques explains the tidally locked systems in our Solar System, including the tidal lock of the Moon to the Earth (the Moon's rotation time is the same as it's revolution time).

The rate of deceleration of the Earth's rotation is so small that it can't be easily measured real-time. In fact, over the short time, changes in atmospheric and ocean currents can temporarily cause the Earth's rotation to speed up!

However, over the long term, the minor changes to the rotation rate even out. And the fossil record supports the theory of gradual deceleration of the Earth's rotation! From Deines and Williams 2016:

enter image description here

Similar to using tree rings, various sea creatures have daily as well as yearly (seasonal) deviations that can be used to tell how many days are in each year, and hence how long a day is at a particular geologic time. To quote their paper:

Early paleontological papers dealing with geochronometry concentrated on corals, bivalves, and brachiopods, which have general daily growth increments that indicate annual variations. With successive alteration of daylight and darkness, growth occurs faster during daylight. Annual variations could be due to changes in daylight length, water temperature, or even food supply. Shorter periods are found within the annual variations, which can imply seasonal fluctuations or lunar synodic months. Variability in specimen growths was verified in recent animal growth tests. Although the fossil record was reliable, it did require many samples or long, continuous growth histories to get better estimates for Earth's rotation rate in ancient times.

It should also be noted that the Earth's rotation is slowing due to similar tidal interactions with the Sun, though these effects are much smaller than those due to the Moon.

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  • $\begingroup$ Thank you!! Much appreciated!! $\endgroup$
    – Kurt Hikes
    Commented Aug 3, 2021 at 19:02
  • $\begingroup$ Shorter term measurements of the changing Earth rotation from 720 BC to 2015 AD from eclipse and occultation timings were discussed in Stephenson et al. 2015 (free to read) $\endgroup$
    – astrosnapper
    Commented Aug 3, 2021 at 20:46
  • $\begingroup$ Nice explanation of the mechanism to slow down earth’s rotation. BUT: how do they determine the rotation (as provided in the figure) millions of years ago? This is a great confirmation of the theory that tidal forces influence the rotation. But “Fossil rings” is quite a vague explanation and I couldn’t find the paper directly. $\endgroup$
    – Hartmut Braun
    Commented Aug 4, 2021 at 5:22
  • $\begingroup$ It'd be interesting to learn details about how the fossil record supports the theory, but that may fit better over on Earth Science than here. $\endgroup$
    – gerrit
    Commented Aug 4, 2021 at 7:48
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    $\begingroup$ @gerrit I think it is relevant for this answer, so I added an explanation. $\endgroup$
    – Connor Garcia ♦
    Commented Aug 4, 2021 at 15:14
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Good insight to the common understanding of the Earth spinning velocity (The Moon's gravitational pull, wind speeds, movement in the Earth's core, atmospheric pressure, the melting of the ice glaciers), which have lot's of scientific approach to make past time approximation. Today, when we have the best precision of making spinning measurements by atomic clock, scientists observe noticeable fluctuations of how fast the Earth is spinning. Actually, recently speeding up is observed. And this is subject is vital for some technical reasons (e.g. GPS synchronization and all related technology behavior).

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