<p>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 <a href="https://phys.org/news/2021-01-earth-faster.html" rel="nofollow noreferrer">speeding up</a> is observed.
And this is subject is vital for some technical reasons (e.g. GPS synchronization and all related technology behavior).</p>

CC BY-SA 4.0
2021-08-05T15:17:24.867Z
2021-08-05T15:17:24.867Z
Lariliss
<p>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 <a href="https://en.wikipedia.org/wiki/Tidal_acceleration" rel="noreferrer">wikipedia</a> graphic.</p>
<p><a href="https://i.sstatic.net/JFVsm.png" rel="noreferrer"><img src="https://i.sstatic.net/JFVsm.png" alt="enter image description here" /></a></p>
<p>The Earth's rotation rate is gradually decreasing due to this <a href="https://en.wikipedia.org/wiki/Tidal_acceleration" rel="noreferrer">tidal interaction</a> 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).</p>
<p>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 <a href="https://phys.org/news/2021-01-earth-faster.html" rel="noreferrer">Earth's rotation to speed up</a>!</p>
<p>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 <a href="https://iopscience.iop.org/article/10.3847/0004-6256/151/4/103" rel="noreferrer">Deines and Williams 2016</a>:</p>
<p><a href="https://i.sstatic.net/91PtW.jpg" rel="noreferrer"><img src="https://i.sstatic.net/91PtW.jpg" alt="enter image description here" /></a></p>
<p>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:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>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.</p>

CC BY-SA 4.0
2021-08-03T19:02:39.460Z
Thank you!! Much appreciated!!
Kurt Hikes
CC BY-SA 4.0
2021-08-03T20:46:42.497Z
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](https://royalsocietypublishing.org/doi/10.1098/rspa.2016.0404) (free to read)
astrosnapper
CC BY-SA 4.0
2021-08-04T05:22:31.693Z
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.
Hartmut Braun
CC BY-SA 4.0
2021-08-04T07:48:54.100Z
It'd be interesting to learn details about _how_ the fossil record supports the theory, but that may fit better over on [EarthScience.SE] than here.
gerrit
CC BY-SA 4.0
2021-08-04T15:13:40.737Z
@HartmutBraun  I added a link to the paper and some verbiage and a quote for more explanation.
Connor Garcia
CC BY-SA 4.0
2021-08-04T15:14:30.980Z
@gerrit I think it is relevant for this answer, so I added an explanation.
Connor Garcia
CC BY-SA 4.0
2021-08-03T18:51:41.610Z
2021-08-04T15:12:54.507Z
2021-08-04T15:12:54.507Z
Connor Garcia
<p>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....</p>
<p>My question is; how do they know that the Earth used to rotate so much faster, so long ago?</p>
<p>Are they basing this assumption on computer models of planet formation?</p>
<p>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.....</p>

CC BY-SA 4.0
2021-08-03T18:36:58.147Z
Possible duplicate of [Where do we have it from that the Moon is migrating away from Earth?](https://astronomy.stackexchange.com/questions/36925)
David Hammen
CC BY-SA 4.0
2021-08-04T02:35:47.257Z
Related: https://astronomy.stackexchange.com/questions/26253/how-fast-was-the-earth-spinning-directly-after-the-moon-formed
Nilay Ghosh
<solar-system><earth><planetary-formation><solar-system-evolution><archaeoastronomy>
How do scientists know that Earth used to spin much more rapidly?
Kurt Hikes
# How do scientists know that Earth used to spin much more rapidly?

(Asked by: username_0 on 2021-08-03)

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.....

**Tags:** solar-system, earth, planetary-formation, solar-system-evolution, archaeoastronomy

> **Comments:**
> **username_1**: Possible duplicate of [Where do we have it from that the Moon is migrating away from Earth?](https://astronomy.stackexchange.com/questions/36925)  
> **username_2**: Related: https://astronomy.stackexchange.com/questions/26253/how-fast-was-the-earth-spinning-directly-after-the-moon-formed  

---

## 2 answers

### Answer by username_4 on 2021-08-03. Score: 8 | Accepted answer

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](https://en.wikipedia.org/wiki/Tidal_acceleration) graphic.

[![enter image description here](https://i.sstatic.net/JFVsm.png)](https://i.sstatic.net/JFVsm.png)

The Earth's rotation rate is gradually decreasing due to this [tidal interaction](https://en.wikipedia.org/wiki/Tidal_acceleration) 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](https://phys.org/news/2021-01-earth-faster.html)!

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](https://iopscience.iop.org/article/10.3847/0004-6256/151/4/103):

[![enter image description here](https://i.sstatic.net/91PtW.jpg)](https://i.sstatic.net/91PtW.jpg)

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.

> **Comments:**
> **username_0**: Thank you!! Much appreciated!!  
> **username_5**: 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](https://royalsocietypublishing.org/doi/10.1098/rspa.2016.0404) (free to read)  
> **username_6**: 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.  
> **username_7**: It'd be interesting to learn details about _how_ the fossil record supports the theory, but that may fit better over on [EarthScience.SE] than here.  
> **username_4**: @HartmutBraun  I added a link to the paper and some verbiage and a quote for more explanation.  
> **username_4**: @username_7 I think it is relevant for this answer, so I added an explanation.  

---

### Answer by username_3 on 2021-08-05. Score: -2

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](https://phys.org/news/2021-01-earth-faster.html) is observed.
And this is subject is vital for some technical reasons (e.g. GPS synchronization and all related technology behavior).


---

